Human Heart

The human heart is a fist-sized muscular organ that pumps blood throughout the body. It sits at the center of the circulatory system, a network of arteries, veins and capillaries that carries blood to and from every area of the body. Each beat is a muscular contraction that pushes blood to the lungs and to the rest of the body.

The heart occupies the mediastinum, the central region of the thoracic cavity, where it sits between the lungs and behind the sternum. Roughly two-thirds of the organ lies to the left of the midline.

In form the heart resembles a pinecone: broad across its upper surface and narrowing toward the tip. Five surfaces are distinguished — a posterior base, an inferior diaphragmatic surface, an anterior sternocostal surface, and left and right pulmonary surfaces — together with right, left, superior and inferior margins. The superior and inferior venae cavae, the aorta and the pulmonary trunk all attach to the upper surface, the region known as the base. Grooves on the external surface carry branches of the right and left coronary arteries, the vessels that deliver blood to the heart muscle.

The measurements below collect the principal dimensions and mass of the adult heart^{3Anatomy and Physiology 2e, 19.1 Heart Anatomy · OpenStax, Rice University ↗}.

The heart is built around four chambers: two upper atria that receive blood and two lower ventricles that pump it out. The atria push blood into the ventricles, which then send it onward to the lungs or to the body. Together the two sides form a right and a left pump, serving the pulmonary and the systemic circuits.

A septum divides the right side of the heart from the left, separating the chambers into the interatrial and interventricular septa. Blood passes between chambers through the atrioventricular orifices. The left ventricle is the largest and strongest chamber, its wall much thicker than that of the right, although both sides pump the same volume.

Four valves regulate the flow of blood through the heart, and each of them is placed so that blood can move in only one direction. The tricuspid valve sits between the right atrium and the right ventricle, while the mitral valve occupies the same position on the left, between the left atrium and the left ventricle. Blood leaving the ventricles passes through two further valves: the pulmonary valve, which carries blood from the right ventricle to the lungs, and the aortic valve, which opens from the left ventricle into the aorta. The atrioventricular valves — those guarding the entrances to the ventricles — prevent blood from flowing back from the ventricles into the atria.

The two atrioventricular valves are not identical in construction. The tricuspid valve on the right side has three cusps, whereas the left valve is known as the mitral, or bicuspid, valve because it has only two. This arrangement gives each ventricle an inlet valve from its atrium and an outlet valve to its artery, so that the course of blood through the heart is held to a single direction.

The heart wall is built from three layers: a thin inner endocardium, a thick muscular myocardium, and an outer pericardium sac. The pericardium is a serous sac of two layers, and its strong fibrous outer portion is attached to the diaphragm, the mediastinal pleura and the sternum. A small amount of fluid between the layers of the pericardium lowers friction as the beating heart moves against nearby tissues.

Cardiac muscle is built from contractile cells, which form the bulk of atrial and ventricular tissue, and conducting cells, which make up the conduction system. Neighbouring cardiac muscle cells bind together at intercalated discs, structures built from desmosomes and many gap junctions that let ions move between the cells and support a synchronized contraction. The muscle works aerobically and keeps myoglobin, lipids and glycogen stored in its cytoplasm. Its cells contract in single twitches with long refractory periods and brief periods of relaxation that let the heart refill.

The heart muscle is nourished by its own vascular supply, the coronary arteries, which arise from the first part of the aorta just above the aortic semilunar valve and divide into the left and right coronary arteries. Grooves visible on the heart's external surface carry branches of these right and left vessels, conducting blood to the muscle they serve. Of the two main channels, the left main artery carries 80% of the flow to the heart muscle.

The adult heart can generate its own electrical impulse, its rate set by whichever of its cells fires fastest, and the signal spreads rapidly from cell to cell. The components of this conduction system are the sinoatrial node, the atrioventricular node, the atrioventricular bundle, its branches, and the Purkinje cells. The nervous and endocrine systems modulate that intrinsic rate rather than originating it. Each heartbeat begins with pacemaker cells in the sinoatrial node of the right atrium — the heart's main pacemaker, which sits at the junction of the superior vena cava and the right atrium and discharges about 70 times a minute; on reaching the atrioventricular node the signal slows briefly while the ventricles fill, then travels from the top of the heart to the bottom to make it contract.

