Heart

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This article uses anatomical terminology; for an overview, see anatomical terminology.

Heart
Heart rotating.gif
The heart sits at the center of the chest. The left side has a greater muscle mass, and as the apex points slightly to the left the heart is often claimed to be on the left side of the chest.
Heart anterior exterior view.jpg
Anatomical illustration of a human heart
Latin cor
Greek kardía (καρδία)
Gray's p.506
System Circulatory
Artery Right coronary artery, left coronary artery, anterior interventricular artery
Vein Superior vena cava, inferior vena cava, right pulmonary veins, left pulmonary veins
MeSH Heart
Dorlands
/Elsevier
Heart
TA A12.1.00.001
FMA FMA:7088
Anatomical terminology
Normal heart sounds as heard with a stethoscope

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The heart is a vital muscular organ in humans and many other animals, which functions to pump blood around the body through the blood vessels of the circulatory system.[1] In doing so, the blood is able to provide the body with oxygen and nutrients, as well as removing metabolic waste. The heart is located in the chest; specifically in the mediastinum, the middle compartment of the thorax.

The heart is divided into four main chambers: the upper left and right atria, and the lower left and right ventricles.[2] Commonly the right atrium and ventricle together are referred to as the right heart and their left counterparts as the left heart.[3] Between the right and left heart is a dividing wall of muscle the septum. The portion of the septum separating the ventricles, the ventricular septum is thicker than that which separates the atria−the atrial septum. The flow of blood is controlled by heart valves, which can be found roughly level in a roughly horizontal plane referred to as the valve plane.[citation needed] The heart is enclosed in a protective sac, called the pericardium which also contains lubricating fluid. The outer wall of the heart is made up of three layers, the epicardium, the myocardium (which is the actual muscle of the heart), and the endocardium.[4]

With every heartbeat the heart pumps blood through both of the body's double circulatory systems. Deoxygenated blood from the systemic circulation enters though the vena cava to the right atrium, where it passes to the right ventricle, after which it is pumped though the pulmonary circulation, to the lungs where it is oxygenated. Fully oxygenated blood is then pumped back to the left atrium, passes through the left ventricle and is pumped out through the aorta back to the systemic circulation−where the oxygen is used. Normally with each heartbeat, the right ventricle pumps the same amount of blood into the lungs that the left ventricle pumps out into the body. Veins transport blood to the heart, while arteries transport blood from the heart, regardless of oxygenation, (e.g. the pulmonary artery carries deoxygenated blood from the heart to the lungs). Veins are further along the circulatory system than arteries, and generally have lower pressures.

The heart and circulatory system are vital for life and diseases affecting either (CVD - cardiovascular diseases) are majors killers both in the developed and developing world. The human heart beats at a rate of 60 to 90 beats per minute (at rest). Exercise temporarily increases this rate, but lowers resting heart rate in the long term, and is good for heart health. Poor heart health increases the risk of CVD–preventable factors causing poor heart health are smoking, being overweight, not enough exercise, high cholesterol, and high blood pressure among others. Inherited or non-modifiable factors such as genetic disorders involving decreased function of the heart also exist. The human heart and associated cardiovascular diseases are studied and primarily treated by cardiologists–although central to all specialties of medicine.

Contents

Structure

Real-time MRI of the human heart
Surface anatomy of the human heart. The heart is demarcated by:
-A point 9 cm to the left of the midsternal line (apex of the heart)
-The seventh right sternocostal articulation
-The upper border of the third right costal cartilage 1 cm from the right sternal line
-The lower border of the second left costal cartilage 2.5 cm from the left lateral sternal line.[5]

The heart is situated in the middle of the chest beneath the breastbone in the mediastinum, at the level of T5-T8. The largest part of the heart is usually slightly offset to the left (though occasionally it may be offset to the right). The heart is usually felt to be on the left side because the left heart is stronger, since it pumps to all body parts. The left lung in turn is smaller than the right lung because it has to accommodate the heart. The heart is supplied by the coronary circulation and is enclosed in the pericardial sac. The pericardium comprises two parts: the fibrous pericardium, made of dense fibrous connective tissue, and the serous pericardium.[6] Between the outer parietal layer and the visceral serous layer is the pericardial cavity which contains the pericardial fluid which protects and lubricates the heart.[7]

The apex is the blunt point situated in an inferior (pointing down and left) direction. A stethoscope can be placed directly over the apex so that the beats can be counted. It is located posterior to the 5th intercostal space just medial of the left mid-clavicular line. In normal adults, the mass of the heart is 250–350 grams (9–12 oz), but an extremely diseased heart can be up to 1000 g (2 lb) in mass due to hypertrophy. It is about the size of a fist.[8]

Heart being dissected showing chambers

The pericardium separates the heart from the other mediastinal structures and also attaches to the mediastinal fascia, providing anchorage for the heart.[4] The dorsal surface of the heart lies near to the vertebrae, and the anterior surface sits deep to the sternum and costal cartilages.[4] The great veins, the superior and inferior venae cavae, and the great arteries, the aorta and pulmonary trunk, are attached to the superior surface of the heart, called the base. The base of the heart is located at the level of the third costal cartilage.[4] The inferior tip of the heart, the apex, lies just to the left of the sternum between the junction of the fourth and fifth ribs near their articulation with the costal cartilages.[4] The right side of the heart is deflected anteriorly, and the left side is deflected posteriorly.[4]

The shape of the heart is similar to a pinecone, rather broad at the superior surface and tapering to the apex.[4] A typical heart is approximately the size of a fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness.[4] Given the size difference between most members of the sexes, the weight of a female heart is approximately 250–300 grams (9–11 ounces), and the weight of a male heart is approximately 300–350 grams (11–12 ounces). The heart of a well-trained athlete, especially one specializing in aerobic sports, can be considerably larger than this.[4] Cardiac muscle responds to exercise in a manner similar to that of skeletal muscle.[4] That is, exercise results in the addition of protein myofilaments that increase the size of the individual cells without increasing their numbers, a concept called hypertrophy.[4] Hearts of athletes can pump blood more effectively at lower rates than those of nonathletes.[4] Enlarged hearts are not always a result of exercise; they can result from pathologies, such as hypertrophic cardiomyopathy.[4] The cause of an abnormally enlarged heart muscle is unknown, but the condition is often undiagnosed and can cause sudden death in apparently otherwise healthy young people.[4]

The human heart has four chambers two superior atria and two inferior ventricles.

Structure diagram of the human heart from an anterior view. Blue components indicate deoxygenated blood pathways and red components indicate oxygenated pathways.

Chambers

Blood flows from the right atrium to the right ventricle, where it is pumped into the pulmonary circuit.[4] The blood in the pulmonary artery branches is low in oxygen but relatively high in carbon dioxide. Gas exchange occurs in the pulmonary capillaries (oxygen into the blood, carbon dioxide out), and blood high in oxygen and low in carbon dioxide is returned to the left atrium.[4] From here, blood enters the left ventricle, which pumps it into the systemic circuit. Following exchange in the systemic capillaries (oxygen and nutrients out of the capillaries and carbon dioxide and wastes in), blood returns to the right atrium and the cycle is repeated.[4]
Main article: Heart chamber

The heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers. The right atrium receives deoxygenated blood from the body and the left atrium receives oxygenated blood from the lungs. When these contract the blood is pushed into the ventricles, which pump to propel the blood to the lungs and the rest of the body.[4] The right atrium and the right ventricle together are sometimes referred to as the right heart and this sometimes includes the pulmonary trunk. Similarly, the left atrium and the left ventricle together are sometimes referred to as the left heart. The atrioventricular septum separates the left heart from the right heart, giving two functionally and anatomically distinct units.[citation needed]

There are two distinct but linked circuits in the human circulation called the pulmonary and systemic circuits. Although both circuits transport blood and everything it carries, one can view the circuits from the point of view of gases.[4] The pulmonary circuit transports blood to and from the lungs, where it picks up oxygen and delivers carbon dioxide for exhalation.[4] The systemic circuit transports oxygenated blood to virtually all of the tissues of the body and returns relatively deoxygenated blood and carbon dioxide to the heart to be sent back to the pulmonary circulation.[4]

The right ventricle pumps deoxygenated blood into the pulmonary trunk, which leads toward the lungs and bifurcates into the left and right pulmonary arteries.[4] These vessels in turn branch many times before reaching the pulmonary capillaries, where gas exchange occurs: Carbon dioxide exits the blood and oxygen enters.[4] The pulmonary trunk arteries and their branches are the only arteries in the post-natal body that carry relatively deoxygenated blood.[4] Highly oxygenated blood returning from the pulmonary capillaries in the lungs passes through a series of vessels that join together to form the pulmonary veins—the only post-natal veins in the body that carry highly oxygenated blood.[4] The pulmonary veins conduct blood into the left atrium, which pumps the blood into the left ventricle, which in turn pumps oxygenated blood into the aorta and on to the many branches of the systemic circuit.[4] Eventually, these vessels will lead to the systemic capillaries, where exchange with the tissue fluid and cells of the body occurs.[4] In this case, oxygen and nutrients exit the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter the blood.[4]

The blood exiting the systemic capillaries is lower in oxygen concentration than when it entered.[4] The capillaries will ultimately unite to form venules, joining to form ever-larger veins, eventually flowing into the two major systemic veins, the superior vena cava and the inferior vena cava, which return blood to the right atrium.[4] The blood in the superior and inferior venae cavae flows into the right atrium, which pumps blood into the right ventricle. This process of blood circulation continues as long as the individual remains alive.[4] Understanding the flow of blood through the pulmonary and systemic circuits is critical to all health professions.[4]

Membranes, surface features, and layers

Membranes

The pericardial membrane that surrounds the heart consists of three layers and the pericardial cavity. The heart wall also consists of three layers.[4] The pericardial membrane and the heart wall share the epicardium.[4]

The membrane that directly surrounds the heart and defines the pericardial cavity is called the pericardium or pericardial sac. It also surrounds the “roots” of the major vessels, or the areas of closest proximity to the heart.[4] The pericardium, which literally translates as “around the heart,” consists of two distinct sublayers: the sturdy outer fibrous pericardium and the inner serous pericardium.[4] The fibrous pericardium is made of tough, dense connective tissue that protects the heart and maintains its position in the thorax.[4] The more delicate serous pericardium consists of two layers: the parietal pericardium, which is fused to the fibrous pericardium, and an inner visceral pericardium, or epicardium, which is fused to the heart and is part of the heart wall. The pericardial cavity, filled with lubricating serous fluid, lies between the epicardium and the pericardium.[4]

In most organs within the body, visceral serous membranes such as the epicardium are microscopic.[4] However, in the case of the heart, there is a macroscopic layer, consisting of a simple squamous epithelium called a mesothelium, reinforced with loose, irregular, or areolar connective tissue that attaches to the pericardium. This mesothelium secretes the lubricating serous fluid that fills the pericardial cavity and reduces friction as the heart contracts.[4] This enables the heart to both move in response to its own contractions and to the movements of adjacent structures such as the diaphragm and lungs.[9] Figure 3 illustrates the pericardial membrane and the layers of the heart.[4]

Surface features

Inside the pericardium, the surface features of the heart are visible, including the four chambers. There is a superficial leaf- like extension of the atria near the superior surface of the heart, one on each side, called an auricle—a name that means “ear like”—because its shape resembles the external ear of a human. Auricles are relatively thin-walled structures that can fill with blood and empty into the atria or upper chambers of the heart. You may also hear them referred to as atrial appendages. Also prominent is a series of fat-filled grooves, each of which is known as a sulcus (plural = sulci), along the superior surfaces of the heart. Major coronary blood vessels are located in these sulci. The deep coronary sulcus is located between the atria and ventricles. Located between the left and right ventricles are two additional sulci that are not as deep as the coronary sulcus. The anterior interventricular sulcus is visible on the anterior surface of the heart, whereas the posterior interventricular sulcus is visible on the posterior surface of the heart.[4]

Layers

The swirling pattern of cardiac muscle tissue contributes significantly to the heart’s ability to pump blood effectively.[4]
The myometrium in the left ventricle is significantly thicker than that of the right ventricle.[4] Both ventricles pump the same amount of blood, but the left ventricle must generate a much greater pressure to overcome greater resistance in the systemic circuit.[4] The ventricles are shown in both relaxed and contracting states showing the variation in the size of the lumens.[4]

The wall of the heart is composed of three layers of unequal thickness.[4] From superficial to deep, these are the epicardium, the myocardium, and the endocardium (see Figure 3).[4] The outermost layer of the wall of the heart is also the innermost layer of the pericardium, the epicardium, or the visceral pericardium.[4]

The middle and thickest layer is the myocardium, made largely of cardiac muscle cells. It is built upon a framework of collagenous fibers, plus the blood vessels that supply the myocardium and the nerve fibers that help regulate the heart.[4] It is the contraction of the myocardium that pumps blood through the heart and into the major arteries.[4] The muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart.[4] They form a figure 8 pattern around the atria and around the bases of the great vessels.[4] Deeper ventricular muscles also form a figure 8 around the two ventricles and proceed toward the apex. More superficial layers of ventricular muscle wrap around both ventricles.[4] This complex swirling pattern allows the heart to pump blood more effectively than a simple linear pattern would. Figure 4 illustrates the arrangement of muscle cells.[4]

