Volume homeostasis and osmoregulation in human pregnancy

This chapter reviews alterations in volume and sodium homeostasis and osmoregulation during human pregnancy. Pregnant women undergo extracellular and plasma volume increases of 50-70%, and these changes accompany marked cumulative sodium retention shared by both mother and fetus. Pregnancy alters several factors with opposing effects on renal salt handling; however, mechanisms by which gestational sodium accumulation and volume expansion are achieved remain obscure. Furthermore, despite substantial increases in absolute blood volume, considerable uncertainty exists as to how this volume is sensed, particularly in late pregnancy when a rapid increase in volume is associated with decreases in peripheral resistance and blood pressure. Attempts to assess 'effective' intravascular volume suggest that pregnant women sense their volume as normal. Osmoregulation is also changed. Body tonicity and the osmotic thresholds for AVP release and thirst decrease by about 10 mosm/kg. The mechanisms responsible for the osmoregulatory changes are obscure. Haemodynamic stimuli such as decrements in blood pressure and of 'effective circulating volume' do not seem to account for them. Of the many increments in hormone levels known to accompany gestation, only hCG has so far been implicated in these changes. Pregnant women experience three- to fourfold increments in AVP disposal rates between early and mid pregnancy; this may be caused by the striking rise in circulating cystine-aminopeptidase (vasopressinase) which also occurs during this period. The increments in MCR may be one reason why the hormonal response to a given osmotic stimulus appears to decrease in late pregnancy. All these alterations permit speculation on the manner in which the decrease in Posm occurs and is maintained within narrow limits. Lowering the osmotic threshold to drink stimulates a rise in water intake and dilution of body fluids. Since AVP release is not suppressed at the usual level of hypotonicity, AVP continues to circulate at levels sufficient to permit water retention. Posm continues to decline until it decreases below the new osmotic thirst threshold, when a new steady state is established. At this point water turnover, too, resembles that in the non-pregnant state. The change in MCR and the marked increment in plasma vasopressinase may explain certain observations regarding disordered water metabolism during late pregnancy. These are the transient DI syndromes due either to subclinical hypothalamic disease or to a disorder peculiar to pregnancy which is AVP-resistant but dDAVP-responsive; the latter analogue resists degradation by vasopressinase.