Coelenterates and platyhelminths nutrients in sponges, respiratory gases and waste materials can easily diffuse through the intercellular spaces. Though, in higher animals the circulatory system gives a medium through which oxygen and nutrients from the environment reach all the tissues of the body and carbon dioxide also nitrogenous waste materials are removed from the body into the external environment. The circulatory system consists of generally blood, the blood spaces and a series of tubes called vessels. Circulatory systems can be split into a number of components along with similar functions. In addition to the circulating fluid, these contain:
i) A main propulsive organ, generally a heart which forces blood around in the body,
ii) An arterial system that serves to distribute blood and as a pressure reservoir,
iii) Capillaries, or in some animals blood spaces, where transfer of materials take place between blood and tissues, and
iv) The venous system which acts as a blood reservoir and as a system for returning blood to the heart.