The innermost layer of the heart wall, the endocardium, is joined to the myocardium with a thin layer of connective tissue. The endocardium lines the chambers where the blood circulates and covers the heart valves.[4] It is made of simple squamous epithelium called endothelium, which is continuous with the endothelial lining of the blood vessels.[4]

Once regarded as a simple lining layer, evidence indicates that the endothelium of the endocardium and the coronary capillaries may play active roles in regulating the contraction of the muscle within the myocardium.[4] The endothelium may also regulate the growth patterns of the cardiac muscle cells throughout life, and the endothelins it secretes create an environment in the surrounding tissue fluids that regulates ionic concentrations and states of contractility.[4] Endothelins are potent vasoconstrictors and, in a normal individual, establish a homeostatic balance with other vasoconstrictors and vasodilators.[4]

Internal structures

The heart’s contraction cycle follows a dual pattern of circulation—the pulmonary and systemic circuits—because of the pairs of chambers that pump blood into the circulation.[4]

Septa

The word septum is derived from the Latin for “something that encloses;” in this case, a septum (plural = septa) refers to a wall or partition that divides the heart into chambers. The septa are physical extensions of the myocardium lined with endocardium. Located between the two atria is the interatrial septum. Normally in an adult heart, the interatrial septum bears an oval-shaped depression known as the fossa ovalis, a remnant of an opening in the fetal heart known as the foramen ovale. The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium to the left atrium, allowing some blood to bypass the pulmonary circuit. Within seconds after birth, a flap of tissue known as the septum primum that previously acted as a valve closes the foramen ovale and establishes the typical cardiac circulation pattern.[4]

Between the two ventricles is a second septum known as the interventricular septum. Unlike the interatrial septum, the interventricular septum is normally intact after its formation during fetal development. It is substantially thicker than the interatrial septum, since the ventricles generate far greater pressure when they contract.[4]

The septum between the atria and ventricles is known as the atrioventricular septum. It is marked by the presence of four openings that allow blood to move from the atria into the ventricles and from the ventricles into the pulmonary trunk and aorta. Located in each of these openings between the atria and ventricles is a valve, a specialized structure that ensures one-way flow of blood. The valves between the atria and ventricles are known generically as atrioventricular valves. The valves at the openings that lead to the pulmonary trunk and aorta are known generically as semilunar valves. Since these openings and valves structurally weaken the atrioventricular septum, the remaining tissue is heavily reinforced with dense connective tissue called the cardiac skeleton, or skeleton of the heart. It includes four rings that surround the openings between the atria and ventricles, and the openings to the pulmonary trunk and aorta, and serve as the point of attachment for the heart valves. The cardiac skeleton also provides an important boundary in the heart electrical conduction system.[4]

Right atrium

The right atrium serves as the receiving chamber for blood returning to the heart from the systemic circulation. The two major systemic veins, the superior and inferior venae cavae, and the large coronary vein called the coronary sinus that drains the heart myocardium empty into the right atrium. The superior vena cava drains blood from regions superior to the diaphragm: the head, neck, upper limbs, and the thoracic region. It empties into the superior and posterior portions of the right atrium. The inferior vena cava drains blood from areas inferior to the diaphragm: the lower limbs and abdominopelvic region of the body. It, too, empties into the posterior portion of the atria, but inferior to the opening of the superior vena cava. Immediately superior and slightly medial to the opening of the inferior vena cava on the posterior surface of the atrium is the opening of the coronary sinus. This thin-walled vessel drains most of the coronary veins that return systemic blood from the heart.[4]

While the bulk of the internal surface of the right atrium is smooth, the depression of the fossa ovalis is medial, and the anterior surface demonstrates prominent ridges of muscle called the pectinate muscles. The right auricle also has pectinate muscles. The left atrium does not have pectinate muscles except in the auricle.[4]

The atria receive venous blood on a nearly continuous basis, preventing venous flow from stopping while the ventricles are contracting. While most ventricular filling occurs while the atria are relaxed, they do demonstrate a contractile phase and actively pump blood into the ventricles just prior to ventricular contraction. The opening between the atrium and ventricle is guarded by the tricuspid valve.[4]

Right ventricle

In this frontal section, you can see papillary muscles attached to the tricuspid valve on the right as well as the mitral valve on the left via chordae tendineae.[4]

The right ventricle receives blood from the right atrium through the tricuspid valve. Each flap of the valve is attached to strong strands of connective tissue, the chordae tendineae, literally “tendinous cords,” or sometimes more poetically referred to as “heart strings.” There are several chordae tendineae associated with each of the flaps of the valves. They are composed of approximately 80 percent collagenous fibers with the remainder consisting of elastic fibers and endothelium. They connect each of the flaps to a papillary muscle that extends from the inferior ventricular surface. There are three papillary muscles in the right ventricle, called the anterior, posterior, and septal muscles, which correspond to the three sections of the valves.[4]

When the myocardium of the ventricle contracts, pressure within the ventricular chamber rises. Blood, like any fluid, flows from higher pressure to lower pressure areas, in this case, toward the pulmonary trunk and the atrium. To prevent any potential backflow, the papillary muscles also contract, generating tension on the chordae tendineae. This prevents the flaps of the valves from being forced into the atria and regurgitation of the blood back into the atria during ventricular contraction. Figure 6 shows papillary muscles and chordae tendineae attached to the tricuspid valve.[4]

The walls of the ventricle are lined with trabeculae carneae, ridges of cardiac muscle covered by endocardium. In addition to these muscular ridges, a band of cardiac muscle, also covered by endocardium, known as the moderator bandreinforces the thin walls of the right ventricle and plays a crucial role in cardiac conduction. It arises from the inferior portion of the interventricular septum and crosses the interior space of the right ventricle to connect with the inferior papillary muscle.[4]

When the right ventricle contracts, it ejects blood into the pulmonary trunk, which branches into the left and right pulmonary arteries that carry it to each lung. The superior surface of the right ventricle begins to taper as it approaches the pulmonary trunk. At the base of the pulmonary trunk is the pulmonary semilunar valve that prevents backflow from the pulmonary trunk.[4]

Left atrium

After exchange of gases in the pulmonary capillaries, blood returns to the left atrium high in oxygen via one of the four pulmonary veins. While the left atrium does not contain pectinate muscles, it does have an auricle that includes these pectinate ridges. Blood flows nearly continuously from the pulmonary veins back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the ventricular relaxation period, the left atrium will contract, pumping blood into the ventricle. This atrial contraction accounts for approximately 20 percent of ventricular filling. The opening between the left atrium and ventricle is guarded by the mitral valve.[4]

Left ventricle

Although both sides of the heart will pump the same amount of blood, the muscular layer is much thicker in the left ventricle compared to the right (see Figure 5). Like the right ventricle, the left also has trabeculae carneae, but there is no moderator band. The mitral valve is connected to papillary muscles via chordae tendineae. There are two papillary muscles on the left—the anterior and posterior—as opposed to three on the right.[4]

The left ventricle is the major pumping chamber for the systemic circuit; it ejects blood into the aorta through the aortic semilunar valve.[4]

Valves

Main article: Heart valves
With the atria and major vessels removed, all four valves are clearly visible, although it is difficult to distinguish the three separate cusps of the tricuspid valve.[4]

A transverse section through the heart slightly above the level of the atrioventricular septum reveals all four heart valves along the same plane (Figure 7). The valves ensure unidirectional blood flow through the heart. Between the right atrium and the right ventricle is the right atrioventricular valve, or tricuspid valve. It typically consists of three flaps, or leaflets, made of endocardium reinforced with additional connective tissue. The flaps are connected by chordae tendineae to the papillary muscles, which control the opening and closing of the valves.[4]

Emerging from the right ventricle at the base of the pulmonary trunk is the pulmonary semilunar valve, or the pulmonary valve; it is also known as the pulmonic valve or the right semilunar valve. The pulmonary valve is comprised of three small flaps of endothelium reinforced with connective tissue. When the ventricle relaxes, the pressure differential causes blood to flow back into the ventricle from the pulmonary trunk. This flow of blood fills the pocket-like flaps of the pulmonary valve, causing the valve to close and producing an audible sound. Unlike the atrioventricular valves, there are no papillary muscles or chordae tendineae associated with the pulmonary valve.[4]

Located at the opening between the left atrium and left ventricle is the mitral valve, also called the bicuspid valve or the left atrioventricular valve. Structurally, this valve consists of two cusps, known as the anterior medial cusp and the posterior medial cusp, compared to the three cusps of the tricuspid valve. In a clinical setting, the valve is referred to as the mitral valve, rather than the bicuspid valve. The two cusps of the mitral valve are attached by chordae tendineae to two papillary muscles that project from the wall of the ventricle.[4]

At the base of the aorta is the aortic semilunar valve, or the aortic valve, which prevents backflow from the aorta. It normally is composed of three flaps. When the ventricle relaxes and blood attempts to flow back into the ventricle from the aorta, blood will fill the cusps of the valve, causing it to close and producing an audible sound.[4]

In Figure 8a, the two atrioventricular valves (AV valves) are open and the two semilunar valves are closed. This occurs when both atria and ventricles are relaxed and when the atria contract to pump blood into the ventricles. Figure 8b shows a frontal view. Although only the left side of the heart is illustrated, the process is virtually identical on the right.[4]

8. (a) A transverse section through the heart illustrates the four heart valves. The two atrioventricular valves are open; the two semilunar valves are closed. The atria and vessels have been removed.
(b) A frontal section through the heart illustrates blood flow through the mitral valve. When the mitral valve is open, it allows blood to move from the left atrium to the left ventricle. The aortic semilunar valve is closed to prevent backflow of blood from the aorta to the left ventricle.[4]
9. (a) A transverse section through the heart illustrates the four heart valves during ventricular contraction. The two atrioventricular valves are closed, but the two semilunar valves are open. The atria and vessels have been removed.
(b) A frontal view shows the closed mitral (bicuspid) valve that prevents backflow of blood into the left atrium. The aortic semilunar valve is open to allow blood to be ejected into the aorta.[4]

Figure 9a shows the atrioventricular valves closed while the two semilunar valves are open. This occurs when the ventricles contract to eject blood into the pulmonary trunk and aorta. Closure of the two atrioventricular valves prevents blood from being forced back into the atria. This stage can be seen from a frontal view in Figure 9b.[4]

When the ventricles begin to contract, pressure within the ventricles rises and blood flows toward the area of lowest pressure, which is initially in the atria. This backflow causes the cusps of the tricuspid and mitral (bicuspid) valves to close. These valves are tied down to the papillary muscles by chordae tendineae. During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight (see Figure 8b). However, as the myocardium of the ventricle contracts, so do the papillary muscles. This creates tension on the chordae tendineae (see Figure 9b), helping to hold the cusps of the atrioventricular valves in place and preventing them from being blown back into the atria.[4]

The aortic and pulmonary semilunar valves lack the chordae tendineae and papillary muscles associated with the atrioventricular valves. Instead, they consist of pocket-like folds of endocardium reinforced with additional connective tissue. When the ventricles relax and the change in pressure forces the blood toward the ventricles, the blood presses against these cusps and seals the openings.[4]

Coronary circulation

The blood supply of the heart is provided by the coronary circulation. Coronary arteries labeled in red text and other landmarks in blue text.
Main article: Coronary circulation

The heart is a pump composed largely of cardiac muscle cells that are incredibly active throughout life. Like all other cells, a cardiomyocyte requires a reliable supply of oxygen and nutrients, and a way to remove wastes, so it needs a dedicated, complex, and extensive coronary circulation. And because of the critical and nearly ceaseless activity of the heart throughout life, this need for a blood supply is even greater than for a typical cell. However, coronary circulation is not continuous; rather, it cycles, reaching a peak when the heart muscle is relaxed and nearly ceasing while it is contracting.[4]

Coronary arteries

The anterior view of the heart shows the prominent coronary surface vessels. The posterior view of the heart shows the prominent coronary surface vessels.[4]

Coronary arteries supply blood to the myocardium and other components of the heart. The first portion of the aorta after it arises from the left ventricle gives rise to the coronary arteries. There are three dilations in the wall of the aorta just superior to the aortic semilunar valve. Two of these, the left posterior aortic sinus and anterior aortic sinus, give rise to the left and right coronary arteries, respectively. The third sinus, the right posterior aortic sinus, typically does not give rise to a vessel. Coronary vessel branches that remain on the surface of the artery and follow the sulci of the heart are called epicardial coronary arteries.[4]

The left coronary artery distributes blood to the left side of the heart, the left atrium and ventricle, and the interventricular septum. The circumflex artery arises from the left coronary artery and follows the coronary sulcus to the left. Eventually, it will fuse with the small branches of the right coronary artery. The larger anterior interventricular artery, also known as the left anterior descending artery (LAD), is the second major branch arising from the left coronary artery. It follows the anterior interventricular sulcus around the pulmonary trunk. Along the way it gives rise to numerous smaller branches that interconnect with the branches of the posterior interventricular artery, forming anastomoses. An anastomosis is an area where vessels unite to form interconnections that normally allow blood to circulate to a region even if there may be partial blockage in another branch. The anastomoses in the heart are very small. Therefore, this ability is somewhat restricted in the heart so a coronary artery blockage often results in death of the cells (myocardial infarction) supplied by the particular vessel.[4]

