circulatory 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. It was first discovered and described by English physician William Harvey. All animals except for the most simple – such as sponges, jellyfish, sea anemones, and corals – have some type of circulatory system. Some invertebrates (animals without a backbone), such as insects, spiders, and most shellfish, have an ‘open’ circulatory system which consists of a simple network of tubes and hollow spaces. Other invertebrates have pump-like structures that send blood through a system of blood vessels. All vertebrates (animals with a backbone), including humans, have a ‘closed’ circulatory system which principally consists of a pumping organ – the heart – and a network of blood vessels.

Fish have a single circulatory system in which blood passes once around the body before returning to a two-chambered heart. In birds and mammals, there is a double circulatory system – the lung or pulmonary circuit and the body or systemic circuit. Blood is first pumped from the heart to the lungs and back to the heart, before being pumped to the remainder of the body and back. The heart is therefore a double pump and is divided into two halves. 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. A network of tiny capillaries carries the blood from arteries to veins. It is through the walls of capillaries that materials are transported to and from the blood.

Although most animals have a heart or hearts to pump the blood, in some small invertebrates normal body movements circulate the fluid. In the open system, found in snails and other molluscs, the blood (more correctly called haemolymph) passes from the arteries into a body cavity (haemocoel), and from here is gradually returned by other blood vessels to the heart, via the gills. 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.

The human circulatory system performs a number of functions: it supplies the cells of the body with the food and oxygen they need to survive (see nutrition); it carries carbon dioxide and other waste products away from the cells; it helps to regulate the temperature of the body; and protects the body from disease. In addition, the system transports hormones, which help to regulate the activities of various parts of the body.