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Recumbant Standing Salt loaded Salt deprived

FIGURE 7 Changes in (A) plasma renin activity and (B) aldosterone in response to postural changes and salt loading in 17 pregnant and 11 nonpregnant women. (Drawn from data of Weinberger MH, Kramer NJ, Grim CE, Petersen LP. J Clin Endocrinol Metab 1977; 44:69-77.)

Recumbant Standing Salt loaded Salt deprived

FIGURE 7 Changes in (A) plasma renin activity and (B) aldosterone in response to postural changes and salt loading in 17 pregnant and 11 nonpregnant women. (Drawn from data of Weinberger MH, Kramer NJ, Grim CE, Petersen LP. J Clin Endocrinol Metab 1977; 44:69-77.)

Hypothalamic osmoreceptors monitor plasma osmolal-ity and signal secretion of arginine vasopressin (AVP) when osmolality increases. AVP signals the kidneys to conserve water by mechanisms described in Chapter 28, but water conservation can only maintain osmolality constant when water intake is adequate. When plasma osmolality increases, osmoreceptors also signal thirst and drinking behavior and thus coordinate water intake and water conservation.

Under basal conditions the concentration of sodium in the plasma of pregnant women is maintained at 3 to 5 mEq lower than that in plasma of nonpregnant women (Fig. 8). Despite the nearly 10 mOsm difference, plasma concentrations of AVP are quite similar in pregnant and nonpregnant women, indicative of a change in the relationship between osmolality and AVP secretion. The seemingly inappropriately higher concentration of AVP does not imply independence of AVP secretion from plasma osmolality or breakdown of normal feedback relationships. Pregnancy does not interfere with the ability to respond normally to changes in osmolality that follow periods of water deprivation or water loading (Fig. 9). When plasma osmolality was increased in pregnant and nonpregnant women by infusions of

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