a. a closed circulatory system
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In a closed circulatory system, a muscular heart or hearts pump blood throughout the body within a system of vessels. Exchange of gases, nutrients, and wastes occur through the walls of the capillaries, the tiniest of these vessels. This closed system is found in all vertebrates. However, the structure of the heart varies among vertebrates; fish have two-chambered hearts, amphibians and many reptiles have three-chambered hearts, and mammals and birds have four-chambered hearts. Many non-vertebrate animals have open circulatory systems where hemolymph, which is a yellowish fluid similar to blood, is pumped by the heart into open vessels.