The state of being hypotonic, i.e. having lesser degree of tone or tension


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.

In general, hypotonicity is a condition characterized by the presence of a lesser degree of tone or tension. In cellular level, hypotonicity may pertain to a property of a solution with a comparatively lower concentration of solutes relative to the amount of solutes in another solution. Also, a solution is described as hypotonic when the other solution being compared with has less osmotic pressure in a fluid compared to another fluid. Solutions that have different tonicities will result in a net flow of water across the cell membrane. For example, blood serum that is hypotonic to a physiologic salt solution would result in the net movement of water molecules towards the area with less water concentration (or higher solute concentration) (i.e. physiologic salt solution).

In the tissue level, hypotonicity in a muscle would imply 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. A muscle whose tension remains relatively constant is described to be isotonic.

Word origin: hypo– ("under") + Greek tonos ("tension")


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