Isotonic Definition

Isotonic is a term used to describe solutions and chemistry and, sometimes, muscles in human biology.

In chemistry, a solution is said to be isotonic when it has the same concentration of solutes as another solution across a semipermeable membrane.

The use of isotonic in human anatomy is used more rarely. However, it describes muscles that have the same tone as “normal” comparisons, or interior-organ fluid levels that are the same as the fluid levels outside that organ’s outer wall or exterior membrane.

Examples of Isotonic

Alcohol Proof

It’s no secret that a shot (appx. 1 oz.) of beer produces affects us less, physically, than a shot of whiskey. This is because whiskey has more alcohol content by volume, or a higher proof, than beer. 

When expressed in scientific terms, whiskey has a greater concentration of ethanol, the molecule responsible for alcohol’s effect, than beer. More simply, whiskey is hypertonic to beer and beer, conversely, is hypotonic to whiskey.

The difference in proofs between different types of alcoholic beverage is not always so severe. For example, there are several low-alcohol varieties of wine that have the same proof as beer. If a low-alcohol wine has the same proof as beer, it is isotonic to, or having the same ethanol concentration as, beer.

Relieving Dehydration

A saline solution consists of water mixed with sodium chloride (NaCl). Saline solutions are often administered to hospital patients experiencing severe dehydration, or who cannot achieve adequate fluid intake through traditional consumption.

An isotonic saline solution is most commonly administered to patients experiencing extreme dehydration. Unlike hypertonic and hypotonic saline solutions, isotonic saline solutions have the same concentration of NaCl as cellular fluids in the human body.

Because they have the same concentration of NaCl as the isotonic saline solution, cells do not shrink by losing more water, nor do they risk edema, or swelling, with excess water. By maintaining consistency of cellular size, isotonic saline solutions reduce the risk of potentially-fatal health problems, like heart failure, associated with extreme rehydration.

Isotonic Exercises

When a physical activity is considered isotonic, it involves a lifting phase and a lowering phase. Isotonic muscles, on the other hand, have the same muscle tone.

Bicep curls are an isotonic activity, because they involve raising the arm and lowering it, to build muscle. However, they may not contribute to isotonic muscles, because the person performing the exercise may dedicate more focus to the right or the left bicep.

Push-ups, on the other hand are an isotonic activity that contribute to isotonic muscles. Push-ups are isotonic because they involve raising the body and lowering it in a plank position. They produce isotonic muscles because they work both sides of the body equally, when performed correctly.

To clarify, push-ups are not isotonic in the sense that they are of the same concentration or tone as another exercise. However, they produce isotonic muscles, which have the same tone.

Related Biology Terms

  • Hypertonic – A solution having a greater concentration of a solute than a solution across a semipermeable membrane, or a muscle having greater tone than another, like muscle.
  • Hypotonic – A solution having a lower concentration of a solute than a solution across a semipermeable membrane, or a muscle having less tone than another, like muscle.

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