Oxygen diffusion in the pulmonary capillaries

We determined the overall pulmonary diffusing capacity (D_L ), membrane diffusing capacity (D_M), and red cell (RBC) diffusing capacity (D_E) for oxygen (O₂,) using a finite element model representing the sheet flow characteristics of the pulmonary capillaries. The results showed that the membrane contributing the major resistance (1/D_M), with the RBC resistance (1/D_E) had increasing significance as O₂ saturation rises during RBC transit, from 7% at the capillary inlet to 30% towards the exit. D_M and D_L increased with increasing hematocrit (Hct) but gradually approached a plateau at higher Hcts (Hct>35%). Axisymmetric model results were similar to the 2D model but significantly overestimated D_M and D_L (~2.2 times), due to an exaggerated air-tissue surface area available for gas transport associated with the axisymmetric geometry. The 2D model geometry correlated reasonably well with the experimental data and better represents the oxygen uptake at the pulmonary capillary bed