CIRCULATION

the process by which nutrients, respiratory gases, and metabolic products are transported throughout a living organism, permitting integration among the various tissues. The process includes the intake of metabolic materials, the conveyance of these materials throughout the organism, and the return of harmful by-products to the environment. Invertebrate animals have a great variety of liquids, cells, and modes of circulation, though many invertebrates have what is called an open system, in which fluid passes more or less freely throughout the tissues or defined areas of tissue. All vertebrates, however, have a closed systemthat is, their circulatory system transmits fluid through an intricate network of vessels. This system contains two fluids, blood and lymph, and functions by means of two interacting modes of circulation, the cardiovascular system and the lymphatic system; both the fluid components and the vessels through which they flow reach their greatest elaboration and specialization in the mammalian systems and, particularly, in the human body. A full treatment of human blood and its various components can be found in the article human blood. A discussion of how the systems of circulation, respiration, and metabolism work together within an animal organism is found in the article respiration. The Editors of the Encyclopdia Britannica the process by which nutrients, respiratory gases, and metabolic products are transported throughout a living organism. In unicellular and small multicellular organisms, simple diffusion across the cell membranes often suffices to carry oxygen and carbon dioxide, and a streaming movement of the cell substance (cytoplasm), called cyclosis, serves to conduct larger molecules like sugars, fats, and proteins to different parts of the cell. In more complex organisms, however, specialized systems have developed to provide adequate circulation to all tissues. Sponges and cnidarians (e.g., jellyfish and hydras) circulate water from the surrounding medium through internal spaces in their bodies. Cells in contact with the water take up food and oxygen directly from it; these materials then pass to other cells largely by diffusion. In the flatworm, which has no organized circulatory structure, a fluid carrying nutrients bathes the animal's tissues while moving passively through the spaces between its component cells. Oxygen is taken up directly from the environment at the animal's surface. Some invertebrates, such as arthropods and most mollusks, have open circulations, in which the heart pumps blood into a rudimentary network of vessels that open into one or more interior body cavities (sinuses), where the blood washes all of the animal's tissues. Blood is then channeled back to the heart through a series of collecting vessels or sinuses. The heart in some invertebrates may be little more than a muscular swelling along the blood vessels, and there may even be additional swellings, or hearts, at the entry to long or broad appendages, such as insect wings or antennae. Invertebrates with a true circulatory system have a closed circulation, as do all vertebrates. In a closed system, blood remains within a network of vessels. Oxygenated blood is carried away from the heart through arteries, and deoxygenated blood is returned to the heart by way of veins. In most vertebrates, the large arteries that exit the heart branch into progressively smaller channels, ultimately reaching a network of microscopic vessels called the capillaries, across the walls of which the exchange of nutrients and waste takes place. The capillaries then coalesce into veins. All vertebrate embryos display the same basic circulatory system. In this embryonic system, venous blood returns to the central heart through a series of veins emptying into the sinus venosus, the first of four embryonic heart chambers. Blood then passes in turn into the atrium, the ventricle, and the bulbus cordis, from which it exits into the arterial system through a large artery, the aorta. The ventricle is the chief muscular pump forcing blood throughout the body. The arteries may be distinguished from veins by the presence of smooth muscle and elastic fibres that help to absorb the surges of blood from the heart. Among adult vertebrates, the basic circulatory model is progressively modified to meet respiratory requirements. Fish exchange respiratory gases through their gills and have retained, for the most part, the embryonic pattern of the circulatory system. In the fishes the four embryonic heart chambers remain intact. The whole circulatory system of fishes is a one-way arrangement. The heart pumps deoxygenated blood to the gills, where it is oxygenated and distributed to the body. Except for sharks, most fishes have developed a swim bladder (the structure from which lungs developed), which controls buoyancy and often acts as an oxygen reserve. Adult amphibians have a dual circulation. The amphibian heart is a tripartite structure in which the atrium is partitioned by a wall, but the ventricle is not physically divided and some mixing of the pulmonary and systemic circulation takes place. But because most adult amphibians exchange oxygen and carbon dioxide through their moist skin as well as through their lungs, respiratory exchange takes place in both the pulmonary and systemic circulation. As a result, there is less need to avoid mixing of the blood in the heart. Reptiles rely exclusively on lungs for respiratory exchange and therefore have a more complete separation of the pulmonary and systemic circulation than is found in amphibians. In the reptile heart the sinus venosus is comparatively small, the atrium is completely divided, and a muscular wall extends across part of the ventricle. This partial wall tends to keep blood from the left atrium (oxygenated) from mixing with that from the right atrium (deoxygenated). The conus arteriosus has disappeared, and in its place three vessels lead directly from the ventriclethe pulmonary artery and the left and right systemic trunks. Temporary closure of a muscular flap directs at least a portion of the deoxygenated blood into the cavum pulmonale (part of the ventricle) and from there into the pulmonary artery leading to the lung. In both birds and mammals there is a complete separation of both ventricle and atrium into a two-sided heart. The conus arteriosus is absent, and only two vessels leave the heart: the aorta from the left ventricle, supplying fresh blood to the rest of the body, and the pulmonary artery from the right ventricle, carrying spent blood to the lungs for oxygenation. The sinus venosus is also eliminated, and blood returns directly to the atria. In mammals, venous blood from the rest of the body empties into the right atrium through the superior and inferior venae cavae, and freshly oxygenated blood enters the left atrium through four pulmonary veins. 