Renal hemodynamics and volume homeostasis in pregnancy.

Maternal hemodynamic adaptation to pregnancy consists of profound changes in various interdependent systems. Of crucial importance in the early adaptation of the volume homeostatic mechanisms to pregnancy is the resetting of the volume and osmoreceptors. This resetting may be induced by a reduction in vascular tone and leads to early changes in plasma osmolality and glomerular filtration rate. After this initial adaptation other volume-regulating mechanisms such as the renin-angiotensin-aldosterone system, pregnancy hormones, and alpha-ANP adapt to the rising blood volume. The initial adaptation results in a state of relative vascular underfill, inducing secondary compensations in the volume homeostasis. The initially increased vascular capacitance in pregnancy is compatible with the signals of vascular overfill. Contrarily, the protracted filling of this enlarged vascular bed triggers signals compatible with vascular underfill.