Alveolar-capillary CO2 and O2 gradients due to uneven hematocrits.

The effect of uneven hematocrit of the blood perfusing the different pulmonary capillaries on gas exchange in the lung was studied in vitro. Venous blood was separated anaerobically into plasma and red cell fractions, arterialized in separate tonometers, and finally reconstituted anaerobically. This simulated the effect of complete separation of plasma and red cells on pulmonary gas exchange. The reconstituted blood had a significantly higher carbon dioxide tension and a lower oxygen tension compared with whole blood similarly arterialized. Experiments in which plasma and red cells were incompletely separated yielded qualitatively similar results; the changes in gas tensions were smaller but still significant. While the degree and extent of separation of plasma and red cells in the pulmonary capillary in vivo is uncertain, direct observation and hemodynamic consideration suggest it must occur to some extent. Thus unevenness in the hematocrit of the perfusing pulmonary capillary blood is a mechanism in the genesis of the alevolar-arterial gradients which should be considered in addition to the known mechanisms of uneven ventilation-perfusion ratios and diffusion defects.