Water Balance

Abstract

Our kidneys excrete urine of varying volume—dilute and concentrated urines. The excreted solutes must be dissolved in water (we obviously cannot urinate solids!). The range of the volume and osmotic concentration of urine reflects the ability of the kidneys to regulate and maintain a constant extracellular fluid (ECF) osmolarity. Changes in ECF are rectified by the kidneys and translated to changes in urine volume/osmolarity. Osmoregulation is the same as regulating water balance. Since ECF sodium concentration is the main determinant of ECF osmolarity, we have three controlled parameters that refer to the same physiologic homeostasis. These are water balance, osmoregulation, and sodium concentration (like a three- in -two coffee pack!). We can quite correctly consider the hypothalamic osmoreceptors as receptors that monitor ECF- sodium concentration.