Isotonic Solutions

solutions with the same osmotic pressure; in biology and medicine, natural or artificially prepared solutions with the same osmotic pressure as that of the content of animal and plant cells, blood, and interstitial fluids.

In normally functioning animal cells the intracellular content is usually isotonic with the extracellular fluid. When there is severe disruption of the isotonicity of the solutions in a plant cell and its surrounding medium, water and dissolved substances are transferred freely into or out of the cell, which may lead to a disturbance of normal cell functions. As a rule, isotonic solutions are close to sea water in composition and concentration. For warm-blooded animals, a 0.9 percent solution of NaCl and a 4.5 percent solution of glucose are isotonic. Isotonic solutions that are close in composition, pH, buffering capacity, and other properties to blood serum are called physiological solutions (Ringer’s solution for cold-blooded animals; Ringer-Locke’s and Ringer-Tyrode’s solutions for warm-blooded animals). Compounds of high molecular weight (dextran, polyvinol) are introduced into isotonic hemosubstitutive solutions to create colloidal-osmotic pressure.