Ejection fraction expresses the share of the end diastolic volume that the heart pumps out with each contraction; it is found by dividing stroke volume by the end diastolic volume and is normally reported as a percentage.^{11Anatomy and Physiology 2e, 19.4 Cardiac Physiology · OpenStax, Rice University ↗}

Cardiac output measures how much blood one ventricle pumps in a minute, and it is obtained by multiplying the stroke volume by the heart rate.^{11Anatomy and Physiology 2e, 19.4 Cardiac Physiology · OpenStax, Rice University ↗}

In a healthy adult the stroke volume usually falls between about 55 and 100 millilitres and the resting heart rate is around 75 beats per minute, which gives a cardiac output near 5 litres per minute.^{11Anatomy and Physiology 2e, 19.4 Cardiac Physiology · OpenStax, Rice University ↗}

Stroke volume is governed mainly by three factors: the preload, or stretch on the ventricles before they contract; the contractility, or force of the contraction itself; and the afterload, or pressure the ventricles must overcome. Under the Frank-Starling mechanism, stretching the ventricular muscle within its physiological limits makes it contract more forcefully, so a larger filling volume produces a larger stroke volume.

The behavioural risk factors that matter most for heart disease and stroke are an unhealthy diet, physical inactivity, tobacco use and the harmful use of alcohol, while air pollution stands out as an important environmental factor. Among the everyday habits that raise a person's risk are diets high in saturated fat, refined carbohydrates and salt, along with physical inactivity, heavy drinking, smoking and stress. Cutting the burden of cardiovascular disease depends on embedding management interventions in universal health coverage packages, yet many countries still require significant investment in their health systems.

An estimated 19.8 million deaths from cardiovascular diseases occurred worldwide in 2022 — approximately 32 percent of all deaths.

In weightlessness, blood and other bodily fluids that gravity normally pulls downward shift toward the head, so the cardiovascular system works less to keep blood flowing to the brain. Adaptation to weightlessness can reduce blood volume and the function of the heart and blood vessels. Back on Earth, astronauts may feel weak, dizzy, or faint when standing.

The Engineered Heart Tissues study used 3D heart tissue grown from stem cells to examine cardiac function in microgravity. A magnetic sensor beneath the culture chamber tracked the tissue in real time.

• 1628
  De Motu Cordis published
  William Harvey's work on the circulation of blood was dedicated to the king, following his recognition that blood circulates fully through the body, supported by experiments and arguments and aided by access to the royal herd of deer and the presentation of medical cases to the king.
• 1893
  First open heart surgery
  Daniel Hale Williams performed the world's first open heart surgery at Provident Hospital, the Chicago institution he founded as the first black owned and operated hospital in the United States, and became chief surgeon at Freedman's Hospital in Washington, D.C. the same year. ^{16Daniel Hale Williams, M.D. (Opening Doors: Contemporary African American Academic Surgeons) · U.S. National Library of Medicine ↗}
• 1924
  Nobel Prize for the electrocardiogram
  The Nobel Prize in Physiology or Medicine was awarded solely to Willem Einthoven for discovering the mechanism of the electrocardiogram.
• 1956
  Nobel Prize for heart catheterization
  The Nobel Prize in Physiology or Medicine was divided equally between Andre Frederic Cournand, Werner Forssmann and Dickinson Richards for discoveries concerning heart catheterization and pathological changes in the circulatory system. ^{17The Nobel Prize in Physiology or Medicine 1956: Summary · Nobel Prize Outreach ↗}
• December 1967
  First human heart transplant
  Christiaan Barnard led twenty surgeons in replacing the heart of the incurably ill South African grocer Louis Washkansky with a heart from a fatally injured accident victim; the transplant succeeded, though Washkansky died eighteen days later from double pneumonia after anti-rejection drugs destroyed his immunity. ^{18Christiaan Barnard · Encyclopaedia Britannica ↗}

The English physician William Harvey was the first to recognize that blood circulates fully through the human body, and he backed the idea with experiments and arguments^{19William Harvey · Encyclopaedia Britannica ↗}.

Daniel Hale Williams, who lived from 1856 to 1931, was a leading surgeon and educator and the founder of Provident Hospital and Training School in Chicago, the first black owned and operated hospital in the United States^{16Daniel Hale Williams, M.D. (Opening Doors: Contemporary African American Academic Surgeons) · U.S. National Library of Medicine ↗}.

Einthoven was the sole laureate of the 1924 Nobel Prize in Physiology or Medicine, and the citation credits his discovery of the mechanism of the electrocardiogram^{20The Nobel Prize in Physiology or Medicine 1924: Summary · Nobel Prize Outreach ↗}.