Plasma Osmolality (mosm/Kg)
FIGURE 8 Relationship between plasma osmolality and AVP concentrations in eight women before (light blue) and at the end (dark blue) of the first trimester of pregnancy. Arrows indicate plasma osmolality at which a conscious desire to drink (thirst) was experienced. (Drawn from data of Davison JM, Shiells EA, Philips PR, Lindheimer MD. J Clin Invest 1988; 81:798-806.)
saline, plasma levels of AVP increased to a similar extent in both groups with each increment in osmolality. While the AVP concentrations changed in parallel in both groups, at each concentration of AVP observed the osmolality was nearly 10 mOsm/L lower in the pregnant women. Pregnancy shifts the relationship between osmolality and thirst to a similar extent. Thus, osmolal regulation remains intact during pregnancy, but operates at a lower set point.
The factors that bring about changes in sensitivity of the osmoreceptor system have not been identified nor is the underlying mechanism understood. A similar but less pronounced shift in the relationship between AVP secretion and plasma osmolality is seen in nonpregnant women during the luteal phase of the menstrual cycle. Progesterone, relaxin, or some other product of the corpus luteum that continues to be secreted throughout pregnancy may thus be responsible for adjusting osmoregulation. In nonpregnant individuals, the relationship between AVP secretion and osmolarity changes with changes in plasma volume. A leftward shift in the AVP/ osmolality relationship similar to that seen in pregnant women occurs in hypovolemia (reduced plasma volume), while hypervolemia tends to shift the relationship in the opposite direction. Thus, it seems that the hypervolemia of pregnancy appears as hypovolemia to the volume-regulating mechanisms that govern AVP secretion. Changes in the set-points for volume and osmolal regulation cause regulatory centers to interpret the increase in plasma volume and the decreased osmolality in pregnancy as normal and to defend these altered states.
When increased plasma volume is sensed by stretch receptors in the right atrium in men and nonpregnant women, secretion of atrial natriuretic factor (ANF) increases (see Chapter 29). ANF combats volume expansion by increasing urinary sodium loss, through direct actions on the kidney and through inhibition of aldos-terone production (see Chapter 40). Despite the 50% increase in blood volume, only small, if any, increases in plasma ANF concentrations are seen in normal pregnant women before the third trimester, when a modest rise occurs. Pregnancy therefore appears to decrease the sensitivity of the ANF-secreting cells to increased volume. However, rates of ANF secretion increase or decrease to similar extents in pregnant and nonpregnant women in response to the variations in central venous volume that result from changes in posture or saline
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