Реферат: Heart Essay Research Paper HEARTThe human heart

Heart Essay, Research Paper

HEART

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.

STRUCTURE AND FUNCTION OF THE HUMAN HEART

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

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