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A circulatory system consists of a muscular pump (the heart), a fluid (blood), and a series of conduits (blood vessels) through which the fluid can be pumped throughout the body. Heart, blood, and vessels are also known collectively as a cardiovascular system (Greek kardia, “heart,” + Latin vasculum, “vessel”). The function of a circulatory system is to transport things around the body. Preceding chapters discussed how circulatory systems transport heat, hormones, respiratory gases, blood cells, platelets, and cells and molecules of the immune system. Succeeding chapters will add nutrients and waste products to that list. In this section we will describe the general types of circulatory systems found in animals.
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A circulatory system functions to transport materials around the body.
Small or thin aquatic organisms may not have circulatory systems, and the needs of their cells are served by direct exchange with the environment.
Circulatory systems use a muscular heart to distribute extracellular fluid throughout the body to transport nutrients, respiratory gases, wastes, and other materials.
Closed circulatory systems allow more rapid flow and differential control of blood flow to different tissues.