Maximum gas transfer efficacy of models for fish gills, avian lungs and mammalian lungs

Abstract

The performance limits of the following models for vertebrate gas exchange organs have been investigated in theory: (1) counter-current model, for fish gills, (2) cross-current model, for avian lungs, and (3) uniform pool model, for mammalian lungs. In “ideal” conditions, i.e. when diffusion limitation and distributional inhomogeneities are absent, partial pressures of respiratory gases in the external medium and blood leaving the gas exchange organ relative to those in the medium and blood entering it are shown to depend, in a specific manner for each model, on the medium-to-blood conductance ratio X = ( V ̇ β) m ( V ̇ β) b , (V̇, flow rate; β, “capacitance coefficient” ; m refers to respiratory medium; b, to blood). Generally, the effectiveness of the models, in terms of extent of gas transfer for same conductance values, increases in the sequence, uniform pool → cross-current → counter-current. The enhanced efficacy of the counter-current and cross-current models occurs only inside a limited range of X values (not far from 1.0). Measurements have shown that in fishes and birds X values for CO 2 and O 2 are close to their optimum values.