CARDIOVASCULAR SYSTEM

in humans, organ system that conveys blood through vessels to and from all parts of the body, carrying nutrients and oxygen to tissues and removing carbon dioxide and other wastes. It is a closed tubular system in which the blood is propelled by a muscular heart. Two circuits, the pulmonary and the systemic, consist of arterial, capillary, and venous components. The primary function of the heart is to serve as a muscular pump propelling blood into and through vessels to and from all parts of the body. The arteries, which receive this blood at high pressure and velocity and conduct it throughout the body, are thickly walled with elastic fibrous tissue and muscle cells. The arterial treethe branching system of arteriesterminates in short, narrow, muscular vessels called arterioles, from which blood enters simple endothelial tubes (i.e., tubes formed of endothelial, or lining, cells) known as capillaries. These microscopically thin capillaries are permeable to vital cellular nutrients and waste products and distribute and receive nutrients and wastes. From the capillaries, the blood, now depleted of oxygen and burdened with waste products, moving more slowly and under low pressure, enters small vessels called venules, which converge to form veins, ultimately guiding the blood on its way back to the heart. organ system that conveys blood to and from all parts of the body, carrying nutrients and oxygen to the tissues and removing wastes and carbon dioxide. The cardiovascular system is essentially a closed tube through which blood moves in a double loop. Its chief structures are the heart, a muscular pump that propels blood around the circuit; the arteries, thick-walled vessels that carry blood outward from the heart; veins, thinner-walled vessels that return blood to the heart; and the capillaries, minute blood passages connecting the arteries and veins, in which nutrients and wastes are exchanged between the blood and tissues. The cardiovascular system is necessary only to animals of some size and complexity because in small animals physical diffusion is adequate to transport nutrients to the body. Hence, though the circulatory system is found in the larger and higher invertebrates, it reaches a complex development only in the vertebrates. It is filled with blood, which travels in a circuit passing through respiratory organs for the exchange of gases and close to the excretory organs for the emission of wastes. The adult human heart is suspended in the chest cavity within a tough, membranous sac, the pericardium, which contains a fluid that cushions the beating organ. The heart muscle (myocardium) is made up of unique muscle fibres that spontaneously contract to squeeze the blood out of the four main cavities, or chambers, of the heart. Blood first enters the right atrium, a relatively thin-walled muscular chamber that accepts blood returning to the heart from most of the body. It then passes into the right ventricle, a more muscular cavity that pumps blood to the lungs to be oxygenated. On returning from the lungs, the blood enters the left atrium and from there flows into the left ventricle, the largest of the heart's four chambers and the chief pump forcing blood to distant tissues. The atria and ventricles are separated by tissue valves that prevent backflow of blood during ventricular contractions; similar valves at the exits from the ventricles prevent blood from surging back into the chamber when it is relaxed. The heart contracts in two steps. First, the two atria distend to accept the inflow of blood from the veins, then contract to force the blood into the ventricles. A second, stronger contraction by the ventricles closes the atrioventricular valves and squeezes the blood out through the arteries. All heart muscle contracts without any outside stimulation, but an electroconductive tissue (the Purkinje fibres) buried in the myocardium triggers rhythmic beats to provide maximum contractile efficiency. From the left ventricle, blood enters the aorta, the largest artery in the circulatory system. Two coronary arteries branch off almost immediately to supply blood to the myocardium; farther along, the aorta gives off the innominate, left carotid, and left subclavian arteries, which carry blood to the head, neck, and arms. The aorta then turns downward, sending arterial branches to the chest and abdominal organs as it descends, finally dividing into the iliac arteries supplying the legs. As arteries branch off, the diameter of the new vessels becomes