(1) A property of a solution that depends on the osmotic force exerted across the membrane as influenced by the differing concentrations of solutes in and out of the cell; the osmotic pressure or tension of a solution, as in the cells would swell or shrink depending on the tonicity of the environment

(2) A state of normal firmness or functional readiness of body tissues or organs; acondition of sustained partial contraction of resting or relaxed muscles


In biology, tonicity pertains to two definitions. The first one is associated with the osmotic pressure exerted upon a membrane and the other is about tone or tension (in a muscle or an organ). Relevant terms include isotonicity, hypertonicity, and hypotonicity.

At the cellular level, isotonicity refers to a condition or property of a solution in which its solute concentration is the same as the solute concentration of another solution with which it is compared. Hypertonicity is a property of a solution wherein the amount of solutes is higher than that of another solution. In contrast, hypotonicity pertains to a property of a solution with a comparatively lower concentration of solutes relative to the amount of solutes in another solution.

At the tissue level, isotonicity is a condition wherein the muscle remains to be in a relatively constant tension while its length changes. Hypotonicity of a muscle means a lesser degree of tone or tension as its length changes. This is in contrast to a hypertonic muscle that has a greater degree of tone or tension.

Word origin: Greek tonos (“tone")

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