The right coronary artery proceeds along the coronary sulcus and distributes blood to the right atrium, portions of both ventricles, and the heart conduction system. Normally, one or more marginal arteries arise from the right coronary artery inferior to the right atrium. The marginal arteries supply blood to the superficial portions of the right ventricle. On the posterior surface of the heart, the right coronary artery gives rise to the posterior interventricular artery, also known as the posterior descending artery. It runs along the posterior portion of the interventricular sulcus toward the apex of the heart, giving rise to branches that supply the interventricular septum and portions of both ventricles. Figure 11 presents views of the coronary circulation from both the anterior and posterior views.[4]

Coronary veins

Coronary veins drain the heart and generally parallel the large surface arteries (see 11). The great cardiac vein can be seen initially on the surface of the heart following the interventricular sulcus, but it eventually flows along the coronary sulcus into the coronary sinus on the posterior surface. The great cardiac vein initially parallels the anterior interventricular artery and drains the areas supplied by this vessel. It receives several major branches, including the posterior cardiac vein, the middle cardiac vein, and the small cardiac vein. The posterior cardiac vein parallels and drains the areas supplied by the marginal artery branch of the circumflex artery. The middle cardiac vein parallels and drains the areas supplied by the posterior interventricular artery. The small cardiac vein parallels the right coronary artery and drains the blood from the posterior surfaces of the right atrium and ventricle. The coronary sinus is a large, thin-walled vein on the posterior surface of the heart lying within the atrioventricular sulcus and emptying directly into the right atrium. The anterior cardiac veins parallel the small cardiac arteries and drain the anterior surface of the right ventricle. Unlike these other cardiac veins, it bypasses the coronary sinus and drains directly into the right atrium.[4]

Physiology

Starting in the right atrium, the blood flows through the tricuspid valve to the right ventricle. Here, it is pumped out the pulmonary semilunar valve and travels through the pulmonary artery to the lungs. From there, oxygenated blood flows back through the pulmonary vein to the left atrium. It then travels through the mitral valve to the left ventricle, from where it is pumped through the aortic semilunar valve to the aorta. The aorta forks and the blood is divided between major arteries which supply the upper and lower body. The blood travels in the arteries to the smaller arterioles and then, finally, to the tiny capillaries which feed each cell. The (relatively) deoxygenated blood then travels to the venules, which coalesce into veins, then to the inferior and superior venae cavae and finally back to the right atrium where the process began.

The heart is effectively a syncytium, a meshwork of cardiac muscle cells interconnected by contiguous cytoplasmic bridges. This relates to electrical stimulation of one cell spreading to neighboring cells.

Some cardiac cells are self-excitable, contracting without any signal from the nervous system, even if removed from the heart and placed in culture. Each of these cells have their own intrinsic contraction rhythm. A region of the human heart called the sinoatrial (SA) node, or pacemaker, sets the rate and timing at which all cardiac muscle cells contract. The SA node generates electrical impulses, much like those produced by nerve cells. Because cardiac muscle cells are electrically coupled by intercalated discs between adjacent cells, impulses from the SA node spread rapidly through the walls of the artria, causing both artria to contract in unison. The impulses also pass to another region of specialized cardiac muscle tissue, a relay point called the atrioventricular node, located in the wall between the right atrium and the right ventricle. Here, the impulses are delayed for about 0.1s before spreading to the walls of the ventricle. The delay ensures that the artria empty completely before the ventricles contract. Specialized muscle fibers called Purkinje fibers then conduct the signals to the apex of the heart along and throughout the ventricular walls. The Purkinje fibres form conducting pathways called bundle branches. This entire cycle, a single heart beat, lasts about 0.8 seconds. The impulses generated during the heart cycle produce electrical currents, which are conducted through body fluids to the skin, where they can be detected by electrodes and recorded as an electrocardiogram (ECG or EKG).[10] The events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next is called a cardiac cycle.[11]

Blood flow

Main articles: Cardiac cycle and Heart valve
Blood flow through the valves
3D - loop of a heart viewed from the top, with the apical part of the ventricles removed and the mitral valve clearly visible. Due to missing data the leaflets of the tricuspid and aortic valves are not clearly visible, but the openings are; the pulmonary valve is not visible. On the left are two standard 2D views (taken from the 3D dataset) showing tricuspid and mitral valves (above) and aortal valve (below).
Blood flow diagram of the human heart. Blue components indicate de-oxygenated blood pathways and red components indicate oxygenated pathways.

The heart functions as a pump and acts as a double pump to provide a continuous circulation of blood throughout the body. Blood flows through the heart in one direction, from the atria to the ventricles, and out through the pulmonary artery, and the aorta. Blood is prevented from flowing backwards (regurgitation) by the tricuspid, bicuspid, aortic, and pulmonary valves.

The function of the right heart, is to collect de-oxygenated blood, in the right atrium, from the body (via the superior and inferior venae cavae and pump it, through the tricuspid valve, via the right ventricle, into the lungs (pulmonary circulation) where carbon dioxide can be exchanged for oxygen. This happens through the passive process of diffusion.

The left heart collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood flows to the left ventricle, through the bicuspid valve (mitral valve), which pumps it out to the body (via the aorta).

The ventricles are stronger and thicker than the atria, and the muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation. Atria facilitate circulation primarily by allowing uninterrupted venous flow to the heart, preventing the inertia of interrupted venous flow that would otherwise occur at each ventricular systole.[12]

Starting in the right atrium, the blood flows through the tricuspid valve to the right ventricle. Here, it is pumped out of the semilunar pulmonary valve and travels through the pulmonary artery to the lungs. From there, blood flows back through the pulmonary vein to the left atrium. It then travels through the mitral valve to the left ventricle, from where it is pumped through the semilunar aortic valve to the aorta and to the rest of the body. The (mostly) deoxygenated blood finally returns to the heart through the superior and inferior venae cavae (see Figure 13).

Cardiac muscle

Main article: Cardiac muscle

Cardiac muscle shares characteristics with both skeletal muscle and smooth muscle, but it also unique. It has the ability to initiate an electrical potential at a fixed rate - autorhythmicity, spreading the impulse rapidly from cell to cell to trigger the contraction of the entire heart. Even though cardiac muscle has autorhythmicity, heart rate is also modulated by the endocrine and nervous systems.[4]

There are two major types of cardiac muscle cells: myocardial contractile cells and myocardial conducting cells. The myocardial contractile cells constitute the bulk (99%) of cells in the atria and ventricles. Contractile cells conduct impulses and are responsible for contractions that pump blood through the body. The myocardial conducting cells (1% of cells) form the conduction system of the heart. Except for Purkinje cells, they are generally much smaller than the contractile cells and have few of the myofibrils or filaments needed for contraction. Their function is similar in many respects to neurons, though they are specialized muscle cells. Myocardial conduction cells initiate and propagate the action potential - which travels throughout the heart and triggers the contractions that propel the blood.[4]

Structure of cardiac muscle


(a) Cardiac muscle cells have myofibrils composed of myofilaments arranged in sarcomeres, T-tubules to transmit the impulse from the sarcolemma to the interior of the cell, numerous mitochondria for energy, and intercalated discs that are found at the junction of different cardiac muscle cells.
(b) A photomicrograph of cardiac muscle cells shows the nuclei and intercalated discs.
(c) An intercalated disc connects cardiac muscle cells and consists of desmosomes and gap junctions. LM × 1600.[4]

In contrast to the giant cylinders of skeletal muscle, cardiac muscle cells, also known as cardiomyocytes, are considerably shorter and have smaller diameters. Cardiac muscle is characterized by striations - the alternating pattern of dark and light bands attributed to the precise arrangement of the myofilaments and fibrils organized in sarcomeres along the length of the cell. These contractile elements are virtually identical to skeletal muscle. T (transverse) tubules penetrate from the surface cell membrane, also known as the sarcolemma, to the interior of the cell, thereby allowing the electrical impulse to reach the interior. The T tubules are only found at the Z discs, whereas in skeletal muscle, they are found at the junction of the A and I bands. Therefore, there are one-half as many T tubules in cardiac muscle as in skeletal muscle. In addition, the sarcoplasmic reticulum stores few calcium ions, so most of the calcium ions come from outside the cells. The result is a slower onset of contraction. Mitochondria are plentiful, providing energy for the contractions of the heart. Typically, cardiomyocytes have a single, central nucleus, can also have two or more.[4]

Cardiac muscle cells branch freely. A junction between two adjoining cells is marked by a critical structure called an intercalated disc, which helps support the synchronized contraction of the muscle. The sarcolemmas from adjacent cells bind together at the intercalated discs. They consist of desmosomes, specialized linking proteoglycans, tight junctions, and large numbers of gap junctions that allow the passage of ions between the cells and help to synchronize the contraction. Intercellular connective tissue also helps to bind the cells together. The importance of strongly binding these cells together is necessitated by the forces exerted by contraction.[4]

Cardiac muscle undergoes aerobic respiration patterns, primarily metabolizing lipids and carbohydrates. Myoglobin, lipids, and glycogen are all stored within the cytoplasm. Cardiac muscle cells undergo twitch-type contractions with long refractory periods followed by brief relaxation periods. The relaxation is essential so the heart can fill with blood for the next cycle. The refractory period is very long to prevent the possibility of tetany, a condition in which muscle remains involuntarily contracted. In the heart, tetany is not compatible with life, since it would prevent the heart from pumping blood.[4]

Electrical conduction

Transmission of a cardiac action potential through the heart's conduction system
Diagrammatic electrical conduction system of the heart

It is not very well known how the electric signal moves in the atria. It seems that it moves in a radial way, but Bachmann's bundle and coronary sinus muscle play a role in conduction between the two atria, which have a nearly simultaneous systole.[13][14][15] While in the ventricles, the signal is carried by specialized tissue called the Purkinje fibers which then transmit the electric charge to the myocardium.[16] In Figure 16, the heart's conduction system is shown with a cardiac action potential transmitted from the atria to the ventricles.

Conduction System of the Heart- Specialized conducting components of the heart include the sinoatrial node, the internodal pathways, the atrioventricular node, the atrioventricular bundle, the right and left bundle branches, and the Purkinje fibers.[4]

If embryonic heart cells are separated into a Petri dish and kept alive, each is capable of generating its own electrical impulse followed by contraction. When two independently beating embryonic cardiac muscle cells are placed together, the cell with the higher inherent rate sets the pace, and the impulse spreads from the faster to the slower cell to trigger a contraction. As more cells are joined together, the fastest cell continues to assume control of the rate. A fully developed adult heart maintains the capability of generating its own electrical impulse, triggered by the fastest cells, as part of the cardiac conduction system. The components of the cardiac conduction system include the atrial and ventricular syncytium, the sinoatrial node, the atrioventricular node, the atrioventricular bundle, the atrioventricular bundle branches, and the Purkinje cells (Figure 17 & 18).[4]

Sinoatrial (SA) node

Main article: SA node

Normal cardiac rhythm is established by the sinoatrial (SA) node, a specialized clump of myocardial conducting cells located in the superior and posterior walls of the right atrium in close proximity to the orifice of the superior vena cava. The SA node has the highest inherent rate of depolarization and is known as the pacemaker of the heart. It initiates the sinus rhythm, or normal electrical pattern followed by contraction of the heart.[4]

This impulse spreads from its initiation in the SA node throughout the atria through specialized internodal pathways, to the atrial myocardial contractile cells and the atrioventricular node. The internodal pathways consist of three bands (anterior,middle, and posterior) that lead directly from the SA node to the next node in the conduction system, the atrioventricular node (see Figure 18). The impulse takes approximately 50 ms (milliseconds) to travel between these two nodes. The relative importance of this pathway has been debated since the impulse would reach the atrioventricular node simply following the cell-by-cell pathway through the contractile cells of the myocardium in the atria. In addition, there is a specialized pathway called Bachmann’s bundle or the interatrial band that conducts the impulse directly from the right atrium to the left atrium. Regardless of the pathway, as the impulse reaches the atrioventricular septum, the connective tissue of the cardiac skeleton prevents the impulse from spreading into the myocardial cells in the ventricles except at the atrioventricular node. Figure 19 illustrates the initiation of the impulse in the SA node that then spreads the impulse throughout the atria to the atrioventricular node.[4]

Cardiac Conduction
(1) The sinoatrial (SA) node and the remainder of the conduction system are at rest.
(2) The SA node initiates the action potential, which sweeps across the atria.
(3) After reaching the atrioventricular node, there is a delay of approximately 100 ms that allows the atria to complete pumping blood before the impulse is transmitted to the atrioventricular bundle.
(4) Following the delay, the impulse travels through the atrioventricular bundle and bundle branches to the Purkinje fibers, and also reaches the right papillary muscle via the moderator band.
(5) The impulse spreads to the contractile fibers of the ventricle.
(6) Ventricular contraction begins.[4]

The electrical event, the wave of depolarization, is the trigger for muscular contraction. The wave of depolarization begins in the right atrium, and the impulse spreads across the superior portions of both atria and then down through the contractile cells. The contractile cells then begin contraction from the superior to the inferior portions of the atria, efficiently pumping blood into the ventricles.[4]

