Gas exchange in the microcirculation

A) Diffusion of oxygen from the capillaries to the cells

· the PO₂ in the intertitual fluid averages 40 mm Hg

· the PO₂ in the arterial blood entering the capillary is 95 mmHg —> there is an initial net difference for diffusion of oxygen into the capillary of 55 mm Hg

· the capillary PO₂ rapidly equilibrates with the oxygen pressure in the intertitual fluid and thus leaves the capillaries at the venous end with a PO₂ of 40 mmHg

· since the oxygen is always being used by the cells, the intracellular PO₂ always remains lower than the interstitual fluid [averaging 23 mm Hg]

· as only 3 mm Hg is adequate for cellular aerobic processes, the low intracellular PO₂ of 23mm Hg is still more than adequate

B) Diffusion of carbon dioxide from the cells to the tissue capillaries

· large amounts of CO₂ are continuosly formed in the cell thereby raising the intracellular pCO₂ to 46 mm Hg

· Although the intracellular PCO₂ [45 mm Hg] is only 1 mm Hg less than the intracellular PCO₂, due to the 20 x diffusing ability of CO₂ vs O₂, the CO₂ nevertheless rapidly diffuses into the interstitual fluid and thence the capillary blood

· arterial blood entering tissue capillaries have a PCO₂ of 40 mm Hg —> there is an initial net difference for diffusion of CO₂ from the capillary to the capillary blood of 5 mm Hg

· as the blood passes through the capillaries, the blood PCO₂ rises to approach the 45 mm Hg PCO₂ of interstitual fluid