Figure 103

Serum electrolytes in pregnancy. A, During normal gestation, serum osmolality decreases by 10 mosm/L and serum sodium (Na+) decreases by 5 mEq/L. A resetting of the osmoreceptor system occurs, with decreased osmotic thresholds for both thirst and vasopressin release [3]. B, Serum chloride (Cl-) levels essentially are unchanged during pregnancy. C, Despite significant increases in aldosterone levels during pregnancy, in most women serum potassium (K+) levels are either normal or, on average, 0.3 mEq/L lower than are values in women who are not pregnant [4]. The ability to conserve potassium may be a result of the elevated progesterone in pregnancy [5]. D, Arterial pH is slightly increased in pregnancy owing to mild respiratory alkalosis. The hyperventilation is believed to be an effect of progesterone. Plasma bicarbonate (HCO-3) concentrations decrease by about 4 mEq/L [6].

pregnancy and often approaches prepregnancy levels at term. B, Despite the decrease in blood pressure, plasma renin activity (PRA) increases during the first few weeks of pregnancy; on average, close to a fourfold increase in PRA occurs by the end of the first trimester, with additional increases until at least 20 weeks. The source of the increased renin is thought to be the maternal renal release of renin.

(Continued on next page)

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