Circulatory Systems

Abstract

The circulatory system provides the important link between the environment and the cells. With increase in body size the coelom becomes incapable of supplying the circulatory needs of the animal and so the pathway of oxygen to the tissues must be facilitated. This has been achieved in many cases by the development and elaboration of a system of sinuses or vessels which contain a fluid (blood). A contractile mechanism is usually developed to pump the blood around the body. In some cases the larger vessels fulfil this function, but in others a special organ, the heart, has been developed. Pumping may be assisted by accessory booster hearts, especially in large or active animals, and also by body movements. Indeed, many small animals rely on body movements alone to circulate the blood and in these the circulatory system is poorly developed and specialised respiratory structures are absent. The function of the blood is to carry oxygen, usually in combination with a respiratory pigment, to the tissues and to remove the waste products of respiration.