Analysis of the effects of pulsatile capillary blood flow and volume on gas exchange.

Blood flow into the pulmonary capillaries and the volume of blood within the capillary bed are both pulsatile with the cardiac cycle. We have developed a quantitative model of diffusional gas exchange in the lung to investigate the effects of coupling between these two time-varying parameters on lung O2 and CO2 exchange. For normal man breathing room air at rest, the computed results agree well with previous predictions for the constant flow and volume case, and for the case of pulsatile flow alone. When coupled time-varying pulmonary capillary blood flow and volume are included, using the best data available in the literature to define these parameters, diffusional O2 exchange is improved over the cases of pulsatile flow or volume alone, and closely approximates that obtained for the hypothetical constant flow and volume case. CO2 exchanges, and O2 exchange during hypoxia, are not affected by pulsatile flow and/or volume. These results suggest that O2 exchange is efficient in the presence of coupled blood flow and blood volume pulsations as they exist in the lung capillaries, and that these conditions may be optimal for gas exchange under certain physiological (or pathological) conditions.