Sinonimi: cardiovascular system
The organs and tissues involved in circulating blood and lymph through the body; SYN. cardiovascular system.
System of vessels in an animal's body that transports essential substances (blood or other circulatory fluid) to and from the different parts of the body. Except for simple animals such as sponges and coelenterates (jellyfishes, sea anemones, corals), all animals have a circulatory system.
In fishes, blood passes once around the body before returning to a two-chambered heart (single circulation). In birds and mammals, blood passes to the lungs and back to the heart before circulating around the remainder of the body (double circulation). In all vertebrates, blood flows in one direction. Valves in the heart, large arteries, and veins prevent backflow, and the muscular walls of the arteries assist in pushing the blood around the body.
Although most animals have a heart or hearts to pump the blood, normal body movements circulate the fluid in some small invertebrates. In the open system, found in snails and other mollusks, the blood (more correctly called hemolymph) passes from the arteries into a body cavity (hemocoel), and from here is gradually returned to the heart, via the gills, by other blood vessels. Insects and other arthropods have an open system with a heart. In the closed system of earthworms, blood flows directly from the main artery to the main vein, via smaller lateral vessels in each body segment. Vertebrates, too, have a closed system with a network of tiny capillaries carrying the blood from arteries to veins.