What does isotonic mean?
Isotonic is a technical adjective used most often in biology and medicine to describe a solution with the same osmotic pressure as the fluid surrounding it. Such a solution is called isotonic when it does not cause cells to gain water or lose it through osmosis; isotonic saline is a familiar example. In physiology, the same word describes a muscle contraction in which the muscle shortens while maintaining relatively constant tension, as in the lifting phase of a biceps curl. The word comes from the same Greek root as tone and tonic, referring to tension or stretching. Although it is not everyday vocabulary, it is useful in describing the conditions that allow cells and tissues to remain stable and in describing controlled muscular movement. It should not be confused with hypertonic or hypotonic, which describe solutions whose osmotic pressures are higher or lower, respectively.
adjectiveHaving the same osmotic pressure as another solution. In physiology, relating to muscle contraction in which tension remains constant while the muscle shortens.
- Having the same osmotic pressure as a reference solution, especially one that prevents cells from gaining or losing water.
- Relating to a muscle contraction in which tension remains constant as the muscle shortens.
"The cells were placed in an isotonic solution to prevent osmotic damage."
"The cells were placed in an isotonic solution to prevent osmotic damage."
"The trainer demonstrated an isotonic contraction of the biceps."
Isotonic means “equal-toned” in the laboratory: a solution that neither draws water into cells nor pushes water out of them.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does isotonic come from?
Isotonic was formed in the 19th century from the Greek prefix iso-, meaning “equal,” and tonic, which derives from Greek tonos, meaning “tension” or “stretch.” The underlying sense is therefore “having equal tension,” a meaning that developed in both scientific and physiological contexts. Related English words include tone, tonic, tonus, and tension, all reflecting the same Greek source.
Related word forms
How isotonic is actually used
The term is chiefly used in biology, physiology, medicine, and biochemistry. In exercise science, it describes the shortening phase of a muscle contraction under constant tension.
Easily confused with isotonic
An isotonic solution has the same osmotic pressure as the fluid surrounding it, whereas a hypertonic solution has a higher osmotic pressure.
An isotonic solution has the same osmotic pressure as the surrounding fluid, whereas a hypotonic solution has a lower osmotic pressure.