Physiological factors affecting O2 transport by hemoglobin in an in vitro capillary system.

O2 transport was examined by measuring the fractional saturation of concentrated hemoglobin solutions flowing through an artificial capillary that was approximately 27 micron in diameter and embedded in a silicone rubber film approximately 170 micron thick. The effects of pH, hemoglobin concentration, O2 tension, temperature, and organic phosphate were measured and analyzed quantitatively by a rigorous mathematical model that included the geometry of the capillary in the silicone film, parabolic flow velocity distributions inside the lumen, and cooperative O2 binding by hemoglobin. The rates of both oxygenation and deoxygenation were limited by diffusion and governed by the magnitude of the O2 gradient between the intracapillary fluid phase and the external gas space. In uptake experiments, O2 flux is determined primarily by the external O2 tension (16-160 mmHg in our experiments) because the internal O2 pressure is kept small due to chemical combination with hemoglobin. In release experiments, the external O2 tension is maintained at zero, and the transport rate is determined by the intracapillary partial pressure of O2 that is proportional to the O2 half-saturation pressure of hemoglobin value of the hemoglobin sample. As a result, factors that change the affinity of hemoglobin for O2, such as pH, temperature, and organic phosphate concentration, influence strongly the rate of O2 release but have little effect on the rate of O2 uptake. These properties are physiologically advantageous, since a decrease in pH or an increase in temperature during exercise increases both the rate and extent of deoxygenation while not altering the kinetics of oxygenation.