A finite-element model of oxygen diffusion in the pulmonary capillaries

We determined the overall pulmonary diffusing capacity (DL) and the diffusing capacities of the alveolar membrane (Dm) and the red blood cell (RBC) segments (De) of the diffusional pathway for O-2 by using a two-dimensional finite-element model developed to represent the sheet-flow characteristics of pulmonary capillaries. An axisymmetric model was also considered to assess the effect of geometric configuration. Results showed the membrane segment contributing the major resistance, with the RBC segment resistance increasing as O-2 saturation (SO2) rises during the RBC transit: RBC contributed 7% of the total resistance at the capillary inlet (SO2 = 75%) and 30% toward the capillary end (SO2 = 95%) for a 45% hematocrit (Hct). Both Dm and DL increased as the Hct increased but began approaching a plateau near an Hct of 35%, due to competition between RBCs for O-2 influx. Both Dm and DL were found to be relatively insensitive (2 similar to 4%) to changes in plasma protein concentration (28 similar to 45%). Axisymmetric results showed similar trends for all Her and protein concentrations but consistently overestimated the diffusing capacities (similar to 2.2 times), primarily because of an exaggerated air-tissue barrier surface area. The two-dimensional model correlated reasonably well with experimental data and can better represent the O-2 uptake of the pulmonary capillary bed.