Relationship between the transfer rate and the gap in partial pressure of gas molecules at a heterogeneous interface.

Abstract

The partial pressure of a gas corresponds to the kinetic energy per unit volume. At equilibrium, the kinetic energy in the gas phase, or the partial pressure of the gas, must be equal to that in the liquid phase. On the other hand, the number of gas molecules dissolved in a unit volume of solution is usually smaller than the number of molecules in the same volume of gas. Consequently, the kinetic energy of a gas molecule in solution must be higher than that in the gas phase. Thus, in order for the gas molecule to cross a heterogeneous interface, it must undergo a rise of fall in kinetic energy. To counter the change in energy transfer rate across the interface, a gap in partial pressure appears essential.We [18] introduced a term, transfer coefficient, to evaluate the relationship between the magnitude of the gap in partial pressure and the transfer rate across the red blood cell (RBC) membrane. The transfer coefficient (ii) was obtained by dividing the diffusion flux by the size of the gap in partial pressure. Thus, the dimensions are similar to those of permeability. However, , is concerned with the discontinuity in partial pressure at heterogeneous interface caused by the difference in kinetic energy, while permeability is related to the diffusibility within a thin layer such as is found in artificial and some biological membranes. UCHIDA et al. [31] measured the C02 diffusion rate in a thin layer of hemoglobin solution, and found that the inward and outward diffusions were equally retarded at the gas-liquid interface. NIIZEKI et al. [23,24] measured the C02 diffusion rate in the RBC under lowered convective conditions and found that the value of tj for C02, being 2.5 x 10-6 cm • s-1 • Torr-1, was identical to that for 02 [9,10,16].Oxygenation and deoxygenation reactions of hemoglobin are very fast [20] and hydration and dehydration reactions of C02 are also highly accelerated by carbonic anhydrase in RBC [23]. Thus, the rates of 02 and C02 reactions with RBC are limited by the transfer rates of these gases and HC03 -ion across the RBC interface as well as by the intracellular diffusion rate. Therefore, estimation of the i values of the RBC and alveolar membranes is a key problem for the kinetic studies in the