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). Michael Francis Oliver Invertebrate circulatory systems Basic physicochemical considerations To maintain optimum metabolism, all living cells require a suitable environment, which must be maintained within relatively narrow limits. An appropriate gas phase (i.e., suitable levels of oxygen and other gases), an adequate and suitable nutrient supply, and a means of disposal of unwanted products are all essential. Direct diffusion through the body surface supplies the necessary gases and nutrients for small organisms, but even some single-celled protozoa have a rudimentary circulatory system. Cyclosis in many ciliates carries food vacuoleswhich form at the forward end of the gullet (cytopharynx)on a more or less fixed route around the cell, while digestion occurs to a fixed point of discharge. For most animal cells, the supply of oxygen is largely independent of the animal and therefore is a limiting factor in its metabolism and ultimately in its structure and distribution. The nutrient supply to the tissues, however, is controlled by the animal itself, and, because both major catabolic end products of metabolismammonia (NH3) and carbon dioxide (CO2)are more soluble than oxygen (O2) in water and the aqueous phase of the body fluids, they tend not to limit metabolic rates. The diffusion rate of CO2 is less than that of O2, but its solubility is 30 times that of oxygen. This means that the amount of CO2 diffusing is 26 times as high as for oxygen at the same temperature and pressure. The oxygen available to a cell depends on the concentration of oxygen in the external environment and the efficiency with which it is transported to the tissues. Dry air at atmospheric pressure contains about 21 percent oxygen, the percentage of which decreases with increasing altitude. Well-aerated water has the same percentage of oxygen as the surrounding air; however, the amount of dissolved oxygen is governed by temperature and the presence of other solutes. For example, seawater contains 20 percent less oxygen than fresh water under the same conditions. The rate of diffusion depends on the shape and size of the diffusing molecule, the medium through which it diffuses, the concentration gradient, and the temperature. These physicochemical constraints imposed by gaseous diffusion have a relationship with animal respiration. Investigations have suggested that a spherical organism larger than 0.5 millimetre (0.02 inch) radius would not obtain enough oxygen for the given metabolic rate, and so a supplementary transport mechanism would be required. Many invertebrates are small, with direct diffusion distances of less than 0.5 millimetre. Considerably larger species, however, still survive without an internal circulatory system. Animals without independent vascular systems A sphere represents the smallest possible ratio of surface area to volume; modifications in architecture, reduction of metabolic rate, or both may be exploited to allow size increase. Sponges overcome the problem of oxygen supply and increase the chance of food capture by passing water through their many pores using ciliary action. The level of organization of sponges is that of a coordinated aggregation of largely independent cells with poorly defined tissues and no organ systems. The whole animal has a relatively massive surface area for gaseous exchange, and all cells are in direct contact with the passing water current. Among the eumetazoan (multicellular) animals the cnidarians (sea anemones, corals, and jellyfish) are diploblastic, the inner endoderm and outer ectoderm being separated by an acellular mesoglea. Sea anemones and corals may also grow to considerable size and exhibit complex external structure that, again, has the effect of increasing surface area. Their fundamentally simple structurewith a gastrovascular cavity continuous with the external environmental waterallows both the endodermal and ectodermal cells of the body wall access to aerated water, permitting direct diffusion. This arrangement is found in a number of other invertebrates, such as Ctenophora (comb jellies), and is exploited further by jellyfish, which also show a rudimentary internal circulatory system. The thick, largely acellular, gelatinous bell of a large jellyfish may attain a diameter of 40 centimetres (16 inches) or more. The gastrovascular cavity is modified to form a series of water-filled canals that ramify through the bell and extend from the central gastric pouches to a circular canal that follows the periphery of the umbrella. Ciliary activity within the canals slowly passes food particles and water, taken in through the mouth, from the gastric pouches (where digestion is initiated) to other parts of the body. Ciliary activity is a relatively inefficient means of translocating fluids, and it may take up to half an hour to complete a circulatory cycle through even a small species. To compensate for the inefficiency of the circulation, the metabolic rate of the jellyfish is low, and organic matter makes up only a small proportion of the total body constituents. The central mass of the umbrella may be a considerable distance from either the exumbrella surface or the canal system, and, while it contains some wandering amoeboid cells, its largely acellular nature means that its metabolic requirements are small. The vertebrate circulatory system The basic vertebrate pattern The plan All vertebrates have circulatory systems based on a common plan, and so vertebrate systems show much less variety than do those of invertebrates. Although it is impossible to trace the evolution of the circulatory system by using fossils (because blood vessels do not fossilize as do bones and teeth), it is possible to theorize on its evolution by studying different groups of vertebrates and their developing embryos. Many of the variations from the common plan are related to the different requirements of living in water and on land. The heart The vertebrate heart lies below the alimentary canal in the front and centre of the chest, housed in its own section of the body cavity. During the development of an embryo, the heart first appears below the pharynx, and although it may also be in this position in adult animals, the heart often moves posteriorly as the animal grows and matures. The heart is basically a tube made of special muscle (cardiac muscle) that is not found anywhere else in the body. This cardiac muscle beats throughout life with its own automatic rhythm. Deoxygenated blood from the body is brought by veins into the most posterior part of the heart tube, the sinus venosus. From there it passes forward into the atrium, the ventricle, and the conus arteriosus (called the bulbus cordis in embryos), and eventually to the arterial system. The blood is pushed through the heart because the various parts of the tube contract in sequence. As the heart develops from embryo to adult, each part of the tube becomes a chamber, separated from the others by valves, so that blood can neither flow backward in the system nor reenter the heart from the arteries. As the heart grows, it bends into an S shape, so that the sinus venosus and atrium lie above the ventricle and conus arteriosus.