Factors affecting gas transfer in respiratory organs of vertebrates

Abstract

The ability of a gas-exchange organ to transfer respiratory gases (O2 and CO2) between the respiratory medium (air or water) and blood is quantitatively characterized by its transfer conductance, usually termed diffusing capacity, D. The problems in defining and determining D are reviewed. In a blood-perfused gas-exchange organ it is useful to consider the ratio [Formula: see text] ([Formula: see text], blood flow; β, effective solubility) which determines the relative role of diffusion limitation. Poor gas transfer may be due to functional inhomogeneities, and not to low D values. Besides the well-known ventilation–perfusion inequality, other types of inhomogeneity involving diffusion resistances in the medium may be operative. Determination of D in functionally inhomogeneous gas-exchange organs is difficult because of both modeling and measurement problems. In blood to medium transfer of CO2, particular features have been noted. First, the equilibration of CO2 between medium and blood appears to be slower than expected on the basis of high physical solubility, owing to the slowness of some steps in the CO2 exchange process (dehydration of carbonic acid, bicarbonate–chloride exchange of red cells). Second, there is controversial evidence for equilibration of pulmonary capillary blood to a CO2 partial pressure lower than that in lung gas.