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human 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 2 oz of blood per beat and 5 qt per minute, compared to 4-7.3 oz per beat and 21-32 qt per minute during exercise. The adult human heart is about the size of a fist and weighs about 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.

EVOLUTION OF THE HEART

The hearts of primitive people apparently had only one atrium and one ventricle. Since their body temperature and metabolic rate fluctuated with the environmental temperature, they did not need as efficient a circulatory system as mammals and birds. The two-chamber heart is retained by modern fish, but oxygen-rich blood does not mix with oxygen-poor blood, because the blood is aerated at the gills and goes directly into systemic circulation, not to the heart. The sides of the heart were separated when a septum formed to divide the ventricle into two chambers. Birds and mammals have completely separate chambers and have more blood per tissue weight and more pressure, because the tissues of birds and mammals (warm-blooded vertebrates) require a constant perfusion of oxygen-rich blood in order to maintain their high metabolic rates and constant body temperature.

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 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 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 ventricles. 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.
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Human Heart. EssayMania.com. Retrieved on 11 Oct, 2010 from