Atrioventricular (AV) node

Main article: AV node

The atrioventricular (AV) node is a second clump of specialized myocardial conductive cells, located in the inferior portion of the right atrium within the atrioventricular septum. The septum prevents the impulse from spreading directly to the ventricles without passing through the AV node. There is a critical pause before the AV node depolarizes and transmits the impulse to the atrioventricular bundle (see Figure 19, step 3). This delay in transmission is partially attributable to the small diameter of the cells of the node, which slow the impulse. Also, conduction between nodal cells is less efficient than between conducting cells. These factors mean that it takes the impulse approximately 100 ms to pass through the node. This pause is critical to heart function, as it allows the atrial cardiomyocytes to complete their contraction that pumps blood into the ventricles before the impulse is transmitted to the cells of the ventricle itself. With extreme stimulation by the SA node, the AV node can transmit impulses maximally at 220 per minute. This establishes the typical maximum heart rate in a healthy young individual. Damaged hearts or those stimulated by drugs can contract at higher rates, but at these rates, the heart can no longer effectively pump blood.[4]

Atrioventricular bundle, bundle branches, and Purkinje fibers

Arising from the AV node, the atrioventricular bundle, or bundle of His, proceeds through the interventricular septum before dividing into two atrioventricular bundle branches, commonly called the left and right bundle branches. The left bundle branch has two fascicles. The left bundle branch supplies the left ventricle, and the right bundle branch the right ventricle. Since the left ventricle is much larger than the right, the left bundle branch is also considerably larger than the right. Portions of the right bundle branch are found in the moderator band and supply the right papillary muscles. Because of this connection, each papillary muscle receives the impulse at approximately the same time, so they begin to contract simultaneously just prior to the remainder of the myocardial contractile cells of the ventricles. This is believed to allow tension to develop on the chordae tendineae prior to right ventricular contraction. There is no corresponding moderator band on the left. Both bundle branches descend and reach the apex of the heart where they connect with the Purkinje fibers (see Figure 19, step 4). This passage takes approximately 25 ms.[4]

The Purkinje fibers are additional myocardial conductive fibers that spread the impulse to the myocardial contractile cells in the ventricles. They extend throughout the myocardium from the apex of the heart toward the atrioventricular septum and the base of the heart. The Purkinje fibers have a fast inherent conduction rate, and the electrical impulse reaches all of the ventricular muscle cells in about 75 ms (see Figure 19, step 5). Since the electrical stimulus begins at the apex, the contraction also begins at the apex and travels toward the base of the heart, similar to squeezing a tube of toothpaste from the bottom. This allows the blood to be pumped out of the ventricles and into the aorta and pulmonary trunk. The total time elapsed from the initiation of the impulse in the SA node until depolarization of the ventricles is approximately 225 ms.[4]

Membrane potentials and ion movement in cardiac conductive cells

Action potentials are considerably different between cardiac conductive cells and cardiac contractive cells. While Na+ and[4]

K+ play essential roles, Ca2+ is also critical for both types of cells. Unlike skeletal muscles and neurons, cardiac conductive cells do not have a stable resting potential. Conductive cells contain a series of sodium ion channels that allow a normal and slow influx of sodium ions that causes the membrane potential to rise slowly from an initial value of −60 mV up to about –40 mV. The resulting movement of sodium ions creates spontaneous depolarization (or prepotential depolarization).[4]

At this point, calcium ion channels open and Ca2+ enters the cell, further depolarizing it at a more rapid rate until it reaches a value of approximately +5 mV. At this point, the calcium ion channels close and K+ channels open, allowing outflux of K+ and resulting in repolarization. When the membrane potential reaches approximately −60 mV, the K+ channels close and Na+ channels open, and the prepotential phase begins again. This phenomenon explains the autorhythmicity properties of cardiac muscle (Figure 20).[4]

Action Potential at the SA Node - The prepotential is due to a slow influx of sodium ions until the threshold is reached followed by a rapid depolarization and repolarization. The prepotential accounts for the membrane reaching threshold and initiates the spontaneous depolarization and contraction of the cell. Note the lack of a resting potential.[4]

Membrane Potentials and ion movement in cardiac contractile cells

There is a distinctly different electrical pattern involving the contractile cells. In this case, there is a rapid depolarization, followed by a plateau phase and then repolarization. This phenomenon accounts for the long refractory periods required for the cardiac muscle cells to pump blood effectively before they are capable of firing for a second time. These cardiac myocytes normally do not initiate their own electrical potential, although they are capable of doing so, but rather wait for an impulse to reach them.[4]

Contractile cells demonstrate a much more stable resting phase than conductive cells at approximately −80 mV for cells in the atria and −90 mV for cells in the ventricles. Despite this initial difference, the other components of their action potentials are virtually identical. In both cases, when stimulated by an action potential, voltage-gated channels rapidlyopen, beginning the positive-feedback mechanism of depolarization. This rapid influx of positively charged ions raises the membrane potential to approximately +30 mV, at which point the sodium channels close. The rapid depolarization period typically lasts 3–5 ms. Depolarization is followed by the plateau phase, in which membrane potential declines relatively slowly. This is due in large part to the opening of the slow Ca2+ channels, allowing Ca2+ to enter the cell while few K+ channels are open, allowing K+ to exit the cell. The relatively long plateau phase lasts approximately 175 ms. Once the membrane potential reaches approximately zero, the Ca2+ channels close and K+ channels open, allowing K+ to exit the cell. The repolarization lasts approximately 75 ms. At this point, membrane potential drops until it reaches resting levels once more and the cycle repeats. The entire event lasts between 250 and 300 ms (Figure 21).[4]

The absolute refractory period for cardiac contractile muscle lasts approximately 200 ms, and the relative refractory period lasts approximately 50 ms, for a total of 250 ms. This extended period is critical, since the heart muscle must contract to pump blood effectively and the contraction must follow the electrical events. Without extended refractory periods, premature contractions would occur in the heart and would not be compatible with life.[4]

Action Potential in Cardiac Contractile Cells
(a) Note the long plateau phase due to the influx of calcium ions. The extended refractory period allows the cell to fully contract before another electrical event can occur.
(b) The action potential for heart muscle is compared to that of skeletal muscle.[4]

Calcium ions

Calcium ions play two critical roles in the physiology of cardiac muscle. Their influx through slow calcium channels accounts for the prolonged plateau phase and absolute refractory period that enable cardiac muscle to function properly. Calcium ions also combine with the regulatory protein troponin in the troponin-tropomyosin complex; this complex removes the inhibition that prevents the heads of the myosin molecules from forming cross bridges with the active sites on actin that provide the power stroke of contraction. This mechanism is virtually identical to that of skeletal muscle.[4]

Approximately 20 percent of the calcium required for contraction is supplied by the influx of Ca2+ during the plateau phase. The remaining Ca2+ for contraction is released from storage in the sarcoplasmic reticulum.[4]

Comparative rates of conduction system firing

The pattern of prepotential or spontaneous depolarization, followed by rapid depolarization and repolarization just described, are seen in the SA node and a few other conductive cells in the heart. Since the SA node is the pacemaker, it reaches threshold faster than any other component of the conduction system. It will initiate the impulses spreading to the other conducting cells. The SA node, without nervous or endocrine control, would initiate a heart impulse approximately 80–100 times per minute. Although each component of the conduction system is capable of generating its own impulse, the rate progressively slows as you proceed from the SA node to the Purkinje fibers. Without the SA node, the AV node would generate a heart rate of 40–60 beats per minute. If the AV node were blocked, the atrioventricular bundle would fire at a rate of approximately 30–40 impulses per minute. The bundle branches would have an inherent rate of 20–30 impulses per minute, and the Purkinje fibers would fire at 15–20 impulses per minute. While a few exceptionally trained aerobic athletes demonstrate resting heart rates in the range of 30–40 beats per minute (the lowest recorded figure is 28 beats per minute for Miguel Indurain, a cyclist), for most individuals, rates lower than 50 beats per minute would indicate a condition called bradycardia. Depending upon the specific individual, as rates fall much below this level, the heart would be unable to maintain adequate flow of blood to vital tissues, initially resulting in decreasing loss of function across the systems, unconsciousness, and ultimately death.[4]

Electrocardiogram

Main article: Electrocardiogram
Standard Placement of ECG Leads - In a 12-lead ECG, six electrodes are placed on the chest, and four electrodes are placed on the limbs.[4]

By careful placement of surface electrodes on the body, it is possible to record the complex, compound electrical signal of the heart. This tracing of the electrical signal is the electrocardiogram (ECG), also commonly abbreviated EKG (K coming kardiology, from the German term for cardiology). Careful analysis of the ECG reveals a detailed picture of both normal and abnormal heart function, and is an indispensable clinical diagnostic tool. The standard electrocardiograph (the instrument that generates an ECG) uses 3, 5, or 12 leads. The greater the number of leads an electrocardiograph uses, the more information the ECG provides. The term “lead” may be used to refer to the cable from the electrode to the electrical recorder, but it typically describes the voltage difference between two of the electrodes. The 12-lead electrocardiograph uses 10 electrodes placed in standard locations on the patient’s skin (Figure 22). In continuous ambulatory electrocardiographs, the patient wears a small, portable, battery-operated device known as a Holter monitor, or simply a Holter, that continuously monitors heart electrical activity, typically for a period of 24 hours during the patient’s normal routine.[4]

A normal ECG tracing is presented in Figure 23. Each component, segment, and interval is labeled and corresponds to important electrical events, demonstrating the relationship between these events and contraction in the heart.[4]

There are five prominent points on the ECG: the P wave, the QRS complex, and the T wave. The small P wave represents the depolarization of the atria. The atria begin contracting approximately 25 ms after the start of the P wave. The large QRS complex represents the depolarization of the ventricles, which requires a much stronger electrical signal because of the larger size of the ventricular cardiac muscle. The ventricles begin to contract as the QRS reaches the peak of the R wave. Lastly, the T wave represents the repolarization of the ventricles. The repolarization of the atria occurs during the QRS complex, which masks it on an ECG.[4]

The major segments and intervals of an ECG tracing are indicated in Figure 23. Segments are defined as the regions between two waves. Intervals include one segment plus one or more waves. For example, the PR segment begins at the end of the P wave and ends at the beginning of the QRS complex. The PR interval starts at the beginning of the P wave and ends with the beginning of the QRS complex. The PR interval is more clinically relevant, as it measures the duration from the beginning of atrial depolarization (the P wave) to the initiation of the QRS complex. Since the Q wave may be difficult to view in some tracings, the measurement is often extended to the R that is more easily visible. Should there be a delay in passage of the impulse from the SA node to the AV node, it would be visible in the PR interval. Figure 24 correlates events of heart contraction to the corresponding segments and intervals of an ECG.[4]

Electrocardiogram - A normal tracing shows the P wave, QRS complex, and T wave. Also indicated are the PR, QT, QRS, and ST intervals, plus the P-R and S-T segments.[4]
ECG Tracing Correlated to the Cardiac Cycle - This diagram correlates an ECG tracing with the electrical and mechanical events of a heart contraction. Each segment of an ECG tracing corresponds to one event in the cardiac cycle.[4]

Cardiac muscle metabolism

Main article: Metabolism

Normally, cardiac muscle metabolism is entirely aerobic. Oxygen from the lungs is brought to the heart, and every other organ, attached to the hemoglobin molecules within the erythrocytes. Heart cells also store appreciable amounts of oxygen in myoglobin. Normally, these two mechanisms, circulating oxygen and oxygen attached to myoglobin, can supply sufficient oxygen to the heart, even during peak performance.[4]

Fatty acids and glucose from the circulation are broken down within the mitochondria to release energy in the form of ATP. Both fatty acid droplets and glycogen are stored within the sarcoplasm and provide additional nutrient supply.[4]

Cardiac cycle

Main article: Cardiac cycle

The period of time that begins with contraction of the atria and ends with ventricular relaxation is known as the cardiac cycle (Figure 25). The period of contraction that the heart undergoes while it pumps blood into circulation is called systole. The period of relaxation that occurs as the chambers fill with blood is called diastole. Both the atria and ventricles undergo systole and diastole, and it is essential that these components be carefully regulated and coordinated to ensure blood is pumped efficiently to the body.[4]

Overview of the Cardiac Cycle - The cardiac cycle begins with atrial systole and progresses to ventricular systole, atrial diastole, and ventricular diastole, when the cycle begins again. Correlations to the ECG are highlighted.[4]

Pressures and flow

Fluids, whether gases or liquids, are materials that flow according to pressure gradients—that is, they move from regions that are higher in pressure to regions that are lower in pressure. Accordingly, when the heart chambers are relaxed (diastole), blood will flow into the atria from the veins, which are higher in pressure. As blood flows into the atria, the pressure will rise, so the blood will initially move passively from the atria into the ventricles. When the action potential triggers the muscles in the atria to contract (atrial systole), the pressure within the atria rises further, pumping blood into the ventricles. During ventricular systole, pressure rises in the ventricles, pumping blood into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle. Again, as you consider this flow and relate it to the conduction pathway, the elegance of the system should become apparent.[4]

Phases of the cardiac cycle

At the beginning of the cardiac cycle, both the atria and ventricles are relaxed (diastole). Blood is flowing into the right atrium from the superior and inferior venae cavae and the coronary sinus. Blood flows into the left atrium from the four pulmonary veins. The two atrioventricular valves, the tricuspid and mitral valves, are both open, so blood flows unimpeded from the atria and into the ventricles. Approximately 70–80 percent of ventricular filling occurs by this method. The two semilunar valves, the pulmonary and aortic valves, are closed, preventing backflow of blood into the right and left ventricles from the pulmonary trunk on the right and the aorta on the left.[4]

