Slow postcapillary pH changes in blood in anesthetized animals.

To investigate the hypothesis that blood pH and PCO2 continue to change after the blood leaves an exchange capillary, we used a rapidly responding, pressure-insensitive, stopped-flow pH electrode apparatus. Arterial blood from an anesthetized dog or cat is drawn through the apparatus into a syringe. Syringe movement is then suddenly stopped. Temperature and pH of the blood in the electrode assembly are continuously monitored, both before and after blood withdrawal ceases. Hemolysis was reduced by coating all blood contact surfaces with silicone and fasting the animal overnight, anesthetizing it with crystalline pentobarbital sodium, and allowing it to ventilate spontaneously. After stopping withdrawal, pH of blood in the electrode chamber continued to change, rising 0.01 unit with t1/2 of 4.4 s. After lysed blood was returned to the animal to provide carbonic anhydrase to the plasma, no pH change was seen after stopping the flow. The small pH rise occurring in arterial blood in vivo is probably due in large part to disequilibrium of [H+] between red blood cells and plasma at the end of the pulmonary capillary, the equilibration process being rate-limited by the extracellular CO2 hydration-dehydration reaction.