The Circulatory System

Profound changes in the demands on the circulatory system have occurred during the evolution of the vertebrates from the aquatic forms to the more advanced terrestrial forms. These changes reflect a variety of anatomical and functional alterations, and also the adjustment from aquatic life at zero gravity to the demands of terrestrial life. The best documented driving force in the evolution of the vertebrate cardiovascular functions is the need for an efficient transport of respiratory gases between the gas exchanger (skin, gill, lung) and the tissues (Johansen and Burggren 1980, Johansen 1982).