Vasopressin and Renal Regulation of Water Homeostasis

Abstract

Besides the role of fluid intake in osmoregulation — which normally depends mainly on our will and of course on the access to water and our physical ability to ingest it — the most important effector of the regulation of body water are the kidneys. The ability of mammals to excrete urine either more or less concentrated than plasma, permits them to maintain the osmolality of body fluids within a remarkably narrow physiological range (280–300 mOsm/kg) compatible with normal cellular function in the face of wide variations in salt and water intake. This homeostatic process which protects body fluids from life-threatening deviations in osmolality involves 1. a complex cascade of processes of generation and maintenance of a renal cortico-medullary osmotic gradient, and 2. processes connected with utilization of this gradient supposing normal regulation of vasopressin secretion and the presence of the collecting duct epithelia responsive to the hydroosmotic effect of AVP. Vasopressin which acts predominantly on the most terminal segment of the nephron, is doubtlessly the principal, although not the only, factor involved in regulation of renal water excretion. The effect of this hormone no longer can be described by the simplified scheme: “vasopressin = urine concentration”. AVP is a factor, whose action depends on the effectivity of the preceeding, “preparatory” phases of urine generation.