Osmoregulation

Osmoregulation is the physiological control of the osmotic concentration of body fluids. In humans, it helps keep plasma osmolality within a narrow range despite changes in water intake and water loss. This is important because changes in extracellular fluid tonicity cause water to move into or out of cells, altering their volume .^1,2

Specialized osmosensitive cells detect changes in the osmotic concentration of body fluids. Important central osmoreceptors are located in brain regions that respond to circulating osmotic signals. When plasma osmolality rises, these systems promote the release of arginine vasopressin (AVP) and stimulate thirst. AVP acts on the kidneys to increase water reabsorption, reducing urinary water loss, while thirst promotes water intake .^1,2

When plasma osmolality falls, AVP secretion decreases, allowing the kidneys to excrete more water. Together, adjustments in thirst and renal water handling form a feedback system that helps stabilize the concentration of body fluids. These responses can also be modified by signals related to blood pressure and circulating volume, so osmotic and cardiovascular regulation are physiologically interconnected .¹

Osmoregulation is related to, but distinct from, extracellular fluid volume regulation. Osmoregulation is primarily concerned with water balance relative to solute and therefore with the concentration of body fluids. Extracellular fluid volume regulation depends strongly on sodium balance and on signals related to blood volume and pressure. The two systems interact but regulate different aspects of fluid homeostasis .^1,2