Physiology of fluid balance

The body, in broad terms, is divided into intra- and extracellular fluid compartments, of which the extracellular consists of intravascular and interstitial compartments. The osmotic pressure of all of these compartments is equal, but their composition is different. This difference and the shifts in fluid between the intra- and extracellular compartments are brought about, in part, by the presence of intracellular proteins, which are negatively charged but which have no osmotic effect and cannot pass across cell membranes, and also by the inability of charged electrolytes to pass across cell membranes except via specialized transport proteins. Intake of fluid is in part voluntary but fluid is also present in food and is derived from the oxidation of food. Fluid balance is controlled by volume and osmolar mechanisms largely under the control of aldosterone and antidiuretic hormone, respectively.