Journal of Applied Physiology AJP: Endocrinology and Metabolism
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J Appl Physiol 88: 149-157, 2000;
8750-7587/00 $5.00
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Vol. 88, Issue 1, 149-157, January 2000

Plasma acid-base regulation above and below ventilatory threshold in late gestation

Aaron P. Heenan and Larry A. Wolfe

School of Physical and Health Education and Department of Physiology, Queen's University, Kingston, Ontario, Canada K7L 3N6

Stewart's physicochemical approach was used to study the effects of pregnancy on acid-base regulation in arterialized blood. Responses of 15 healthy pregnant women (PG; gestational age, 37.1 ± 0.2 wk) were compared with those of 15 nonpregnant controls (CG) at rest and during cycling at 70 and 110% of the ventilatory threshold (Tvent). Hydrogen ion concentration ([H+]) was lower in the PG vs. CG at rest and during exercise (P < 0.05 at rest and 70% Tvent). Exercise-induced changes in [H+] were similar between groups. Lower resting [H+] values in the PG vs. CG resulted from lower values for arterialized PCO2 (PaCO2) and total weak acid ([A]tot), which were partly offset by a lower strong-ion difference ([SID]). Reductions in [A]tot and [SID] at rest were primarily the result of reductions in albumin [Alb] and sodium [Na+], respectively. In the transition from rest to 70% Tvent, small increases in PaCO2 and [A]tot contributed to moderate increases in [H+] in both groups, however [SID] increased in the PG and decreased in the CG (P < 0.05 between groups). In the transition from rest to 110% Tvent, decreases in [SID] made a significantly greater contribution to changes in [H+] in the CG vs. PG. Exercise-induced increases in [H+] are similar in the pregnant vs. nonpregnant state, but there is a reduced contribution of [SID] both above and below Tvent during pregnancy.

hydrogen ion; carbon dioxide tension; strong-ion difference; total weak acid


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