Atrial systole and diastole

Contraction of the atria follows depolarization, represented by the P wave of the ECG. As the atrial muscles contract from the superior portion of the atria toward the atrioventricular septum, pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular (tricuspid, and mitral or bicuspid) valves. At the start of atrial systole, the ventricles are normally filled with approximately 70–80 percent of their capacity due to inflow during diastole. Atrial contraction, also referred to as the “atrial kick,” contributes the remaining 20–30 percent of filling (see Figure 25). Atrial systole lasts approximately 100 ms and ends prior to ventricular systole, as the atrial muscle returns to diastole.[4]

Ventricular systole

Ventricular systole (see Figure 25) follows the depolarization of the ventricles and is represented by the QRS complex in the ECG. It may be conveniently divided into two phases, lasting a total of 270 ms. At the end of atrial systole and just prior to atrial contraction, the ventricles contain approximately 130 mL blood in a resting adult in a standing position. This volume is known as the end diastolic volume (EDV) or preload.[4]

Initially, as the muscles in the ventricle contract, the pressure of the blood within the chamber rises, but it is not yet high enough to open the semilunar (pulmonary and aortic) valves and be ejected from the heart. However, blood pressure quickly rises above that of the atria that are now relaxed and in diastole. This increase in pressure causes blood to flow back toward the atria, closing the tricuspid and mitral valves. Since blood is not being ejected from the ventricles at this early stage, the volume of blood within the chamber remains constant. Consequently, this initial phase of ventricular systole is known as isovolumic contraction, also called isovolumetric contraction (see Figure 25).[4]

In the second phase of ventricular systole, the ventricular ejection phase, the contraction of the ventricular muscle has raised the pressure within the ventricle to the point that it is greater than the pressures in the pulmonary trunk and the aorta. Blood is pumped from the heart, pushing open the pulmonary and aortic semilunar valves. Pressure generated by the left ventricle will be appreciably greater than the pressure generated by the right ventricle, since the existing pressure in the aorta will be so much higher. Nevertheless, both ventricles pump the same amount of blood. This quantity is referred to as stroke volume. Stroke volume will normally be in the range of 70–80 mL. Since ventricular systole began with an EDV of approximately 130 mL of blood, this means that there is still 50–60 mL of blood remaining in the ventricle following contraction. This volume of blood is known as the end systolic volume (ESV).[4]

Ventricular diastole

Ventricular relaxation, or diastole, follows repolarization of the ventricles and is represented by the T wave of the ECG. It too is divided into two distinct phases and lasts approximately 430 ms.[4]

During the early phase of ventricular diastole, as the ventricular muscle relaxes, pressure on the remaining blood within the ventricle begins to fall. When pressure within the ventricles drops below pressure in both the pulmonary trunk and aorta, blood flows back toward the heart, producing the dicrotic notch (small dip) seen in blood pressure tracings. The semilunar valves close to prevent backflow into the heart. Since the atrioventricular valves remain closed at this point, there is no change in the volume of blood in the ventricle, so the early phase of ventricular diastole is called the isovolumic ventricular relaxation phase, also called isovolumetric ventricular relaxation phase (see Figure 25).[4]

In the second phase of ventricular diastole, called late ventricular diastole, as the ventricular muscle relaxes, pressure on the blood within the ventricles drops even further. Eventually, it drops below the pressure in the atria. When this occurs, blood flows from the atria into the ventricles, pushing open the tricuspid and mitral valves. As pressure drops within the ventricles, blood flows from the major veins into the relaxed atria and from there into the ventricles. Both chambers are in diastole, the atrioventricular valves are open, and the semilunar valves remain closed (see Figure 25). The cardiac cycle is complete.[4]

Figure 26 illustrates the relationship between the cardiac cycle and the ECG.[4]

Relationship between the Cardiac Cycle and ECG - Initially, both the atria and ventricles are relaxed (diastole). The P wave represents depolarization of the atria and is followed by atrial contraction (systole). Atrial systole extends until the QRS complex, at which point, the atria relax. The QRS complex represents depolarization of the ventricles and is followed by ventricular contraction. The T wave represents the repolarization of the ventricles and marks the beginning of ventricular relaxation.[4]

Heart sounds

Main article: Heart sounds

One of the simplest methods of assessing the heart's condition is to listen to it using a stethoscope.[4] In a healthy heart, there are only two audible heart sounds: S1 and S2. S1 is the sound created by the closing of the atrioventricular valves during ventricular contraction and is normally described as a “lub,” or first heart sound. The second heart sound, S2, is the sound of the closing of the semilunar valves during ventricular diastole and is described as a “dub” (Figure 27). In both cases, as the valves close, the openings within the atrioventricular septum guarded by the valves will become reduced, and blood flow through the opening will become more turbulent until the valves are fully closed. There is a third heart sound, S3, but it is rarely heard in healthy individuals. It may be the sound of blood flowing into the atria, or blood sloshing back and forth in the ventricle, or even tensing of the chordae tendineae. S3 may be heard in youth, some athletes, and pregnant women. If the sound is heard later in life, it may indicate congestive heart failure, warranting further tests. Some cardiologists refer to the collective S1, S2, and S3 sounds as the “Kentucky gallop,” because they mimic those produced by a galloping horse. The fourth heart sound, S4, results from the contraction of the atria pushing blood into a stiff or hypertrophic ventricle, indicating failure of the left ventricle. S4 occurs prior to S1 and the collective sounds S4, S1, and S2 are referred to by some cardiologists as the “Tennessee gallop,” because of their similarity to the sound produced by a galloping horse with a different gait. A few individuals may have both S3 and S4, and this combined sound is referred to as S7.[4]

Heart Sounds and the Cardiac Cycle - In this illustration, the x-axis reflects time with a recording of the heart sounds. The y-axis represents pressure.[4]

The term murmur is used to describe an unusual sound coming from the heart that is caused by the turbulent flow of blood. Murmurs are graded on a scale of 1 to 6, with 1 being the most common, the most difficult sound to detect, and the least serious. The most severe is a 6. Phonocardiograms or auscultograms can be used to record both normal and abnormal sounds using specialized electronic stethoscopes.[4]

During auscultation, it is common practice for the clinician to ask the patient to breathe deeply. This procedure not only allows for listening to airflow, but it may also amplify heart murmurs. Inhalation increases blood flow into the right side of the heart and may increase the amplitude of right-sided heart murmurs. Expiration partially restricts blood flow into the left side of the heart and may amplify left-sided heart murmurs. Figure 28 indicates proper placement of the bell of the stethoscope to facilitate auscultation.[4]

Stethoscope Placement for Auscultation - Proper placement of the bell of the stethoscope facilitates auscultation. At each of the four locations on the chest, a different valve can be heard.[4]

Cardiac output

Main article: Cardiac output

Cardiac output (CO) is a measurement of the amount of blood pumped by each ventricle in one minute. To calculate this value, multiply stroke volume (SV), the amount of blood pumped by each ventricle, by heart rate (HR), in contractions per minute (or beats per minute, bpm).[4] It can be represented mathematically by the following equation:

CO=HR*SV[4]

SV is normally measured using an echocardiogram to record EDV and ESV, and calculating the difference: SV = EDV – ESV. SV can also be measured using a specialized catheter, but this is an invasive procedure and far more dangerous to the patient. A mean SV for a resting 70-kg (150-lb) individual would be approximately 70 mL. There are several important variables, including size of the heart, physical and mental condition of the individual, sex, contractility, duration of contraction, preload or EDV, and afterload or resistance. Normal range for SV would be 55–100 mL. An average resting HR would be approximately 75 bpm but could range from 60–100 in some individuals.[4]

Using these numbers, the mean CO is 5.25 L/min, with a range of 4.0–8.0 L/min. Remember, however, that these numbers refer to CO from each ventricle separately, not the total for the heart. Factors influencing CO are summarized in Figure 29.[4]

Major Factors Influencing Cardiac Output - Cardiac output is influenced by heart rate and stroke volume, both of which are also variable.[4]

SVs are also used to calculate ejection fraction, which is the portion of the blood that is pumped or ejected from the heart with each contraction. To calculate ejection fraction, SV is divided by EDV. Despite the name, the ejection fraction is normally expressed as a percentage. Ejection fractions range from approximately 55–70 percent, with a mean of 58 percent.[4]

Exercise and maximum cardiac output

In healthy young individuals, HR may increase to 150 bpm during exercise. SV can also increase from 70 to approximately 130 mL due to increased strength of contraction. This would increase CO to approximately 19.5 L/min, 4–5 times the resting rate. Top cardiovascular athletes can achieve even higher levels. At their peak performance, they may increase resting CO by 7–8 times.[4]

Since the heart is a muscle, exercising it increases its efficiency. The difference between maximum and resting CO is known as the cardiac reserve. It measures the residual capacity of the heart to pump blood.[4]

Heart rates

Main article: Heart rate

HRs vary considerably, not only with exercise and fitness levels, but also with age. Newborn resting HRs may be 120 bpm. HR gradually decreases until young adulthood and then gradually increases again with age.[4]

Maximum HRs are normally in the range of 200–220 bpm, although there are some extreme cases in which they may reach higher levels. As one ages, the ability to generate maximum rates decreases. This may be estimated by taking the maximal value of 220 bpm and subtracting the individual’s age. So a 40-year-old individual would be expected to hit a maximum rate of approximately 180, and a 60-year-old person would achieve a HR of 160.[4]

Correlation between heart rates and cardiac output

Initially, physiological conditions that cause HR to increase also trigger an increase in SV. During exercise, the rate of blood returning to the heart increases. However as the HR rises, there is less time spent in diastole and consequently less time for the ventricles to fill with blood. Even though there is less filling time, SV will initially remain high. However, as HR continues to increase, SV gradually decreases due to decreased filling time. CO will initially stabilize as the increasing HR compensates for the decreasing SV, but at very high rates, CO will eventually decrease as increasing rates are no longer able to compensate for the decreasing SV. Consider this phenomenon in a healthy young individual. Initially, as HR increases from resting to approximately 120 bpm, CO will rise. As HR increases from 120 to 160 bpm, CO remains stable, since the increase in rate is offset by decreasing ventricular filling time and, consequently, SV. As HR continues to rise above 160 bpm, CO actually decreases as SV falls faster than HR increases. So although aerobic exercises are critical to maintain the health of the heart, individuals are cautioned to monitor their HR to ensure they stay within the target heart rate range of between 120 and 160 bpm, so CO is maintained. The target HR is loosely defined as the range in which both the heart and lungs receive the maximum benefit from the aerobic workout and is dependent upon age.[4]

Cardiovascular centers

Nervous control over the heartrate is centralized within the two paired cardiovascular centres of the medulla oblongata (Figure 30). The cardioaccelerator regions stimulate activity via sympathetic stimulation of the cardioaccelerator nerves, and the cardioinhibitory centers decrease heart activity via parasympathetic stimulation as one component of the vagus nerve, cranial nerve X. During rest, both centers provide slight stimulation to the heart, contributing to autonomic tone. This is a similar concept to tone in skeletal muscles. Normally, vagal stimulation predominates as, left unregulated, the SA node would initiate a sinus rhythm of approximately 100 bpm.[4]

Both sympathetic and parasympathetic stimuli flow through the paired cardiac plexus near the base of the heart. The cardioaccelerator center also sends additional fibers, forming the cardiac nerves via sympathetic ganglia (the cervical ganglia plus superior thoracic ganglia T1–T4) to both the SA and AV nodes, plus additional fibers to the atria and ventricles. The ventricles are more richly innervated by sympathetic fibers than parasympathetic fibers. Sympathetic stimulation causes the release of the neurotransmitter norepinephrine at the neuromuscular junction of the cardiac nerves. This shortens the repolarization period, thus speeding the rate of depolarization and contraction, which results in an increased heartrate. It opens chemical- or ligand-gated sodium and calcium ion channels, allowing an influx of positively charged ions.[4]

Norepinephrine binds to the beta-1 receptor. Some cardiac medications (for example, beta blockers) work by blocking these receptors, thereby slowing HR and are one possible treatment for hypertension. Overprescription of these drugs may lead to bradycardia and even stoppage of the heart.[4]

Autonomic Innervation of the Heart - Cardioaccelerator and cardioinhibitory areas are components of the paired cardiac centers located in the medulla oblongata of the brain. They innervate the heart via sympathetic cardiac nerves that increase cardiac activity and vagus (parasympathetic) nerves that slow cardiac activity.[4]

Parasympathetic stimulation originates from the cardioinhibitory region with impulses traveling via the vagus nerve (cranial nerve X). The vagus nerve sends branches to both the SA and AV nodes, and to portions of both the atria and ventricles. Parasympathetic stimulation releases the neurotransmitter acetylcholine (ACh) at the neuromuscular junction. ACh slows HR by opening chemical- or ligand-gated potassium ion channels to slow the rate of spontaneous depolarization, which extends repolarization and increases the time before the next spontaneous depolarization occurs. Without any nervous stimulation, the SA node would establish a sinus rhythm of approximately 100 bpm. Since resting rates are considerably less than this, it becomes evident that parasympathetic stimulation normally slows HR. This is similar to an individual driving a car with one foot on the brake pedal. To speed up, one need merely remove one’s foot from the break and let the engine increase speed. In the case of the heart, decreasing parasympathetic stimulation decreases the release of ACh, which allows HR to increase up to approximately 100 bpm. Any increases beyond this rate would require sympathetic stimulation. Figure 31 illustrates the effects of parasympathetic and sympathetic stimulation on the normal sinus rhythm.[4]

