Isotonic literally translates to equal tension. The term is used in several different contexts.

An Isotonic solution is similar in its chemical composition (and most importantly its concentration) to the human body’s blood, plasma and tears. All fluids in the body have a certain concentration, referred to as osmotic pressure. The body’s common osmotic pressure, which is isotonic, allows a consistent maintenance of body tissues. In order for a substance to be better absorbed and used in the body’s metabolism, it must be transported in an isotonic state.

An isotonic solution that mimicks the osmolality of blood for humans and many other mammals consists of 0.9% (9 g/L) sodium chloride in water and is often referred to as saline. Saline is generally administered via an intravenous drip. Red blood cells will not change their form in such an isotonic medium while they would swell and even burst in pure water.

This term is also used in relation to a sports drink with added salts mimicking the osmolality of blood. This property is claimed by their advertising campaign to increase physical endurance. The statistically insignificant (6 rowers) scientific study often quoted, however, attributes the 10% increase in endurance over pure water to the higher carbohydrate content.