Human heart

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Human heart
Heart rotating.gif
The heart is situated at the centre of the chest, with the larger portion to the left.
Heart anterior exterior view.jpg
anatomical illustration of a human heart.
Latin cor humanum
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
Anatomical terminology

The heart is a muscle and vital organ in humans and other animals, and pumps blood through the blood vessels of the circulatory system. The heart sits in the chest, in the mediastinum contained 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.[1] 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. The flow of blood is controlled by the heart valves.

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.[2]

The human heart and its disorders (cardiovascular diseases) are studied primarily by cardiologists.

Structure[edit]

The human heart has a mass of between 250 and 350 grams and is about the size of a large fist.[3] The heart has a fibrous skeleton also called the cardiac skeleton, of dense connective tissue. The cardiac skeleton forms and anchors the heart valves by encircling the bases with fibrous rings. It also separates the atria from the ventricles. In doing this it forms the primary channel for the conduction of electrical energy, from the top of the heart to the bottom.

Chambers[edit]

Main article: Heart chamber

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.

Valves[edit]

Main article: Heart valves

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.[4]– 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 (also called the heartstrings), which anchor the valves to the papillary muscles of the heart.

Walls[edit]

Heart walls.

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.[5] 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.[6][7]

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.[8] Also, it merges with the inner lining (endothelium) of blood vessels and covers heart valves.[9]

Location[edit]

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.[10]

  • 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.[10]
  • 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.[10]
  • 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.[10] 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.[10]

Blood supply[edit]

Main article: Coronary circulation

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.

Development[edit]

Main article: Heart development
At 21 days the embryonic human heart begins beating at 70 to 80 BPM

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.[11]

Function[edit]

Blood flow[edit]

Main articles: Cardiac cycle and Heart valve
Blue components indicate de-oxygenated blood-flow pathways and red components indicate oxygenated blood-flow pathways.
Blood flow though valves

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, 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 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.[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.

Electrical conduction[edit]

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

The energy that stimulates the heart occurs in the sinoatrial node, in the right atrium, where an action potential is produced, which is sent across the atria.

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]

Clinical significance[edit]

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:

Sectional anatomy

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.

Lifestyle and heart health[edit]

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]

Society and culture[edit]

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.[23] 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.[24]

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.[25] 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.

Other animals[edit]

Additional images[edit]

See also[edit]

References[edit]

  1. ^ Cecie Starr; Christine Evers; Lisa Starr (2 January 2009). Biology: Today and Tomorrow With Physiology. Cengage Learning. pp. 422–. ISBN 978-0-495-56157-6. Retrieved 7 June 2012. 
  2. ^ Phibbs, Brendan (2007). The human heart: a basic guide to heart disease (2nd ed.). Philadelphia: Lippincott Williams & Wilkins. p. 1. ISBN 9780781767774. 
  3. ^ MacDonald, Matthew (2009). Your Body: The Missing Manual. Sebastopol, CA: Pogue Press. ISBN 0-596-80174-2. 
  4. ^ Marieb, Elaine Nicpon (2003). Human Anatomy & Physiology (6th ed.). Upper Saddle River: Pearson Education. ISBN 080535462X. 
  5. ^ Levine, Joseph M.; Miller, Kenneth S. (2002). Biology. Upper Saddle River, NJ: Pearson Prentice Hall. ISBN 0-13-050730-X. 
  6. ^ "the anatomy and applied anatomy of the mediastinal fascia". Retrieved 2013-02-27. 
  7. ^ "The effect of the pericardium on ventricular systolic function in man". Retrieved 2013-02-27. 
  8. ^ Pratiyogita Darpan (April 2009). Competition Science Vision. Pratiyogita Darpan. pp. 55–. Retrieved 7 June 2012. 
  9. ^ "Heart". MedicaLook. Medicalook.com. Retrieved 2010-05-03. 
  10. ^ a b c d e Drake, Richard L.; Vogl, Wayne; Tibbitts, Adam W.M. Mitchell; illustrations by Richard; Richardson, Paul (2005). Gray's anatomy for students. Philadelphia: Elsevier/Churchill Livingstone. pp. 157–159. ISBN 978-0-8089-2306-0. 
  11. ^ DuBose, Miller , Moutos. "Embryonic Heart Rates Compared in Assisted and Non-Assisted Pregnancies". Obgyn.net. Retrieved 2010-10-18. 
  12. ^ Anderson, RM. The Gross Physiology of the Cardiovascular System (2nd ed., 2012). See "Chapter 1: Normal Physiology."
  13. ^ Antz, Matthias; et al. (1998). "Electrical Conduction Between the Right Atrium and the Left Atrium via the Musculature of the Coronary Sinus". Circulation 98 (17): 1790–1795. doi:10.1161/01.CIR.98.17.1790. 
  14. ^ De Ponti, Roberto; et al. (2002). "Electroanatomic Analysis of Sinus Impulse Propagation in Normal Human Atria". Journal of Cardiovascular Electrophysiology 13 (1): 1–10. doi:10.1046/j.1540-8167.2002.00001.x. 
  15. ^ "SA node definition – Medical Dictionary definitions of popular medical terms easily defined on MedTerms". Medterms.com. 27 April 2011. Retrieved 7 June 2012. 
  16. ^ "Purkinje Fibers". Biology.about.com. 9 April 2012. Retrieved 7 June 2012. 
  17. ^ "Heart attack/coronary artery disease". Mount Sinai Hospital, New York. 
  18. ^ a b c Marieb, Elaine N. (2009). Essentials of Human Anatomy & Physiology 9th Edition. San Francisco, CA: Pearson Benjamin Cummings. ISBN 978-0-321-51353-3. 
  19. ^ "Eating for a healthy heart". MedicineWeb. Retrieved 2009-03-31. 
  20. ^ Division of Vital Statistics; Arialdi M. Miniño, M.P.H., Melonie P. Heron, Ph.D., Sherry L. Murphy, B.S., Kenneth D. Kochanek, M.A. (21 August 2007). "Deaths: Final data for 2001" (PDF). National Vital Statistics Reports (United States: Center for Disease Control) 55 (20): 7. Retrieved 2007-12-30. 
  21. ^ White House News. "American Heart Month, 2007". Retrieved 16 July 2007. 
  22. ^ National Statistics Press Release 25 May 2006
  23. ^ Britannica, Ib; Slider, Ab, Egyptian heart and soul conception. The word was also transcribed by Wallis Budge as Ab.
  24. ^ Aristotle. On the Parts of Animals. book 3, ch. 4 (De partibus animalium) 
  25. ^ Galen, De usu partium corporis humani ("The Use of the Parts of the Human Body"), book 6.

External links[edit]