The human heart is a specialized, four-chambered muscle that maintains BLOOD flow in the CIRCULATORY
SYSTEM. Located in the thorax, it lies left of the body's midline, above and in contact with the
diaphragm. It is situated immediately behind the breastbone, or sternum, and between the lungs, with its
apex tilted to the body cavity's left side. In most people the apex can be felt during each heart
contraction. At rest, the heart pumps about 59 cc (2 oz) of blood per beat and 5 l (5 qt) per minute,
compared to 120-220 cc (4-7.3 oz) per beat and 20-30 l (21-32 qt) per minute during exercise. The adult
human heart is about the size of a fist and weighs about 250-350 gm (9 oz).
Blood supplies food and oxygen to the cells of the body for their life needs and removes the waste
products of their chemical processes. It also helps to maintain a consistent body temperature, circulate
hormones, and fight infections. The brain cells are very dependent on a constant supply of oxygen. If the
circulation to the brain is stopped, death ensues shortly. Since heart attacks are the number-one cause
of death in the United States, the heart gets a great deal of attention.
The role of the heart was long considered a mystery and often given elevated importance. Some thought it
was the seat of the soul. Others thought it was the center of love, courage, joy, and sadness. Primitive
man must have been aware of the heartbeat and probably recognized the heart as an organ whose malfunction
could cause sudden death.
The Hippocratic De Corde, which probably dates from the time of ARISTOTLE, describes the construction of
the heart's valves. LEONARDO DA VINCI made exquisite drawings of the heart, but it was not until the
publication of William HARVEY's De Motu Cordis (1628) that the heart's specific role in relation to
circulation was widely understood.
The heart's wall has three parts. Muscle tissue, or myocardium, is the middle layer. The inner layer, or
endocardium, that lines the inside of the heart muscle consists of a thin layer of endothelial tissue
overlying a thin layer of vascularized connective tissue. The outside of the heart, the epicardium, is in
intimate contact with the pericardium; this serous membrane is a closed sac covering the heart muscle's
outside wall. Within the sac, a small amount of fluid reduces the friction between the two layers of
tissue. In addition to muscular and connective tissue, the heart muscle contains varying amounts of fatty
tissue, especially on the outside. Both anatomically and functionally, the heart is divided into a left
and a right half by the cardiac septum. Each half contains two separate spaces: the atrium (pl. atria),
or auricle, and the ventricle. The upper reservoirs, or collecting chambers, are the thin-walled atria,
and the lower pumping chambers are the thick-walled ven!
tricles. The total thickness of the ventricular walls is about three times that of the atria; the wall of
the heart's left half is approximately twice as thick as that of the right half. The thickness of the
heart muscle varies from 2 to about 20 mm (0.1 to 0.8 in). This thickness is correlated with the maximum
pressure that can be attained in each chamber.
The right atrium receives oxygen-poor blood from two major veins: the superior and inferior vena cava,
which enter the atrium through separate openings. From the right atrium the blood passes through the
tricuspid valve, which consists of three flaps, or cusps, of tissue. This valve directs blood flow from
the right atrium to the right ventricle. The tricuspid valve remains open during diastole, or ventricular
filling; however, when the ventricle contracts, the valve closes, sealing the opening and preventing
backflow into the right atrium. Five cords attached to small muscles (papillary muscles) on the
ventricles' inner surface prevent the valves' flaps from being pushed backward. From the right ventricle
blood is pumped through the pulmonary, or semilunar, valve, which has three half-moon-shaped flaps, into
the pulmonary artery. This valve prevents backflow from the artery into the right ventricle. From the
pulmonary artery, blood is pumped to the lungs, where it gives up ca!
rbon dioxide and receives oxygen, and then is returned to the heart's left side through four pulmonary
veins (two from each lung) to the left atrium and then through the mitral valve, a two-flapped valve also
called a bicuspid valve, to the left ventricle. As the ventricles contract, the