adjective

isotonic

eye-suh-TON-ik

Isotonic (adjective) means of a solution, having the same concentration of dissolved particles as the fluid it is compared with, usually blood or the inside of a cell, so that water does not move across the cell membrane in either direction. Example: “Saline at 0.9 per cent is isotonic with human blood.”

adjective

Of a solution, having the same concentration of dissolved particles as the fluid it is compared with, usually blood or the inside of a cell, so that water does not move across the cell membrane in either direction.

"Saline at 0.9 per cent is isotonic with human blood."

"Red blood cells keep their shape in an isotonic solution."

Of a sports drink, formulated with roughly the same particle concentration as body fluids so that it is absorbed quickly while replacing fluid, salts and some carbohydrate.

"She switched to an isotonic drink for runs over an hour."

Of muscle contraction, occurring while the muscle changes length against a constant load, as in lifting a weight through a range of motion.

"Bicep curls are an isotonic exercise; a plank is isometric."

How to Use Isotonic

Learner’s notes

In plain EnglishHaving the same strength or concentration as the fluid it is being compared to.

When to use it

Technical, used in biology, medicine and sports science, plus drink marketing.

Memory tip

Remember iso- means equal: isotonic solutions are equal in strength, and isobars are lines of equal pressure.

Trace the full origin ↓

Easily confused with

isotonic vs isometric isotonic vs hypotonic isotonic vs hypertonic

Common pairings

isotonic solution isotonic saline isotonic sports drink isotonic contraction

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Fill the Gap

Can you complete this real example?

Saline at 0.9 per cent is _____ with human blood.

Etymology

From Greek isos 'equal' plus tonos 'tension, tone'.

Origin: Greek

Antonyms

hypertonic hypotonic isometric

Rhymes for isotonic

tonic ionic sonic ironic bionic iconic laconic Masonic chronic moronic demonic bubonic

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Frequently Asked Questions

What is the difference between isotonic, hypotonic and hypertonic?

The three describe a solution compared with the fluid inside a cell. Isotonic has the same particle concentration, so the cell stays as it is; hypotonic is more dilute, so water flows in and the cell swells; hypertonic is more concentrated, so water flows out and the cell shrinks.

What is the difference between isotonic and isometric exercise?

Isotonic exercise moves a joint through a range while the muscle shortens or lengthens under a steady load, like a squat or a press. Isometric exercise holds a fixed position with no change in muscle length, like a wall sit or a plank.

Are isotonic drinks better than water?

For everyday activity and sessions under about an hour, water is generally enough. Isotonic drinks are aimed at longer or hotter efforts, where the added sodium and carbohydrate help with fluid absorption and energy.