How is the tonicity related to the osmosis?

With a few exceptions, the body fluids are osmotically isotonic (~ 0.3 Osm / l) - corresponding to a "physiological saline solution" (0.9%, i.e. 9 g NaCl / l), which is usually used as the basis for infusion solutions. Osmotic effects can occur at physiological interfaces; Cell membranes are permeable to water and can compensate for differences in osmotic concentration through osmosis. If different concentrations of osmotically active substances occur (intra- vs. extracellular), cells either absorb water (cell swelling or cytolysis in a hypotonic environment) or they shrink (in a hypertonic environment).

The osmotic concentration is important in relation to the whole organism, since the milieu for biochemical and cell-physiological processes depends on it. It is precisely regulated; the center for this is the hypothalamus, which has neuronal and humoral access to correct the osmolality - if necessary. Starting points for this regulation are on the one hand the intake and on the other hand the excretion of water and salt - i.e. thirst, possibly salt appetite, and kidney function.

Dehydration means too little and overhydration an increased amount of fluid in the body. Usually one means the condition of the extracellular space (in the case of intracellular deviations this is called). Changes in hydration can be isotonic, hypotonic, or hypertonic. The water always moves over (cellular, epithelial) boundary layers to the side of the higher concentration of dissolved substances (osmosis); there are characteristic clinical examples of each of these disorders.

The normal range of osmolality is narrow, roughly between 280 and 295 mOsm (corresponding to a sodium concentration of around 145 mM). Vasopressin secretion and blood levels change with osmolality; Below ~ 280 mOsm they are practically zero, above ~ 290 mOsm they increase, and above ~ 290 mOsm a feeling of thirst begins to have an additional effect (drinking water reduces the osmolality of the body fluids).