Diffusion limitation in comparative models of gas exchange

Piiper and Scheid (Resp. Physiol. 23: 209-221, 1975) compared different models of external gas exchange with performance indices defined as functions of ventilatory/perfusive and diffusive/perfusive conductance ratios (Gvent/Gperf and Gdiff/Gperf, where Gdiff is diffusing capacity). We expanded their analysis to include: (1) Δp_D, the average partial pressure gradient driving diffusion across the exchange barrier, normalized to the maximum gradient available (Pi-Pv), and (2) Jdiff, the sensitivity of total conductance to changes in Gdiff, where total conductance is the ratio of gas flux to the maximum gradient [Gtot = M ̇/(Pi-Pv)]. Although the counter-current model is most efficient, it is more sensitive than cross-current or ventilated pool models to changes in Gdiff. For given Gvent, Gperf and Pi-Pv, maximum Gtot may not be achieved in the counter-current model until Gdiff is over ten-fold greater than that necessary for maximum Gtot in the other models. Experimental data also shows greater Jdiff and diffusion limitation in fish than in birds or mammals. We conclude that counter-current O₂ exchange cannot approach ideal levels as closely as the ventilated pool or cross-current models in nature.