Emeritus Professor, Kyushu University
In fish, flow of respiratory medium is large, and oxygen utilization in the respiratory organ is high. The regular decrease in the mass-specific rate of metabolism with increasing body mass is qualitatively and quantitatively explained principally by a combination of an increase in the relative size of tissues of low metabolic activity and a decrease in the rate of tissue respiration with increasing body mass. Mass-specific metabolism is lower in a grouped fish than in a solitary one in group-living fishes, and vice versa in solitary-living fishes. The snakehead takes O2 mainly from air and eliminates CO2 almost exclusively to water. The fish morphologically and physiologically shows a double circulation composed of respiratory circulation and systemic circulation. Large carangid fishes show myomere subsegmentation and double-tracking of the segmental vessels. These modifications are considered to overcome the restriction imposed by a small number of myomeres in developing a large body and an active swimming ability. Erythrocytes are supplied from the spleen into the circulating blood during swimming and in hypoxic water. Fish spleen has microcirculatory pathways which are considered to function for rapid drainage of erythrocytes.