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By Mary Klouda
A human heart consists of four chambers. There are two receiving chambers — the right atrium and the left atrium. And there are two pumping chambers — the right ventricle and the left ventricle. Blood enters the heart by way of the two atria (plural of atrium) and is pumped out by the two ventricles. The walls of these chambers are composed of a special type of muscle called cardiac muscle. The ventricles contract approximately once every second in order to pump blood through the blood vessels of the body. That amounts to approximately 32 million contractions in just one year.
In order to produce the energy needed for these contractions, the cardiac muscle uses large quantities of oxygen and nutrients. These nutrients and oxygen must be delivered to the cardiac muscle by a special system of blood vessels. At first glance it might seem that, since the chambers of the heart are full of blood, the cardiac muscle can get all the oxygen and nutrients it needs from the blood inside the chambers. However the distance the nutrients and oxygen can travel is microscopic. And the walls of the chambers, especially the ventricles, are extremely thick. Therefore the walls of the chambers contain a vast network of blood vessels whose purpose is to deliver oxygen and nutrients to the cardiac muscle. These blood vessels are called coronary vessels.
The aorta is the large blood vessel which leaves the left ventricle to carry blood to all parts of the body. Vessels branching off the aorta are called arteries. At the very beginning of the aorta, the right coronary artery and the left coronary artery branch off the aorta and travel over the outside of the heart. Both of these arteries branch many times to form a network of blood vessels which bring blood to every minute part of the cardiac muscle. That blood is rich in nutrients and oxygen, which the cardiac muscle uses to produce the energy needed to power the contractions of the heart.
If the cardiac muscle does not receive sufficient blood supply, the result could be a heart attack. The usual cause of a heart attack is a "coronary occlusion". The word, coronary, refers to the coronary blood vessels. The word, occlusion, means that the size of the opening inside the coronary vessel has been significantly diminished, thus causing the vessel to carry less blood. The most common cause of a coronary occlusion is the build-up of a material called plaque inside the coronary vessel.
The condition in which plaque forms inside arteries is called atherosclerosis. The atherosclerotic plaque is composed of fat deposits, cholesterol, calcium, and fibers. Depending on how much plaque is present, the coronary vessel may have a slightly narrower opening or it could be effectively closed. When the opening in the artery becomes narrower, a certain area of cardiac muscle does not receive sufficient oxygen and nutrients. This causes a painful sensation called "angina pectoris" or simply angina. This pain is the sign of a heart attack.
If there is severe narrowing or even closure of the coronary artery, the area of cardiac muscle being served by that artery dies. The area of dead cardiac muscle is called a "myocardial infarction" or simply an infarct. The consequences of a myocardial infarction depend upon how much cardiac muscle is involved. If the occlusion occurs in a very small coronary artery, only a small area of muscle will be affected. In this case, the rest of the cardiac muscle may be able to compensate, allowing the heart to continue pumping blood. However if a large coronary artery is occluded. a large area of muscle will be affected and the heart will be unable to function.
While the presence of atherosclerotic plaque is the most common cause of coronary occlusion, there are other less common causes. A blood clot which formed somewhere else in the body could be carried into a coronary artery and get stuck, thus occluding the vessel. Or a spasm in the wall of a coronary artery could cause the opening inside the vessel to become narrower.
Mary Ann Klouda Ph.D. has had 40 years of experience teaching Anatomy and Physiology at numerous colleges and universities. She is the author of "Understanding the Amazing Human Body" an easy-to-understand guide to the workings of the human body, available from Wheatmark Publisher (http://www.wheatmark.com) or from any online bookseller.
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This page created 10 Feb 2011 14:55
Last updated 16 Feb 2011 02:11
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