Acid-base regulation and control of ventilation in human pregnancy

The purposes of this review were two-fold: to apply modern physicochemical principles to explain changes in acid-base regulation and the control of ventilation in human pregnancy; and to demonstrate the value of pregnancy as a model for the study of endocrine effects on physiological control systems. Application of P.A. Stewart's approach (P.A. Stewart. Can. J. Physiol. Pharmacol. 61: 1444-1461, 1983) shows that lower values of plasma hydrogen ion concentration ([H⁺]) observed at rest and in association with exercise in pregnancy are the result of lower values for carbon dioxide tension (P(CO₂)) and total weak acid ([A(tot)]). This effect is partly offset by a lower strong ion difference ([SID]). The ability to predict plasma [H⁺] at rest and following strenuous exercise in pregnancy (J.G. Kemp, F.A. Greer, and L.A. Wolfe. J. Appl. Physiol. 83: 644-651, 1997) supports the validity of Stewart's approach. Jennings and associates (D.B. Jennings. Can. J. Physiol. Pharmacol. 72: 1499-1512, 1994) have further demonstrated in animal models the involvement of plasma osmolality and circulating levels of angiotensin II (ANG II) and arginine vasopressin (AVP) in the chemical control of ventilation. We hypothesize that pregnancy- induced increases in respiratory sensitivity to carbon dioxide are the combined result of reduced plasma osmolality, reduced cerebrospinal fluid [SID], and augmented circulating levels of progesterone, ANG II, and AVP.