Effect of pulsatile flow on gas exchange in the fish gill: theory and experimental data.

A model for gas exchange in the fish gill allowing for time-varying water and blood flow is presented. An analysis based on this mathematical model shows that pulsatile water and blood flow potentially may reduce the efficiency of gas exchange significantly. The degree of inefficiency imposed on gas exchange is, however, determined by the physical dimensions of the gill and the gas capacitance coefficients of water and blood. Using anatomical and physiological data it is shown to be likely that for a large group of fishes, including the salmonids, pulsatility of water and blood flow affects gas exchange efficiency only marginally. A close coupling between cardiac and respiratory rhythms is therefore only of marginal advantage to gas exchange efficiency. Due to their exceptional gill dimensions tunas, and to a lesser extent mackerels, are susceptible to the negative effect of pulsatility on gas exchange, which may be one of the factors favouring ram ventilation in these species.