The cardiovascular system is a collection of organs, tissues, and cells with three main components: the heart, blood vessels, and blood. Its function is to deliver oxygen and nutrients to cells and to remove carbon dioxide and waste products from cells.
The active component of the cardiovascular system is the heart, a fist-sized muscular organ in humans weighing about 250–300 grams (9–11 oz.). All animals with a circulatory system have a heart as part of that system. The human heart consists of four chambers, the left and right atrium and the left and right ventricle. Blood that is low in oxygen passes from the veins into the heart through two main portals, the superior and inferior vena cava, which empty into the right atrium. Blood then flows out of the right atrium into the right ventricle, and then into the pulmonary artery. The pulmonary artery carries deoxygenated blood from the heart into the lungs, where it absorbs oxygen from that organ. Oxygenated blood then leaves the lungs through pulmonary veins, which empty into the left atrium. That blood then flows out of the left atrium into the left ventricle, and then into the aorta. The aorta is the largest artery in the body, through which blood flows out of the heart into the body.
The heart is an amazing organ that beats on average at a rate of 72 times per minute, or more than 100,000 times a day. In all, the heart moves more than 2,000 gallons of blood through a 60,000 mile network of blood vessels in a single day. During the average lifespan of a human, the heart will have beat more than 2.5 billion times to keep blood moving through the circulatory system.
When a red blood cell—lacking its oxygen molecules—reaches the end of a capillary, it passes into another type of blood vessel, a venule. A venule is a smaller version of the next portion of the circulatory system, a vein. A vein looks similar to an artery except that it is thinner because it does not have to withstand the force produced by heart contractions pushing blood through the vessels. Veins differ from arteries also in that they contain one-way valves that prevent blood from backing up and flowing away from the heart, rather than towards it. Arteries do not have valves as do veins because the force of heart contractions is sufficient to keep blood flowing in the right direction. Blood continues to flow through veins towards the heart until they reach the superior and inferior vena cava, at which point, the pattern of blood flow repeats.
The third component of the cardiovascular system is blood, the life-giving fluid that is pumped by the heart through the blood vessels. Blood consists of two parts, the first of which is a watery solution called plasma that contains about 91% water, in which are dissolved a number of substances, such as proteins and amino acids, sugars, lipids, hormones, and electrolytes. Plasma makes up about 55% of the total volume of blood. The remaining portion of blood consists of white blood cells (leukocytes), red blood cells (erythrocytes), and platelets (cell fragments important for blood clotting).
An animal's body stays alive, carries out a host of essential functions, grows, and reproduces by means of a complex set of chemical reactions collectively known as metabolic reactions, or metabolism. Metabolic reactions are of two general types: catabolic and anabolic. Catabolic reactions are those in which organic compounds such as glucose and amino acids are broken down to produce the energy needed to carry out all essential body functions. Anabolic reactions are those in which cells use simple raw materials such as simple sugars and amino acids to make the complex structures of which cells, hormones, enzymes, and other essential body constituents are composed. Both catabolic and anabolic reactions result in the formation of waste products such as carbon dioxide, urea, and water that must be removed from the body.
The role of the cardiovascular system is to provide animals such as humans with a mechanism for supplying cells with the raw materials they need to carry out essential metabolic functions (raw materials such as oxygen, simple sugars, and amino acids) and to remove from cells the waste products produced during metabolic reactions.
The cardiovascular system is subject to a very large number and variety of diseases and disorders. Some of these conditions are caused by genetic factors, others by infectious diseases, others by environmental factors, and still others by the process of aging. One example of a genetic disorder of the cardiovascular system is hemophilia, a condition that develops because a person's body lacks one of a number of substances needed in the process of blood clotting. Therefore their blood does not coagulate when a cut occurs in the skin. If not quickly treated, a person with this condition may bleed to death. Other genetic conditions may develop when the heart fails to form properly prior to birth. Such abnormalities may cause blood to flow in the wrong direction or in inadequate amounts, preventing the heart from functioning properly.
Infectious agents, such as viruses, bacteria, and fungi, can cause damage to the cardiovascular system in many different ways. In the condition known as infectious endocarditis, for example, infectious agents enter the interior of the heart and attack heart valves or the interior lining of the heart (the endocardium). These agents may cause scarring of heart tissue or may produce holes in heart tissue that can lead to heart disease or death. Infectious agents may cause damage in other ways also, such as initiating the formation of blood clots or plaques on blood vessel walls that can lead to heart attack or stroke.
Environmental factors play a critical role in the development of cardiovascular disorders that make such disorders the leading cause of death in most developed nations in the world. For example, individuals whose diets are high in some forms of cholesterol and other fatty materials are at risk for the formation of plaques on blood vessel walls. Plaques are accumulations of fatty materials that, over time, gradually narrow the opening of vessels, inhibiting the flow of blood. When plaques grow large enough to close off a blood vessel, or when they break lose to form a clot in the bloodstream, a heart attack or stroke may result, which can be seriously debilitating or even fatal.
Aging has many effects on the human body, including the cardiovascular system. As one example, the heart's normal function is controlled by a group of tissues known as the sinoatrial (SA) node, which sets the pace for the heart's beating and is called, therefore, the heart's pacemaker. As one ages, the SA node may lose some of its cells or have its function inhibited by the development of fatty deposits. When this change occurs, the heart may begin to beat too rapidly or slowly, resulting in a variety of unpleasant and often dangerous symptoms. Individuals in whom this condition is well advanced may require the surgical installation of an artificial pacemaker, an electronic device that takes over the heart's contractions and maintains them at the proper level.
A list of diseases and disorders of the cardiovascular system is very long but contains conditions such as the following:
See also Cardiorespiratory fitness tests ; Skeletal system .
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National Heart Blood and Lung Institute, PO Box 30105, Bethesda, MD, 20824-0105, (301) 592-8573, nhlbiinfo@ nhlbi.nih.gov, http://www.nhlbi.nih.gov .
David E. Newton, AB, MA, EdD