relative constancy of osmotic pressure in the fluid media and tissues of the body, a result of the maintenance at a given level of the concentrations of electrolytes, proteins, and other substances.

Isotonicity is one of the body’s most important physiological constants, which are ensured by mechanisms of self-regulation. Deviation of osmotic pressure from the normal physiological level (≈76–0.81 meganewtons per sq m, or 7.6–8.1 atmospheres) leads to the disruption of metabolic processes between the blood and the interstitial fluid.