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The ability of the heart to keep hypoxic (“venous”) blood that returns to the heart via the large systemic veins separate from the oxygenated blood from gills and/or lungs increases the efficiency of gas transport for the organism. A phylogenetic trend towards a higher degree of blood separation within the heart, and a more sophisticated control of respiratory and systemic blood flow distribution, is readily discernible among the vertebrates. The classical view, based primarily on deductions from anatomical studies, regarded the situation found in birds and mammals as the “final” stage, with a heart completely separated into four compartments with no possibility of intracardiac mixing of oxygenated and hypoxic blood. While preserving the separation of the flow of the two bloodstreams, the arrangement does not allow reduction of pulmonary flow during periods of apnoea. The avian and mammalian heart can be regarded as a set of two pumps arranged in series, and any change in blood flow in either the pulmonary or systemic circuit must obviously be replicated in the other circuit (Johansen and Burggren 1980; Burggren 1985, 1987; Johansen 1985).