Effect of Boundary Layers on Cutaneous Gas Exchange

Abstract

Boundary layers may offer significant resistance to cutaneous oxygen uptake by amphibians in water. This hypothesis was tested by measuring resistance to oxygen uptake as a function of water velocity in bullfrogs submerged at 5.degree. C and by direct measurments of the boundary layer with oxygen microelectrodes. The oxygen diffusion boundary layer was easily measurable with oxygen microelectrodes. The proportion of the total resistance to oxygen uptake represented by the boundary layer increased from 35% in a water velocity of 5 cm s-1 to over 90% at 0.1 cm s-1. At water velocities below 1 cm s-1 oxygen uptake was limited by the resistance of the boundary layer. At 0.1 cm s-1, the partial pressure of oxygen immediately adjacent to the skin was only 2 kPa (15 mmHg); placing an immobilized frog is still water was tantamount to placing it in anoxic water. Body movements disrupted boundary layers efficiently; even occasional small movements by the animal (1 min-1) were sufficient to maintain oxygen uptake in still water.