Analysis of the effects of hematocrit on pulmonary CO2 transfer

A mathematical model of the chemical and transport events in blood during and after gas exchange has been used to examine the rates of CO2 excretion (Vco2) and O2 uptake (Vo2) in the lung at different levels of hematocrit (Hct), assuming fixed mixed venous blood O2 and CO2 contents and alveolar gases and constant cardiac output. The results show that a reduction in Hct from 45 to 30% leads to approximately 25% reduction in Vco2 compared with approximately 30% reduction in Vo2. Reduction of Hct from 45 to 15% results in approximately 50% reduction in Vco2 and approximately 63% reduction in Vo2. An increase in Hct from 45 to 60% results in approximately 25% increase in Vco2, accompanied by approximately 30% increase in Vo2. These fractional changes in gas exchange are only slightly affected by the extent of catalysis of the plasma CO2-H2CO3 hydration-dehydration reactions in the pulmonary vasculature. The reduction in Vco2 with reductions in Hct are due to 1) decrease in the total quantity of Bohr protons released during diminution of Vo2, 2) decrease in the size of the high buffer capacity intraerythrocytic pool, and 3) decrease in the total surface area available for HCO-3/Cl- exchange across the red cell membrane. We conclude that hitherto unrecognized changes in Vco2 (in addition to the well-known changes in Vo2) may occur as a consequence of alterations in Hct.