Renal Function and Volume Homeostasis

This section focuses on the remarkable physiological changes in kidney structure and function, as well as volume homeostasis during normal pregnancy.  All of these changes, many being large, have important clinical ramifications for the physician caring for pregnant women as highlighted in the chapter.  The kidney volume virtually doubles during pregnancy due to 1 cm increases in the length, width and thickness.  The urinary tract dilates particularly on the right side as a consequence of changes in hormones and physiological ureteral obstruction.  Both the glomerular filtration rate and renal plasma flow increase during the first trimester by 50-85%.  These increases result from a profound reduction in the renal afferent and efferent arteriolar resistances. Plasma osmolality declines due to simultaneous reductions in the osmotic threshold for both thirst and arginine vasopressin release.  Emerging investigations in both animals and humans implicate the ovarian hormone relaxin in the renal hemodynamic and osmoregulatory changes of pregnancy.  Renal tubular handling of glucose, amino acids, vitamins, as well as uric acid is altered; in addition to increased filtered load, there is a physiological reduction in proximal tubular absorption of these solutes frequently resulting in increased urinary excretion.  Because of chronic respiratory alkalosis mediated through stimulation of the central nervous system by progesterone during pregnancy, a metabolic compensation occurs resulting in slight increases in urinary bicarbonate excretion and consequent reductions in plasma bicarbonate concentration.  Both potassium and sodium are retained during pregnancy ultimately due to renal retention.  However, the hormones responsible for renal retention of these solutes remain uncertain.  Water absorption follows sodium resulting in marked expansion of plasma and extracellular fluid volume.  Despite the absolute increase in plasma and blood volume, it is unclear how gravidas “sense” their circulating volume.  Indeed, this may be a function of the gestational age, i.e., during early gestation, circulatory “underfilling” may exist due to the rapid vasodilation of non-reproductive organs at that time.  As pregnancy proceeds, compensation occurs by renal retention of sodium and water resulting in “normofill”, and perhaps at the end of pregnancy, a slight “overfill” condition pertains. 

This manuscript was revised based on an original book chapter, authored for a prior edition by Marshall D. Lindheimer and William M. Barron.