The transport system of an animal moves substances to where they are needed in the body. Even the smallest animal must have the means of transporting substances around its body. Oxygen and food molecules must move to all the cells, and the waste products must be removed from the cells and expelled into the environment. A variety of fluid systems, called vascular systems, help such transport in most members of the animal kingdom. A circulatory system is a vascular system (i.e. with tubes or vessels) in which the transport fluid moves rhythmically in a particular direction, since it is propelled by a muscular pumping structure.

Unicellular organisms (Protozoa) have no specialized system for circulation of fluid. Sponges and coelenterates also lack a specialized circulatory system. Part of the evolution of multicellular animals was the development of the body fluids (tissue fluids, blood and lymph). These fluids provide all body cells a stable and relatively non-fluctuating environment, and fill in the space between cells and cell layers.

The body fluids are divided into (a) intracellular (inside the cells) and (b) extracellular, (blood plasma and interstitial fluid). The blood plasma is present in blood vessels and the interstitial fluid in the spaces around the cells. Nutrients and gases passing between blood vessels and cells have to cross this fluid.