progressively smaller. The small arteries, called arterioles, feed into the microscopic capillary beds, where the vessel walls are but a few microns thick. The thin capillary walls act as a semipermeable membrane, allowing small molecules such as oxygen, water, carbon dioxide, and glucose to pass in both directions. Nutrients diffuse outward through this membrane while tissue waste products diffuse in, to be carried to the kidneys, liver, and lungs for removal from the body. Capillaries converge to form small venules, which eventually join to become larger veins, generally following the same path as the arteries. Because most of the pressure of the heart contractions has been attenuated by the time the blood passes through the capillaries, fluid pressure in the veins is fairly low, so the vessels contain a number of valves that help control the direction of flow. Blood from the venous network eventually reaches the heart through the superior and inferior venae cavae, from the upper and lower body respectively, which empty into the right atrium. That blood is expelled through the right ventricle into the short pulmonary artery leading to the lungs. The capillaries of the lungs are organized around the air pockets or alveoli at the end of the airway passages, so that carbon dioxide is given up by the blood into the air and oxygen is taken up. The oxygenated blood then returns to the left atrium through the pulmonary vein. Diseases of the heart and blood vessels are a major source of human mortality and morbidity. The single largest cause of death in developed countries is atherosclerotic heart disease, in which the buildup of fatty deposits in the coronary arteries closes off the blood supply to part of the myocardium, destroying vital heart tissue. Atherosclerosis also contributes to strokes by narrowing the arteries supplying the brain, thus making them susceptible to blockage by spasm or blood clots. Other major diseases of the cardiovascular system include congenital heart diseases, rheumatic heart disease, and vascular inflammation. Additional reading General accounts and elementary descriptions of circulatory systems are found in many biology textbooks, including the following: Raymond F. Oram, Biology: Living Systems, 5th ed. (1986); Karen Arms and Pamela S. Camp, Biology, 3rd ed. (1986); and Paul B. Weisz and Richard N. Keogh, The Science of Biology, 5th ed. (1982). Textbooks dealing with animal structure at a more advanced level include the following: Ralph M. Buchsbaum, Animals Without Backbones, 3rd ed. (1987); Robert D. Barnes, Invertebrate Zoology, 5th ed. (1987); Alfred Sherwood Romer and Thomas S. Parsons, The Vertebrate Body, 6th ed. (1986); and Charles K. Weichert, Anatomy of the Chordates, 4th ed. (1970); Knut Schmidt-Nielsen, Animal Physiology: Adaptation and Environment, 3rd ed. (1983); and Milton Hildebrand, Analysis of Vertebrate Structure, 2nd ed. (1982).For the history of circulation studies, see Helen Rapson, The Circulation of Blood (1982); David J. Furley and J.S. Wilkie (eds.), Galen on Respiration and the Arteries (1984); The Selected Writings of William Gilbert, Galileo Galilei, William Harvey (1952), in The Great Books of the Western World series; Fredrick A. Willius and Thomas J. Dry, A History of the Heart and the Circulation (1948); and Alfred P. Fishman and Dickinson W. Richards, Circulation of the Blood: Men and Ideas (1964, reprinted 1982). Special studies of circulation include Donald A. McDonald, Blood Flow in Arteries, 2nd ed. (1974); David I. Abramson and Philip B. Dobrin (eds.), Blood Vessels and Lymphatics in Organ Systems (1984); Colin L. Schwartz, Nicholas T. Werthessen, and Stewart Wolf, Structure and Function of the Circulation, 3 vol. (198081); and Jerry Franklin Green, Fundamental Cardiovascular and Pulmonary Physiology, 2nd ed. (1987). Stanley W. Jacob, Clarice Ashworth Francone, and Walter J. Lossow, Structure and Function in Man, 5th ed. (1982); and Gary A. Thibodeau, Anatomy and Physiology (1987), are basic texts. Arthur C. Guyton, Human Physiology and Mechanisms of Disease, 4th ed. (1987), is a technical description of the physiology of cardiac muscle, heart function, and hemodynamics. Also see Peter F. Cohn, Clinical Cardiovascular Physiology (1985); James J. Smith and John P. Kampine, Circulatory Physiology: The Essentials (1984); Harvey V. Sparks, Jr., and Thom W. Rooke, Essentials of Cardiovascular Physiology (1987); and Peter Harris and Donald Heath, The Human Pulmonary Circulation, 3rd ed. (1986). Michael Francis Oliver