Chapter 7 - Regulation of Sodium and Water Balance

Abstract

Sodium and water balance are regulated by the endocrine system. Osmolality of the extracellular fluid is monitored and adjusted by regulating water excretion by the kidney in response to antidiuretic hormone (ADH), which is secreted by the posterior lobe of the pituitary gland. Maintenance of vascular volume depends on maintenance of sodium balance. The rennin–angiotensin–aldosterone system and the atrial natriuretic hormone regulate renal mechanisms that govern retention or loss of sodium. ADH also contributes directly to volume regulation, and when demands for constancy of osmolality are in conflict with demands for constancy of volume, the latter prevail. These hormonal mechanisms operate largely by regulating renal function, but they also regulate salt and water intake. Salt and water balance are maintained at constant despite wide variations in intake and loss of both sodium and water. Osmolarity of blood plasma can be increased or decreased by adjusting the proportion of water relative to solute that is excreted in the urine, thus producing reciprocal changes in the osmolality of the urine. The most important stimulus for ADH secretion is an increase in blood osmolality. Aldosterone, an adrenal steroid, plays a pivotal role in maintaining salt and water balance. Atrial natriuretic factor (ANF), as its name implies, promotes the excretion of sodium in the urine. The three hormones, ADH, angiotensin II, and aldosterone collaborate to maintain or increase the effective volume of the blood plasma.