Effects of Parasympathetic and Sympathetic Stimulation on Normal Sinus Rhythm - The wave of depolarization in a normal sinus rhythm shows a stable resting HR. Following parasympathetic stimulation, HR slows. Following sympathetic stimulation, HR increases.[4]

Input to the cardiovascular center

The cardiovascular center receives input from a series of visceral receptors with impulses traveling through visceral sensory fibers within the vagus and sympathetic nerves via the cardiac plexus. Among these receptors are various proprioreceptors, baroreceptors, and chemoreceptors, plus stimuli from the limbic system. Collectively, these inputs normally enable the cardiovascular centers to regulate heart function precisely, a process known as cardiac reflexes. Increased physical activity results in increased rates of firing by various proprioreceptors located in muscles, joint capsules, and tendons. Any such increase in physical activity would logically warrant increased blood flow. The cardiac centers monitor these increased rates of firing, and suppress parasympathetic stimulation and increase sympathetic stimulation as needed in order to increase blood flow.[4]

Similarly, baroreceptors are stretch receptors located in the aortic sinus, carotid bodies, the venae cavae, and other locations, including pulmonary vessels and the right side of the heart itself. Rates of firing from the baroreceptors represent blood pressure, level of physical activity, and the relative distribution of blood. The cardiac centers monitor baroreceptor firing to maintain cardiac homeostasis, a mechanism called the baroreceptor reflex. With increased pressure and stretch, the rate of baroreceptor firing increases, and the cardiac centers decrease sympathetic stimulation and increase parasympathetic stimulation. As pressure and stretch decrease, the rate of baroreceptor firing decreases, and the cardiac centers increase sympathetic stimulation and decrease parasympathetic stimulation.[4]

There is a similar reflex, called the atrial reflex or Bainbridge reflex, associated with varying rates of blood flow to the atria. Increased venous return stretches the walls of the atria where specialized baroreceptors are located. However, as the atrial baroreceptors increase their rate of firing and as they stretch due to the increased blood pressure, the cardiac center responds by increasing sympathetic stimulation and inhibiting parasympathetic stimulation to increase HR. The opposite is also true.[4]

Increased metabolic byproducts associated with increased activity, such as carbon dioxide, hydrogen ions, and lactic acid, plus falling oxygen levels, are detected by a suite of chemoreceptors innervated by the glossopharyngeal and vagus nerves. These chemoreceptors provide feedback to the cardiovascular centers about the need for increased or decreased blood flow, based on the relative levels of these substances.[4]

The limbic system can also significantly impact HR related to emotional state. During periods of stress, it is not unusual to identify higher than normal HRs, often accompanied by a surge in the stress hormone cortisol. Individuals experiencing extreme anxiety may manifest panic attacks with symptoms that resemble those of heart attacks. These events are typically transient and treatable. Meditation techniques have been developed to ease anxiety and have been shown to lower HR effectively. Doing simple deep and slow breathing exercises with one’s eyes closed can also significantly reduce this anxiety and HR.[4]

Other factors influencing heart rate

Using a combination of autorhythmicity and innervation, the cardiovascular center is able to provide relatively precise control over HR. However, there are a number of other factors that have an impact on HR as well, including epinephrine, NE, and thyroid hormones; levels of various ions including calcium, potassium, and sodium; body temperature; hypoxia; and pH balance (Table 1 and Table 2). After reading this section, the importance of maintaining homeostasis should become even more apparent.[4]

Table 1: Major Factors Increasing Heart Rate and Force of Contraction[4]
Factor Effect
Cardioaccelerator nerves Release of norepinephrine
Proprioreceptors Increased rates of firing during exercise
Chemoreceptors Decreased levels of O2; increased levels of H+, CO2, and lactic acid
Baroreceptors Decreased rates of firing, indicating falling blood volume/pressure
Limbic system Anticipation of physical exercise or strong emotions
Catecholamines Increased epinephrine and norepinephrine
Thyroid hormones Increased T3 and T4
Calcium Increased Ca2+
Potassium Decreased K+
Sodium Decreased Na+
Body temperature Increased body temperature
Nicotine and caffeine Stimulants, increasing heart rate
Table 2: Factors Decreasing Heart Rate and Force of Contraction[4]
Factor Effect
Cardioinhibitor nerves (vagus) Release of acetylcholine
Proprioreceptors Decreased rates of firing following exercise
Chemoreceptors Increased levels of O2; decreased levels of H+ and CO2
Baroreceptors Increased rates of firing, indicating higher blood volume/pressure
Limbic system Anticipation of relaxation
Catecholamines Decreased epinephrine and norepinephrine
Thyroid hormones Decreased T3 and T4
Calcium Decreased Ca2+
Potassium Increased K+
Sodium Decreased Na+
Body temperature Decrease in body temperature

Epinephrine and norepinephrine

The catecholamines, epinephrine and NE, secreted by the adrenal medulla form one component of the extended fight-or- flight mechanism. The other component is sympathetic stimulation. Epinephrine and NE have similar effects: binding to the beta-1 receptors, and opening sodium and calcium ion chemical- or ligand-gated channels. The rate of depolarization is increased by this additional influx of positively charged ions, so the threshold is reached more quickly and the period of repolarization is shortened. However, massive releases of these hormones coupled with sympathetic stimulation may actually lead to arrhythmias. There is no parasympathetic stimulation to the adrenal medulla.[4]

Thyroid hormones

In general, increased levels of thyroid hormone, or thyroxin, increase cardiac rate and contractility. The impact of thyroid hormone is typically of a much longer duration than that of the catecholamines. The physiologically active form of thyroid hormone, T3 or triiodothyronine, has been shown to directly enter cardiomyocytes and alter activity at the level of the genome. It also impacts the beta adrenergic response similar to epinephrine and NE described above. Excessive levels of thyroxin may trigger tachycardia.[4]

Calcium

Calcium ion levels have great impacts upon both HR and contractility; as the levels of calcium ions increase, so do HR and contractility. High levels of calcium ions (hypercalcemia) may be implicated in a short QT interval and a widened T wave in the ECG. The QT interval represents the time from the start of depolarization to repolarization of the ventricles, and includes the period of ventricular systole. Extremely high levels of calcium may induce cardiac arrest. Drugs known as calcium channel blockers slow HR by binding to these channels and blocking or slowing the inward movement of calcium ions.[4]

Caffeine and nicotine

Caffeine and nicotine are not found naturally within the body. Both of these nonregulated drugs have an excitatory effect on membranes of neurons in general and have a stimulatory effect on the cardiac centers specifically, causing an increase in HR. Caffeine works by increasing the rates of depolarization at the SA node, whereas nicotine stimulates the activity of the sympathetic neurons that deliver impulses to the heart.[4]

Although it is the world’s most widely consumed psychoactive drug, caffeine is legal and not regulated. While precise quantities have not been established, “normal” consumption is not considered harmful to most people, although it may cause disruptions to sleep and acts as a diuretic. Its consumption by pregnant women is cautioned against, although no evidence of negative effects has been confirmed. Tolerance and even physical and mental addiction to the drug result in individuals who routinely consume the substance.[4]

Nicotine, too, is a stimulant and produces addiction. While legal and nonregulated, concerns about nicotine’s safety and documented links to respiratory and cardiac disease have resulted in warning labels on cigarette packages.[4]

Factors decreasing heart rate

HR can be slowed when a person experiences altered sodium and potassium levels, hypoxia, acidosis, alkalosis, and hypothermia (see Table 1). The relationship between electrolytes and HR is complex, but maintaining electrolyte balance is critical to the normal wave of depolarization. Of the two ions, potassium has the greater clinical significance. Initially, both hyponatremia (low sodium levels) and hypernatremia (high sodium levels) may lead to tachycardia. Severely high hypernatremia may lead to fibrillation, which may cause CO to cease. Severe hyponatremia leads to both bradycardia and other arrhythmias. Hypokalemia (low potassium levels) also leads to arrhythmias, whereas hyperkalemia (high potassium levels) causes the heart to become weak and flaccid, and ultimately to fail.[4]

Heart muscle relies exclusively on aerobic metabolism for energy. Hypoxia (an insufficient supply of oxygen) leads to decreasing HRs, since metabolic reactions fueling heart contraction are restricted.[4]

Acidosis is a condition in which excess hydrogen ions are present, and the patient’s blood expresses a low pH value. Alkalosis is a condition in which there are too few hydrogen ions, and the patient’s blood has an elevated pH. Normal blood pH falls in the range of 7.35–7.45, so a number lower than this range represents acidosis and a higher number represents alkalosis. Enzymes, being the regulators or catalysts of virtually all biochemical reactions - are sensitive to pH and will change shape slightly with values outside their normal range. These variations in pH and accompanying slight physical changes to the active site on the enzyme decrease the rate of formation of the enzyme-substrate complex, subsequently decreasing the rate of many enzymatic reactions, which can have complex effects on HR. Severe changes in pH will lead to denaturation of the enzyme.[4]

The last variable is body temperature. Elevated body temperature is called hyperthermia, and suppressed body temperature is called hypothermia. Slight hyperthermia results in increasing HR and strength of contraction. Hypothermia slows the rate and strength of heart contractions. This distinct slowing of the heart is one component of the larger diving reflex that diverts blood to essential organs while submerged. If sufficiently chilled, the heart will stop beating, a technique that may be employed during open heart surgery. In this case, the patient’s blood is normally diverted to an artificial heart-lung machine to maintain the body’s blood supply and gas exchange until the surgery is complete, and sinus rhythm can be restored. Excessive hyperthermia and hypothermia will both result in death, as enzymes drive the body systems to cease normal function, beginning with the central nervous system.[4]

Stroke volume

Many of the same factors that regulate HR also impact cardiac function by altering SV. While a number of variables are involved, SV is ultimately dependent upon the difference between EDV and ESV. The three primary factors to consider are preload, or the stretch on the ventricles prior to contraction; the contractility, or the force or strength of the contraction itself; and afterload, the force the ventricles must generate to pump blood against the resistance in the vessels. These factors are summarized in Table 1 and Table 2.[4]

Preload

Preload is another way of expressing EDV. Therefore, the greater the EDV is, the greater the preload is. One of the primary factors to consider is filling time, or the duration of ventricular diastole during which filling occurs. The more rapidly the heart contracts, the shorter the filling time becomes, and the lower the EDV and preload are. This effect can be partially overcome by increasing the second variable, contractility, and raising SV, but over time, the heart is unable to compensate for decreased filling time, and preload also decreases.[4]

With increasing ventricular filling, both EDV or preload increase, and the cardiac muscle itself is stretched to a greater degree. At rest, there is little stretch of the ventricular muscle, and the sarcomeres remain short. With increased ventricular filling, the ventricular muscle is increasingly stretched and the sarcomere length increases. As the sarcomeres reach their optimal lengths, they will contract more powerfully, because more of the myosin heads can bind to the actin on the thin filaments, forming cross bridges and increasing the strength of contraction and SV. If this process were to continue and the sarcomeres stretched beyond their optimal lengths, the force of contraction would decrease. However, due to the physical constraints of the location of the heart, this excessive stretch is not a concern.[4]

The relationship between ventricular stretch and contraction has been stated in the well-known Frank-Starling mechanism or simply Starling’s Law of the Heart. This principle states that, within physiological limits, the force of heart contractionis directly proportional to the initial length of the muscle fiber. This means that the greater the stretch of the ventricular muscle (within limits), the more powerful the contraction is, which in turn increases SV. Therefore, by increasing preload, you increase the second variable, contractility.[4]

Any sympathetic stimulation to the venous system will increase venous return to the heart, which contributes to ventricular filling, and EDV and preload. While much of the ventricular filling occurs while both atria and ventricles are in diastole, the contraction of the atria, the atrial kick, plays a crucial role by providing the last 20–30 percent of ventricular filling.[4]

Contractility

It is virtually impossible to consider preload or ESV without including an early mention of the concept of contractility. Indeed, the two parameters are intimately linked. Contractility refers to the force of the contraction of the heart muscle, which controls SV, and is the primary parameter for impacting ESV. The more forceful the contraction is, the greater the SV and smaller the ESV are. Less forceful contractions result in smaller SVs and larger ESVs. Factors that increase contractility are described as positive inotropic factors, and those that decrease contractility are described as negative inotropic factors (ino- = “fiber;” -tropic = “turning toward”).[4]

Not surprisingly, sympathetic stimulation is a positive inotrope, whereas parasympathetic stimulation is a negative inotrope. Sympathetic stimulation triggers the release of NE at the neuromuscular junction from the cardiac nerves and also stimulates the adrenal cortex to secrete epinephrine and NE. In addition to their stimulatory effects on HR, they also bind to both alpha and beta receptors on the cardiac muscle cell membrane to increase metabolic rate and the force of contraction. This combination of actions has the net effect of increasing SV and leaving a smaller residual ESV in the ventricles. In comparison, parasympathetic stimulation releases ACh at the neuromuscular junction from the vagus nerve. The membrane hyperpolarizes and inhibits contraction to decrease the strength of contraction and SV, and to raise ESV. Since parasympathetic fibers are more widespread in the atria than in the ventricles, the primary site of action is in the upper chambers. Parasympathetic stimulation in the atria decreases the atrial kick and reduces EDV, which decreases ventricular stretch and preload, thereby further limiting the force of ventricular contraction. Stronger parasympathetic stimulation also directly decreases the force of contraction of the ventricles.[4]

