Hydrogen Ion Concentration and Acid-base Equilibrium in Normal Pregnancy

In 1912 Hasselbalch, with Lundsgaard and Gammeltoft (6, 7), described a lowering of the alveolar carbon dioxide and of the carbon dioxide-combining power of the blood in the later months of normal pregnancy. These changes have been abundantly confirmed by others and found to appear early in the course of pregnancy. These findings together with an increased ammonia to total nitrogen ratio in the urine described by Hasselbalch and Gammeltoft (6) have long been considered as an expression of an increased acid production in pregnancy. This conception has arisen partly from the impression that Hasselbalch and his associates detected a reduction in the pH of the blood in pregnancy. In actual point of fact they did not determine the pH of the blood directly, but only after exposure to a known concentration of COZ in air. This is equivalent, of course, to measuring the bicarbonate content or alkaline reserve and yields no information concerning the actual reaction of the blood in the body. The distinction was clearly recognized by Hasselbalch who concluded that the hydrion concentration of the blood in pregnancy remained normal. Hasselbalch did, however, interpret the increased ammonia to nitrogen ratio as an indication of an abnormally great acid production. It is now recognized that acid production is proportional not to the urinary ammonia to nitrogen ratio, but to the ab-