Regulation of fluid and electrolyte balance by the kidney

The kidneys match renal excretion to intake of water and electrolytes to regulate the osmolality and volume of body fluids. Deficits of water or electrolytes can be compensated for by increases in intake and retention, whereas excesses are compensated for by increases in urinary excretion. It is usually considered that regulation of water excretion determines osmolality, and regulation of electrolyte excretion (principally NaCl) determines fluid volume; however, regulation of water excretion influences fluid volume and osmolality. For example, an increase in osmolality, through excessive sweating or diarrhoea, triggers water retaining and fluid intake mechanisms such as antidiuretic hormone (ADH) release or thirst to return osmolality to normal and restore body fluid volume.