Several synthetic drugs, including dopamine and isoproterenol, have been developed that mimic the effects of epinephrine and NE by stimulating the influx of calcium ions from the extracellular fluid. Higher concentrations of intracellular calcium ions increase the strength of contraction. Excess calcium (hypercalcemia) also acts as a positive inotropic agent. The drug digitalis lowers HR and increases the strength of the contraction, acting as a positive inotropic agent by blocking the sequestering of calcium ions into the sarcoplasmic reticulum. This leads to higher intracellular calcium levels and greater strength of contraction. In addition to the catecholamines from the adrenal medulla, other hormones also demonstrate positive inotropic effects. These include thyroid hormones and glucagon from the pancreas.[4]

Negative inotropic agents include hypoxia, acidosis, hyperkalemia, and a variety of synthetic drugs. These include numerous beta blockers and calcium channel blockers. Early beta blocker drugs include propranolol and pronethalol, and are credited with revolutionizing treatment of cardiac patients experiencing angina pectoris. There is also a large class of dihydropyridine, phenylalkylamine, and benzothiazepine calcium channel blockers that may be administered decreasing the strength of contraction and SV.[4]

Afterload

Afterload refers to the tension that the ventricles must develop to pump blood effectively against the resistance in the vascular system. Any condition that increases resistance requires a greater afterload to force open the semilunar valves and pump the blood. Damage to the valves, such as stenosis, which makes them harder to open will also increase afterload. Any decrease in resistance decreases the afterload. Tables 3&4 summarize cardiac responses to restore homeostasis upon increased and decreased blood flow and pressure.[4]

Major Factors Influencing Stroke Volume - Multiple factors impact preload, afterload, and contractility, and are the major considerations influencing SV.[4]
Summary of Major Factors Influencing Cardiac Output - The primary factors influencing HR include autonomic innervation plus endocrine control. Not shown are environmental factors, such as electrolytes, metabolic products, and temperature. The primary factors controlling SV include preload, contractility, and afterload. Other factors such as electrolytes may be classified as either positive or negative inotropic agents.[4]
Table 3: Cardiac Response to Decreasing Blood Flow and Pressure Due to Decreasing Cardiac Output[4]
Baroreceptors (aorta, carotid arteries, venae cavae, and atria) Chemoreceptors (both central nervous system and in proximity to baroreceptors)
Sensitive to Decreasing stretch[4] Decreasing O2 and increasing CO2, H+, and lactic acid[4]
Target Parasympathetic stimulation suppressed[4] Sympathetic stimulation increased[4]
Response of heart Increasing heart rate and increasing stroke volume[4] Increasing heart rate and increasing stroke volume[4]
Overall effect Increasing blood flow and pressure due to increasing cardiac output; hemostasis restored[4] Increasing blood flow and pressure due to increasing cardiac output; hemostasis restored[4]
Table 4: Cardiac Response to Increasing Blood Flow and Pressure Due to Increasing Cardiac Output[4]
Baroreceptors (aorta, carotid arteries, venae cavae, and atria) Chemoreceptors (both central nervous system and in proximity to baroreceptors)
Sensitive to Increasing stretch[4] Increasing O2 and decreasing CO2, H+, and lactic acid[4]
Target Parasympathetic stimulation increased[4] Sympathetic stimulation suppressed[4]
Response of heart Decreasing heart rate and decreasing stroke volume[4] Decreasing heart rate and decreasing stroke volume[4]
Overall effect Decreasing blood flow and pressure due to decreasing cardiac output; hemostasis restored<[4] Decreasing blood flow and pressure due to decreasing cardiac output; hemostasis restore

[4]

Clinical significance

The stethoscope is used for auscultation of the heart, and is one of the most iconic symbols for medicine. Auscultation of heart sounds is done in order to diagnose a number of diseases - primarily by listening for heart murmurs.
Atherosclerosis is a condition affecting the cardiovascular system. If atherosclerosis occurs in the coronary arteries (which supply the heart) the result may be angina pectoris, or in worse cases a heart attack.

Being a complex organ the heart is prone to several cardiovascular diseases some becoming more prevalent with ageing. The following are just some of these disorders:

Coronary artery disease, is also known as ischemic heart disease (IHD), and more usually as atherosclerosis. This disease is caused by a build-up of plaque along the inner walls of the arteries which has the effect of narrowing the arteries and so reducing the blood flow to the heart. It is the most common form of heart disease, the cause of heart attacks and the most common cause of death, globally.[17] Coronary artery bypass surgery to improve the blood supply to the heart is often the only treatment option.

Because the heart is a double pump, each side can fail independently of the other. Peripheral congestion can occur as a result of blood backing up in the systemic circulation. Edema is most noticeable in the feet, ankles, and fingers which become swollen. Failure of one side of the heart puts a strain on the opposite side, and eventually the whole heart fails. When the left side of the heart fails, Pulmonary congestion occurs. The right side of the heart continues to propel blood to the lungs, but the left side is unable to eject the returning blood into the systemic circulation. As blood vessels within the lungs become swollen with blood, the pressure within them increases, and fluid leaks from the circulation into the lung tissue, causing pulmonary edema. If untreated, the person will suffocate because they are drowning in their own blood.[18]

Other conditions can interfere with the regular conduction of impulses across the heart. Damage to the sinoatrial node (SA), (the pace maker of the heart), can result in a slower heart rate. Ischemia, or an inadequate blood supply to the heart muscle, may lead to fibrillation - a rapid, uncoordinated shuddering of the heart muscle, a major cause of fatal heart attacks.[18]

Heart murmurs are abnormal or unusual heart sounds which can be caused by an obstruction in the blood flow. These murmurs can be heard with a stethoscope. Heart murmurs are common in young children and the elderly even if they have perfectly healthy hearts. They may have heart murmurs because their heart walls are thin and vibrate with the rushing blood. However, murmurs in patients that do not fall into either of those categories most often have a valve issue. For example, if a valve does not close tightly enough, a swishing sound will be heard after that valve has (supposedly) closed, as the blood flows back through the partially open valve. Distinct sounds also can be heard when blood flows turbulently through stenosed (narrowed) valves.[18]

Heart failure which can also be congestive heart failure, happens when the heart is pumping insufficiently and cannot meet the need of blood flow required by the body. It can cause shortness of breath and edema in the legs. Common causes are a heart attack, valvular heart disease and hypertension. Heart failure is described as congestive heart failure when due to insufficient pumping, the body becomes congested with fluid (which does not always occur with heart failure).

Cardiac tamponade, also known as pericardial tamponade, is the condition of an abnormal build-up of fluid in the pericardium which can adversely affect the function of the heart.

Cardiac arrest is the sudden cessation of normal heart rhythm which can include a number of pathologies such as tachycardia, an extremely rapid heart beat which prevents the heart from effectively pumping blood, which is an irregular and ineffective heart rhythm, and asystole, which is the cessation of heart rhythm entirely.

Cardiac tamponade is a condition in which the pericardium fills with excess fluid or blood, suppressing the heart's ability to beat properly. Tamponade is treated by pericardiocentesis, the gentle insertion of the needle of a syringe into the pericardial sac (avoiding the heart itself) on an angle, usually from just below the sternum, and gently withdrawing the tamponading fluids.

Lifestyle and heart health

Obesity, high blood pressure, and high cholesterol can increase the risk of developing heart disease. However, half the number of heart attacks occur in people with normal cholesterol levels. Heart disease is a major cause of death.

It is generally accepted that factors such as exercise or the lack of it, good or poor diet, and overall well-being, including both emotional and physiological components, affect heart health in humans.[19][20][21][22]

Development

Main articles: Heart development and Embryogenesis

The embryology and early development of the human and mammalian heart:

The heart derives from embryonic mesoderm germ-layer cells that differentiate after gastrulation into mesothelium, endothelium, and myocardium. Mesothelial pericardium forms the outer lining of the heart. The inner lining of the heart, lymphatic and blood vessels, develop from endothelium. Heart muscle is termed myocardium.[23]

From splanchnopleuric mesoderm tissue, the cardiogenic plates develop cranially and laterally to the neural plates. In the cardiogenic plates, two separate angiogenic cell clusters form on either side of the embryo. The cell clusters coalesce to form an endocardial heart tube continuous with a dorsal aorta and a vitteloumbilical vein. As embryonic tissue continues to fold, the two endocardial tubes are pushed into the thoracic cavity, begin to fuse together, and complete the fusing process at approximately 22 days.[24]

The human heart is the first functional organ to develop.[4] It begins beating and pumping blood around day 21 or 22, a mere three weeks after fertilization.[4] This emphasizes the critical nature of the heart in distributing blood through the vessels and the vital exchange of nutrients, oxygen, and wastes both to and from the developing baby.[4] The critical early development of the heart is reflected by the prominent heart bulge that appears on the anterior surface of the embryo.[4]

The heart forms from an embryonic tissue called mesoderm around 18 to 19 days after fertilization. Mesoderm is one of the three primary germ layers that differentiates early in development that collectively gives rise to all subsequent tissues and organs.[4] The heart begins to develop near the head of the embryo in a region known as the cardiogenic area.[4] Following chemical signals called factors from the underlying endoderm (another of the three primary germ layers), the cardiogenic area begins to form two strands called the cardiogenic cords (Figure 34).[4] As the cardiogenic cords develop, a lumen rapidly develops within them. At this point, they are referred to as endocardial tubes.[4] The two tubes migrate together and fuse to form a single primitive heart tube. The primitive heart tube quickly forms five distinct regions.[4] From head to tail, these include the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus.[4] Initially, all venous blood flows into the sinus venosus, and contractions propel the blood from tail to head, or from the sinus venosus to the truncus arteriosus. This is a very different pattern from that of an adult.[4]

Development of the Human Heart - This diagram outlines the embryological development of the human heart during the first eight weeks and the subsequent formation of the four heart chambers.[4]

The five regions of the primitive heart tube develop into recognizable structures in a fully developed heart.[4] The truncus arteriosus will eventually divide and give rise to the ascending aorta and pulmonary trunk.[4] The bulbus cordis develops into the right ventricle.[4] The primitive ventricle forms the left ventricle.[4] The primitive atrium becomes the anterior portions of both the right and left atria, and the two auricles.[4] The sinus venosus develops into the posterior portion of the right atrium, the SA node, and the coronary sinus.[4]

35. At 22 days after conception, the human heart begins beating at 70 to 80 beats per minute and accelerates linearly for the first month of beating. Heart rates measured by motion mode (M-mode) sonography.[citation needed]

As the primitive heart tube elongates, it begins to fold within the pericardium, eventually forming an S shape, which places the chambers and major vessels into an alignment similar to the adult heart.[4] This process occurs between days 23 and 28.[4] The remainder of the heart development pattern includes development of septa and valves, and remodeling of the actual chambers.[4] Partitioning of the atria and ventricles by the interatrial septum, interventricular septum, and atrioventricular septum is complete by the end of the fifth week, although the fetal blood shunts remain until birth or shortly after.[4] The atrioventricular valves form between weeks five and eight, and the semilunar valves form between weeks five and nine.[4]

Embryonic heart rate (EHR)

The human embryonic heart begins beating at around 22 days after conception, or five weeks after the last normal menstrual period (LMP). The first day of the LMP is normally used to date the start of the gestation (pregnancy). The human heart begins beating at a rate near the mother’s, about 75–80 beats per minute (BPM). The embryonic heart rate (EHR) then accelerates linearly by approximately 100 BPM during the first month to peak at 165–185 BPM during the early 7th week after conception, (early 9th week after the LMP). This acceleration is approximately 3.3 BPM per day, or about 10 BPM every three days, which is an increase of 100 BPM in the first month.[25][26][27]

The regression formula which describes this linear acceleration before the embryo reaches 25mm in crown-rump length, or 9.2 LMP weeks, is: the Age in days = EHR(0.3)+6.

After 9.1 weeks after the LMP, it decelerates to about 152 BPM (±25 BPM) during the 15th week post LMP. After the 15th week, the deceleration slows to an average rate of about 145 (±25 BPM) BPM, at term. There is no difference in female and male heart rates before birth.[28]

The human heart begins beating at a rate near to that of the mother’s heart rate, which is about 75–80 beats per minute (BPM). The embryonic heart rate (EHR) then accelerates by approximately 100 BPM during the first month to peak at 165–185 BPM during the early seventh week. This acceleration is approximately 3.3 BPM per day, or about 10 BPM every three days, which gives an increase of 100 BPM in the first month.[25]

History

Ancient

Heart and its blood vessels, by Leonardo da Vinci, 15th century

The valves of the heart were discovered by a physician of the Hippocratean school around the 4th century BC, although their function was not fully understood. On dissection, arteries are typically empty of blood because blood pools in the veins after death. Ancient anatomists subsequently assumed they were filled with air and served to transport it around the body.

Philosophers distinguished veins from arteries, but thought the pulse was a property of arteries themselves. Erasistratos observed that arteries cut during life bleed. He ascribed the fact to the phenomenon that air escaping from an artery is replaced with blood which entered by very small vessels between veins and arteries. Thus he apparently postulated capillaries, but with reversed flow of blood.

