Isotoonic

Isotonic refers to a state of equal osmotic pressure. In biology and medicine, this term is most commonly used to describe solutions and their effects on cells, particularly red blood cells.

An isotonic solution is one that has the same concentration of solutes as another solution, typically the intracellular fluid of a cell. When a cell is placed in an isotonic solution, there is no net movement of water across the cell membrane. This is because the rate at which water enters the cell is equal to the rate at which water leaves the cell. As a result, the cell maintains its normal shape and volume.

For example, a 0.9% sodium chloride solution is considered isotonic to human blood. This is why intravenous (IV) fluids used for hydration or medication delivery are often isotonic. Administering an isotonic solution helps to prevent damage to red blood cells, which could occur if they were placed in a hypotonic (lower solute concentration) or hypertonic (higher solute concentration) solution. In a hypotonic solution, water would rush into the cell, causing it to swell and potentially burst (hemolysis). In a hypertonic solution, water would leave the cell, causing it to shrink and shrivel (crenation).

The concept of isotonicity is also relevant in other fields, such as food science, where it relates to the osmotic pressure of food products. However, its primary application remains in biological and medical contexts.