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Word Count: 603
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 bodys 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 cavitys 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 4-73 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
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