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Human heart

 

Human heart

This article is about the muscular organ. For the psychological concept, see Heart (symbol). For other uses, see Heart (disambiguation).

Human heart
External view
Surface view of the heart
Latin cor humanum
Gray's subject #135 506
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

The human heart is an organ that provides a continuous blood circulation through the cardiac cycle and is a vital organ in the human body.[1] 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.[2] There is a thick wall of muscle separating the right side and the left side of the heart called the 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.[3]

The electric energy that stimulates the heart occurs in the sinoatrial node, which produces a definite potential and then discharges, sending an impulse 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.[4][5][6] While in the ventricles, the signal is carried by specialized tissue called the Purkinje fibers which then transmit the electric charge to the myocardium.[7]

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 the mother’s, 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 after conception (early ninth 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.[8][9][10] The regression formula, which describes this acceleration before the embryo reaches 25 mm in crown-rump length, or 9.2 LMP weeks, is: the age in days = EHR(0.3)+6.[11]

After 9.1 weeks after the LMP, it decelerates to about 152 BPM (±25 BPM) during the 15th week after 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.

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

Structure

The human heart has a mass of between 250 and 350 grams and is about the size of a large fist.[12]

It is enclosed in a double-walled protective sac called the pericardium. The superficial part of this sac is called the parietal pericardium. The inner pericardium layer is called the visceral pericardium. Together they are usually called the serous pericardium because they contain the pericardial fluid. Outside the parietal pericardium there is a fibrous layer which depends from the mediastinal fascia and is called the fibrous pericardium.[13] The pericardium protects the heart, from physical knocks and shocks, and from infection. It anchors its surrounding structures, but has no effect over the heart function in normal individuals.[14]

The double membrane of the pericardium contains the pericardial fluid which provides a smooth lubricated sliding surface within which the heart organ can move in response to its own contractions and to movement of adjacent structures such as the diaphragm and lungs.[15]


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.[16] Also, it merges with the inner lining (endothelium) of blood vessels and covers heart valves.[17]

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 pathways of blood through the human heart are part of the pulmonary and systemic circuits. These pathways include the tricuspid valve, the mitral valve, the aortic valve, and the pulmonary valve.[18] 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 semi-lunar valves separate the left and right ventricle from the pulmonary artery and the aorta respectively. These valves are attached to the chordae tendinae (literally the heartstrings), which anchors the valves to the papilla muscles of the heart.

The interatrioventricular septum separates the left atrium and ventricle from the right atrium and ventricle, dividing the heart into two functionally separate and anatomically distinct units.

Functioning


Blood flows through the heart in one direction, from the atria to the ventricles, and out of the great arteries, or the aorta for example. 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 (see right heart) is to collect de-oxygenated blood, in the right atrium, from the body (via superior and inferior vena cavae) and pump it, via the right ventricle, into the lungs (pulmonary circulation) so that carbon dioxide can be dropped off and oxygen picked up (gas exchange). This happens through the passive process of diffusion.

The left side (see left heart) collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood moves to the left ventricle which pumps it out to the body (via the aorta).

On both sides, 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.[19]

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 inferior vena cava and superior vena cava.

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.[20][21][22][23]

Cultural significance

F34
jb (F34) "heart"
in hieroglyphs


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 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.[24] 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", , 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.[25]

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.[26] 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.

Gallery

See also

References

External links

  • Prenatal human heart development
  • Anatomy of the Human Heart - Texas Heart Institute
  • The Gross Physiology of the Cardiovascular System (2nd Ed., 2012)
  • Dissection review showing vessels of the heart as well as internal and external features
  • Heart Pain / Attack Treatments and Home Remedies
  • The Human Heart: An Online Exploration from The Franklin Institute
  • Blood Flow Through the Human Heart

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