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Surface view of the heart
|Artery||Right coronary artery, left coronary artery, anterior interventricular artery|
|Vein||Superior vena cava, inferior vena cava, right pulmonary veins, left pulmonary veins|
The human heart is a vital organ that functions as a pump, providing a continuous circulation of blood through the body, by way of the cardiac cycles. The heart is contained in the mediastinum in the thoracic cavity of the thorax.
The heart is enclosed in a protective sac, the pericardium which also contains a lubricating pericardial fluid. The outer wall of the heart is made up of three layers, the epicardium, the myocardium which is the muscle of the heart, and the endocardium. The heart is divided into four main chambers: the two upper chambers are called the left atrium and the right atrium (plural atria) and the two lower chambers are called the right and the left ventricle. There is a dividing wall of muscle, called the septum, which separates the right side of the heart from the left side of the heart. The part of the septum that separates the ventricles, the ventricular septum is thicker than that which separates the atria, the atrial septum.
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. Physicians commonly refer to the right atrium and right ventricle together as the right heart and to the left atrium and left ventricle as the left heart.
The human heart has four chambers, two superior atria and two inferior ventricles. The atria are the receiving chambers and the ventricles are the discharging chambers. 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 interatrioventricular septum separates the left heart from the right heart, giving two functionally and anatomically distinct units.
The pathways of blood through the human heart are part of the pulmonary and systemic circuits. These pathways include the tricuspid valve, the bicuspid mitral valve, the aortic valve, and the pulmonary valve. The aortic valve and the pulmonary valve are the two semilunar valves of the heart.– The mitral and tricuspid valves are classified as the atrioventricular (AV) valves. This is because they are found between the atria and ventricles. The aortic and pulmonary semilunar valves separate the left and right ventricle from the aorta and the pulmonary arterty respectively. These valves are attached to the chordae tendineae (literally the heartstrings), which anchor the valves to the papillary muscles of the heart.
It is enclosed in a double-walled protective sac called the pericardium. The two membranes of this sac enclose the pericardial cavity. As with other body cavities the pericardial cavity is lined with two serous membranes, or serosas. The serosas secrete a serous fluid the pericardial fluid which fills the cavity, and acts as a lubricant to prevent friction. This enables the heart to move in response to its own contractions and to the movements of adjacent structures such as the diaphragm and lungs. It also serves as protection from infection and knocks.The outer serous membrane is that of the wall, the parietal membrane, and the serosa closest to the organ is the visceral membrane. Each serosa is made up of a single layer of squamous epithelial tissue as mesothelium which produces the serous fluid. The membranes of mesothelium are tightly bound to an underlying layer of connective tissue which provides the blood vessels and nerves for the overlying secretory cells. On the outer side of the parietal pericardium there is a fibrous layer the fibrous pericardium which is joined to the mediastinal fascia. This fibrous layer anchors the heart's surrounding structures but generally has no effect on heart function.
The outer wall of the human heart is composed of three layers. The outer layer is called the epicardium, or visceral pericardium since it is also the inner wall of the (serous) pericardium. The middle layer of the heart is called the myocardium and is composed of muscle which contracts. The inner layer is called the endocardium and is in contact with the blood that the heart pumps. Also, it merges with the inner lining (endothelium) of blood vessels and covers heart valves.
The heart is located in the middle mediastinum, one of the divisions of the thoracic cavity. The heart is at the level of the thoracic vertebrae T5-8. The heart rests on the diaphragm, beneath the sternum and ribs, and has two sides adjacent to the right and left lungs.
- The left ventricle, and some of the right ventricle, rest on the central tendon of the diaphragm. At the posterior edge of the surface is the coronary sulcus.
- The side opposite the apex is the base of the heart, and is the most posterior section of the heart. It consists mainly of the left atrium and some of the parts closest to the heart of the right atrium, inferior and superior vena cavae, and pulmonary veins.
- The side of the heart facing the sternum and ribs consists mostly of the right ventricle, with some of the right atrium and left ventricle also present. The coronary sulcus runs down this surface.
- Facing the left lung is the left ventricle and some of the left atrium, and facing the right lung is the right ventricle and some of the right atrium.
As well as the blood being pumped within the heart, the heart has its own blood supply that surrounds it. This is the coronary circulation.
The heart is the first functional organ to develop in the embryo, with the pericardium being the first of its structures to form, from the initial activation of cardiac muscle cells. Heart development involves five stages. The first stage is that of the development of a heart tube, which sees the early stages of the five regions that will develop into the adult heart structures. In the second stage the heart chambers begin to take form and the heart tube loops into an asymmetrical structure. Stage three involves the formation of the two septa, the interatrial septum and the intraventricular septum. These are critical for the correct positioning and functioning of the heart valves, the proper chamber formation and subsequent blood flow. Several structures join in order to form the primitive atrium, which only becomes the final atrial septum after birth. The atrioventricular canal develops into the AV septum separating the atria from the ventricles, it will also develop into the associated valves; the right side of the AV canal will become the tricuspid valve and the left side the bicuspid valve. There are endocardial cushions (cells which have collected together as swellings), and these fuse to form the septa of the four heart chambers. In stage four, the aorticopulmonary septum forms to separate the aorta and pulmonary arteries; this septum will fuse with the intraventricular septum during development. Stage five sees the heart completed with the formation of the heart valves.
The human embryonic heart begins beating at around 21 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 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.
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 by the tricuspid, bicuspid, aortic, and pulmonary valves.
The heart acts as a double pump. The function of the right side of the heart (also called the right heart, which includes the right atrium and right ventricle and sometimes the pulmonary trunk) is to collect de-oxygenated blood, in the right atrium, from the body (via the superior and inferior venae cavae and pump it, 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 side of the heart (also called the left heart which includes the left atrium and left ventricle), collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood flows to the left ventricle which pumps it out to the body (via the aorta).
The lower ventricles are thicker and stronger than the upper atria. 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.
Starting in the right atrium, the blood flows through the tricuspid valve to the right ventricle. Here, it is pumped out of the pulmonary semilunar 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 aortic semilunar valve to the aorta and to the rest of the body. The (relatively) deoxygenated blood finally returns to the heart through the superior and inferior venae cavae.
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. While in the ventricles, the signal is carried by specialized tissue called the Purkinje fibers which then transmit the electric charge to the myocardium.
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. 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.
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.
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.
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.
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.
Society and culture
|jb (F34) "heart"
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. 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"). 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: 心), 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 神 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.
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. 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.
Real-time MRI - Human Heart (short-axis view)
anterior view, coronal section.
The blood supply of the heart is provided by the coronary circulation. Coronary arteries labeled in red text and other landmarks in blue text.
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- The Human Heart: An Online Exploration from The Franklin Institute
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