Respiration and blood circulation of fish

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 ver sa in solitary-living fishes. The snakehead takes O-2, mainly from air and eliminates CO2 almost exclusively to water. The fish morphologically and ph ysiologically shows a double circulation composed of respiratory circulatio n and systemic circulation. Large carangid fishes show myomere subsegmentat ion and double-tracking of the segmental vessels. These modifications are c onsidered to overcome the restriction imposed by a small number of myomeres in developing a large body and an active swimming ability. Erythrocytes ar e supplied from the spleen into the circulating blood during swimming and i n hypoxic water. Fish spleen has microcirculatory pathways which are consid ered to function for rapid drainage of erythrocytes.