The Greek physician Galen (2nd century AD) knew blood vessels carried blood and identified venous (dark red) and arterial (brighter and thinner) blood, each with distinct and separate functions. Growth and energy were derived from venous blood created in the liver from chyle, while arterial blood gave vitality by containing pneuma (air) and originated in the heart. Blood flowed from both creating organs to all parts of the body, where it was consumed and there was no return of blood to the heart or liver. The heart did not pump blood around, the heart's motion sucked blood in during diastole and the blood moved by the pulsation of the arteries themselves.

Galen believed the arterial blood was created by venous blood passing from the left ventricle to the right through 'pores' in the interventricular septum, while air passed from the lungs via the pulmonary artery to the left side of the heart. As the arterial blood was created, "sooty" vapors were created and passed to the lungs, also via the pulmonary artery, to be exhaled.

Pre-modern

The earliest descriptions of the coronary circulation and pulmonary circulation systems can be found in the Commentary on Anatomy in Avicenna's Canon, published in 1242 by Ibn al-Nafis.[29] In his manuscript, al-Nafis wrote that blood passes through the pulmonary circulation instead of moving from the right to the left ventricle as previously believed by Galen.[30] His work was later translated into Latin by Andrea Alpago.[31]

In Europe, the teachings of Galen continued to dominate the academic community and his doctrines were adopted as the official canon of the Church. Andreas Vesalius questioned some of Galen's beliefs of the heart in De humani corporis fabrica (1543), but his magnum opus was interpreted as a challenge to the authorities and he was subjected to a number of attacks.[32] Michael Servetus wrote in Christianismi Restitutio (1553) that blood flows from one side of the heart to the other via the lungs, but his book was considered heresy and he was condemend and burned at the stake in Geneva.[32] The breakthrough came with the publication of De Motu Cordis (1628) by the English physician William Harvey. Harvey's book completely describes the systemic circulation and the mechanical force of the heart, leading to an overhaul of the Galenic doctrines.[33]

Modern

Otto Frank (1865–1944) was a German physiologist; among his many published works are detailed studies of this important heart relationship. Ernest Starling (1866–1927) was an important English physiologist who also studied the heart. Although they worked largely independently, their combined efforts and similar conclusions have been recognized in the name “Frank-Starling mechanism.”[4]

Although Purkinje fibres and the Bundle of His were discovered as early as the 19th century, their specific role in the electrical conduction system of the heart remained unknown until Sunao Tawara published his monograph, titled Das Reizleitungssystem des Säugetierherzens, in 1906. Tawara's discovery of the atrioventricular node prompted Arthur Keith and Martin Flack to look for similar structures in the heart, leading to their discovery of the sinoatrial node several months later. These structures from the anatomical basis of the electrocardiogram, whose inventor, Willem Einthoven, was awarded the Nobel Prize in Medicine or Physiology in 1924.[34]

The first successful heart transplantation was performed in 1967 by the South African surgeon Christiaan Barnard at Groote Schuur Hospital in Cape Town. This marks an important milestone in cardiac surgery, capturing the attention of both the medical profession and the world at large. However, long-term survival rates of patients were initially very low. Louis Washkansky, the first recipient of a donated heart, died 18 days after the operation while other patients did not survive for more than a few weeks.[35] The American surgeon Norman Shumway has been credited for his efforts in improving the transplantation techniques, along with pioneers Richard Lower, Vladimir Demikhov and Adrian Kantrowitz. As of March 2000, more than 55,000 heart transplantations have been performed worldwide.[36]

By the middle of the 20th century, heart disease had surpassed infectious disease as the leading cause of death in the United States, and it is currently the leading cause of deaths worldwide. Since 1948, the ongoing Framingham Heart Study has shed light on the effects of various influences on the heart, including diet, exercise, and common medications such as aspirin. Although the introduction of ACE inhibitors and beta blockers has improved the management of chronic heart failure, the disease continues to be an enormous medical and societal burden, with 30 to 40% of parients dying within a year of receiving the diagnosis.[37] There is a need for better treatments to reduce the huge societal burdens and high medical expenses that are incurred as a result of heart disease.

Society and culture

F34
jb (F34) "heart"
in hieroglyphs
Letter of the Georgian script is often used as a "heart" symbol.
The seal script glyph for "heart" (Middle Chinese sim)

As one of the vital organs, the heart was long identified as the center of the entire body, the seat of life, or emotion, or reason, will, intellect, purpose or the mind. Thus, in the Hebrew Bible, the word for "heart" לָבַב lebab is used in these meanings (paralleling the use of φρήν "diaphragm" in Homeric Greek).

An important part of the concept of the soul in Ancient Egyptian religion was thought to be the heart, or ib. The ib or metaphysical heart was believed to be formed from one drop of blood from the child's mother's heart, taken at conception.[38] To ancient Egyptians, the heart was the seat of emotion, thought, will and intention. This is evidenced by Egyptian expressions which incorporate the word ib, such as Awt-ib for "happiness" (literally, "wideness of heart"), Xak-ib for "estranged" (literally, "truncated of heart").[citation needed] In Egyptian religion, the heart was the key to the afterlife. It was conceived as surviving death in the nether world, where it gave evidence for, or against, its possessor. It was thought that the heart was examined by Anubis and the deities during the Weighing of the Heart ceremony. If the heart weighed more than the feather of Maat, it was immediately consumed by the monster Ammit.

The Chinese character for "heart", (Chinese: ), derives from a comparatively realistic depiction of a heart (indicating the heart chambers) in seal script. The Chinese word Chinese word xīn also takes the metaphorical meanings of "mind, intelligence", "soul" or "center, core". In Chinese medicine, the heart is seen as the center of shén "spirit, soul, consciousness".

The Sanskrit word for heart, hRd (हृद्) dates at least as far back as the Rigveda and is a cognate of the word for heart in Greek, Latin and English. The same word is used to mean "mind" or "soul" depending on the context.

Many classical philosophers and scientists, including Aristotle, considered the heart the seat of thought, reason or emotion, often disregarding the brain as contributing to those functions.[39]

The identification of the heart as the seat of emotions in particular is due to the Roman physician Galen, who also located the seat of the passions in the liver, and the seat of reason in the brain.[40] However these "emotional properties" of the heart were later discovered to be solely centered in the brain.

This tradition influenced the development of the medieval Christian devotion to the Sacred Heart of Jesus and the Immaculate Heart of Mary.

The idiomatic expression of "pierced" or "broken" hearts ultimately derive from devotional Christianity, where the hearts of Mary or Jesus are depicted as suffering various tortures (symbolizing the pain suffered by Christ for the sins of the world, and the pain of Mary at the crucifixion of her son, respectively), but from an early time the metaphor was transferred to unfullfilled romantic love, in late medieval literature dealing with the ideals of courtly love. The notion of "Cupid's arrows" is ancient, due to Ovid, but while Ovid describes Cupid as wounding his victims with his arrows, it is not made explicit that it is the heart that is wounded. The familiar iconography of Cupid shooting little heart symbols is Baroque.

Heart as food

Animal hearts are widely consumed as food. As they are almost entirely muscle, they are high in protein. They are often included in dishes with other offal, for example in the pan-Ottoman kokoretsi.

Chicken hearts are considered to be giblets, and are often grilled on skewers: Japanese hāto yakitori, Brazilian churrasco de curacao, Indonesian chicken heart satay.[41] They can also be pan-fried, as in Jerusalem mixed grill. In Egyptian cuisine, they can be used, finely chopped, as part of stuffing for chicken.[42] Many recipes combined them with other giblets, such as the Mexican pollo en menudencias[43] and the Russian ragu iz kurinyikh potrokhov.[44]

The hearts of beef, pork, and mutton can generally be interchanged in recipes. As heart is a hard-working muscle, it makes for "firm and rather dry" meat,[45] so is generally slow-cooked. Another way of dealing with toughness is to julienne the meat, as in Chinese stir-fried heart.[46]

Beef heart may be grilled or braised.[45] In the Peruvian anticuchos de corazón, barbecued beef hearts are grilled after being tenderized through long marination in a spice and vinegar mixture. An Australian recipe for "mock goose" is actually braised stuffed beef heart.[47]

Pig heart is stewed, poached, braised,[48] or made into sausage. The Balinese oret is a sort of blood sausage made with pig heart and blood. A French recipe for coeur de porc à l'orange is made of braised heart with an orange sauce.

Other animals

The structure of the heart can vary among the different animal species. (See Circulatory system.) Cephalopods have two "gill hearts" and one "systemic heart". In vertebrates, the heart lies in the anterior part of the body cavity, dorsal to the gut. It is always surrounded by a pericardium, which is usually a distinct structure, but may be continuous with the peritoneum in jawless and cartilaginous fish. Hagfish, uniquely among vertebrates, also possess a second heart-like structure in the tail.[49]

The SA node is found in all amniotes but not in more primitive vertebrates. In these animals, the muscles of the heart are relatively continuous and the sinus venosus coordinates the beat which passes in a wave through the remaining chambers. Indeed, since the sinus venosus is incorporated into the right atrium in amniotes, it is likely homologous with the SA node. In teleosts, with their vestigial sinus venosus, the main centre of coordination is, instead, in the atrium. The rate of heartbeat varies enormously between different species, ranging from around 20 beats per minute in codfish to around 600 in hummingbirds.[49]

Double circulatory systems

In amphibians and most reptiles, a double circulatory system is used but the heart is not completely separated into two pumps. The development of the double system is necessitated by the presence of lungs which deliver oxygenated blood directly to the heart.

In living amphibians, the atrium is divided into two separate chambers by the presence of a muscular septum even though there is only one ventricle. The sinus venosus, which remains large in amphibians but connects only to the right atrium, receives blood from the venae cavae, with the pulmonary vein by-passing it entirely to enter the left atrium.

In the heart of lungfish, the septum extends part-way into the ventricle. This allows for some degree of separation between the de-oxygenated bloodstream destined for the lungs and the oxygenated stream that is delivered to the rest of the body. The absence of such a division in living amphibian species may be at least partly due to the amount of respiration that occurs through the skin in such species; thus, the blood returned to the heart through the vena cavae is, in fact, already partially oxygenated. As a result, there may be less need for a finer division between the two bloodstreams than in lungfish or other tetrapods. Nonetheless, in at least some species of amphibian, the spongy nature of the ventricle seems to maintain more of a separation between the bloodstreams than appears the case at first glance. Furthermore, the conus arteriosus has lost its original valves and contains a spiral valve, instead, that divides it into two parallel parts, thus helping to keep the two bloodstreams separate.[49]

The heart of most reptiles (except for crocodilians; see below) has a similar structure to that of lungfish but, here, the septum is generally much larger. This divides the ventricle into two halves but, because the septum does not reach the whole length of the heart, there is a considerable gap near the openings to the pulmonary artery and the aorta. In practice, however, in the majority of reptilian species, there appears to be little, if any, mixing between the bloodstreams, so the aorta receives, essentially, only oxygenated blood.[49]

The fully divided heart

Archosaurs (crocodilians and birds) and mammals show complete separation of the heart into two pumps for a total of four heart chambers; it is thought that the four-chambered heart of archosaurs evolved independently from that of mammals. In crocodilians, there is a small opening, the foramen of Panizza, at the base of the arterial trunks and there is some degree of mixing between the blood in each side of the heart, during a dive underwater;[50][51] thus, only in birds and mammals are the two streams of blood – those to the pulmonary and systemic circulations – permanently kept entirely separate by a physical barrier.[49]

Fish

Schematic of simplified fish heart

Primitive fish have a four-chambered heart, but the chambers are arranged sequentially so that this primitive heart is quite unlike the four-chambered hearts of mammals and birds. The first chamber is the sinus venosus, which collects deoxygenated blood, from the body, through the hepatic and cardinal veins. From here, blood flows into the atrium and then to the powerful muscular ventricle where the main pumping action will take place. The fourth and final chamber is the conus arteriosus which contains several valves and sends blood to the ventral aorta. The ventral aorta delivers blood to the gills where it is oxygenated and flows, through the dorsal aorta, into the rest of the body. (In tetrapods, the ventral aorta has divided in two; one half forms the ascending aorta, while the other forms the pulmonary artery).[49]

In the adult fish, the four chambers are not arranged in a straight row but, instead form an S-shape with the latter two chambers lying above the former two. This relatively simpler pattern is found in cartilaginous fish and in the ray-finned fish. In teleosts, the conus arteriosus is very small and can more accurately be described as part of the aorta rather than of the heart proper. The conus arteriosus is not present in any amniotes, presumably having been absorbed into the ventricles over the course of evolution. Similarly, while the sinus venosus is present as a vestigial structure in some reptiles and birds, it is otherwise absorbed into the right atrium and is no longer distinguishable.[49]

Invertebrates

In invertebrates that possess a circulatory system, the heart is typically a tube or small sac and pumps fluid that contains water and nutrients such as proteins, fats, and sugars. In insects, the "heart" is often called the dorsal tube and insect "blood" is almost always not oxygenated since they usually respirate (breathe) directly from their body surfaces (internal and external) to air. However, the hearts of some other arthropods (including spiders and crustaceans such as crabs and shrimp) and some other animals pump hemolymph, which contains the copper-based protein hemocyanin as an oxygen transporter similar to the iron-based hemoglobin in red blood cells found in vertebrates.

Additional Images

References

This article incorporates text from the CC-BY book: OpenStax College, Anatomy & Physiology. OpenStax CNX. 30 jul 2014..

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