Final answer:
In animals with closed circulatory systems, the transport medium is blood, which is pumped by the heart through blood vessels distinct from the body's interstitial fluid. This efficient system supports complex organisms, including vertebrates like fish, amphibians, reptiles, mammals, and birds, with variations in heart chamber number and blood mixing.
Step-by-step explanation:
The transport medium of animals with closed circulatory systems is called blood. In these systems, the heart pumps blood through vessels that are separate from the interstitial fluid of the body. This is in contrast to an open circulatory system, in which the circulating fluid, called hemolymph, is not confined entirely to vessels but is partially mixed with the interstitial fluid.
In closed circulatory systems, most vertebrates, such as fish, amphibians, reptiles, mammals, and birds, have blood that circulates through a network of blood vessels. Fish uniquely possess a two-chambered heart and unidirectional circulation. Amphibians and most non-avian reptiles have a three-chambered heart with varying levels of blood mixing, while mammals and birds have a four-chambered heart with no mixing of the blood, ensuring efficient double circulation.