circulationthe process by which circulatory systemsystem that transports nutrients, respiratory gases, and metabolic products are transported throughout a living organism, permitting integration among the various tissues. The process of circulation includes the intake of metabolic materials, the conveyance of these materials throughout the organism, and the return of harmful by-products to the environment.



Main features of circulatory systems
General features of circulation









Body fluids

The fluid compartments of animals consist of intracellular and extracellular components. The intracellular component includes the body cells and, where present, the blood cells, while the extracellular component includes the tissue fluid, coelomic fluid, and blood plasma. In all cases the major constituent is water derived from the environment. The composition of the fluid varies markedly depending on its source and is regulated more or less precisely by homeostasis.

Blood and coelomic fluid are often physically separated by the blood-vessel walls; where a hemocoel (a blood-containing body cavity) exists, however, blood rather than coelomic fluid occupies the cavity. The composition of blood may vary from what is little more than the environmental water containing small amounts of dissolved nutrients and gases to the highly complex tissue containing many cells of different types found in mammals.

Lymph essentially consists of blood plasma that has left the blood vessels and has passed through the tissues. It is generally considered to have a separate identity when it is returned to the bloodstream through a series of vessels independent of the blood vessels and the coelomic space. Coelomic fluid itself may circulate in the body cavity. In most cases this circulation has an apparently random nature, mainly because of movements of the body and organs. In some phyla, however, the coelomic fluid has a more important role in internal distribution and is circulated by ciliary tracts.

Fluid compartments

Blood is circulated through vessels of the blood vascular system. Blood is moved through this system by some form of pump. The simplest pump, or heart, may be no more than a vessel along which a wave of contraction passes to propel the blood. This simple, tubular heart is adequate where low blood pressure and relatively slow circulation rates are sufficient to supply the animal’s metabolic requirements, but it is inadequate in larger, more active, and more demanding species. In the latter animals, the heart is usually a specialized, chambered, muscular pump that receives blood under low pressure and returns it under higher pressure to the circulation. Where the flow of blood is in one direction, as is normally the case, valves in the form of flaps of tissue prevent backflow.

A characteristic feature of hearts is that they pulsate throughout life and any prolonged cessation of heartbeat is fatal. Contractions of the heart muscle may be initiated in one of two ways. In the first, the heart muscle may have an intrinsic contractile property that is independent of the nervous system. This myogenic contraction is found in all vertebrates and some invertebrates. In the second, the heart is stimulated by nerve impulses from outside the heart muscle. The hearts of other invertebrates exhibit this neurogenic contraction.

Chambered hearts, as found in vertebrates and some larger invertebrates, consist of a series of interconnected muscular compartments separated by valves. The first chamber, the auricle, acts as a reservoir to receive the blood that then passes to the second and main pumping chamber, the ventricle. Expansion of a chamber is known as diastole and contraction as systole. As one chamber undergoes systole the other undergoes diastole, thus forcing the blood forward. The series of events during which blood is passed through the heart is known as the cardiac cycle.

Contraction of the ventricle forces the blood into the vessels under pressure, known as the blood pressure. As contraction continues in the ventricle, the rising pressure is sufficient to open the valves that had been closed because of attempted reverse blood flow during the previous cycle. At this point the ventricular pressure transmits a high-speed wave, the pulse, through the blood of the arterial system. The volume of blood pumped at each contraction of the ventricle is known as the stroke volume, and the output is usually dependent on the animal’s activity.

After leaving the heart, the blood passes through a series of branching vessels of steadily decreasing diameter. The smallest branches, only a few micrometres (there are about 25,000 micrometres in one inch) in diameter, are the capillaries, which have thin walls through which the fluid part of the blood may pass to bathe the tissue cells. The capillaries also pick up metabolic end products and carry them into larger collecting vessels that eventually return the blood to the heart. In vertebrates there are structural differences between the muscularly walled arteries, which carry the blood under high pressure from the heart, and the thinner walled veins, which return it at much reduced pressure. Although such structural differences are less apparent in invertebrates, the terms artery and vein are used for vessels that carry blood from and to the heart, respectively.

The closed circulatory system found in vertebrates is not universal; a number of invertebrate phyla have an “open” system. In the latter animals, the blood leaving the heart passes into a series of open spaces, called sinuses, where it bathes internal organs directly. Such a body cavity is called a hemocoel, a term that reflects the amalgamation of the blood system and the coelom.