The renin-angiotensin system and regulation of fluid volume

The renin-angiotensin system is a key component of the homeostatic mechanisms for regulating effective circulating volume. It acts primarily by controlling renal reabsorption of sodium and, because sodium is the main extracellular solute (and because osmoregulation controls the solute concentration), this determines the extracellular fluid volume. Renin is a proteolytic enzyme which, when released (in response to a number of signals indicating decreased effective circulating volume) initiates a cascade of enzyme reactions leading to the production of the octapeptide angiotensin II. Angiotensin II has many actions, all of which increase the effective circulating volume. These actions include systemic vasoconstriction, vasopressin release (to promote renal water retention), increased reabsorption of sodium in the renal proximal tubule, aldosterone release (leading to increased reabsorption of sodium in the distal nephron) and thirst. Interventions in the renin-angiotensin system can be used to treat hypertension because the actions described above also contribute to the maintenance of blood pressure. Such interventions include blocking the synthesis of angiotensin II and blocking angiotensin II receptors. This review also explains why the avid reabsorption of sodium elicited by the renin-angiotensin system does not disturb potassium homeostasis.