The flow of blood through the body of an animal, enabling the transport of oxygen and nutrients to the tissues and the removal of waste products.
Blood is pumped around the body by a heart .
Smaller animals, such as crabs and insects, have a tubular heart that pumps blood into the body cavity. The organs suspended within it thus come into direct contact with the blood and materials are exchanged by diffusion. This is described as an open circulation . With an increase in body size, diffusion is no longer efficient, and an active circulatory system is needed. The vertebrate cardiovascular system, with arteries, veins, and a powerful muscular heart, supplies this need.