Regulation of fluid and electrolyte balance

The three fluid compartments of the body are interdependent. Their homeostasis relies on systems that regulate water balance and, as the principal extracellular solute, sodium balance. Maintenance of plasma volume is essential for adequate tissue perfusion. Regulation of plasma osmolality, which is determined primarily by the serum sodium concentration, is essential for the preservation of normal cell volume and function. The importance of osmoregulation is best illustrated by the consequences of a rapid fall or rise in serum osmolality, which can cause permanent neurological damage and death through shrinkage or swelling of cells. It is tempting to attribute control of plasma sodium concentration to sodium balance, but there is no direct relationship between plasma sodium and renal sodium excretion. Osmolality and volume are, therefore, regulated by separate mechanisms. It is important to recognize that osmoregulation occurs through changes in water balance, whereas volume regulation is principally determined by changes in sodium excretion.