isosmotic

Isosmotic is a descriptor used in physiology and biochemistry to denote two solutions that have the same osmotic concentration, measured as osmolarity (osm/L) or osmolality (osm/kg). The osmolarity of a solution is determined by the total concentration of all solute particles that contribute to osmotic pressure, including electrolytes and non-electrolytes. In humans, blood plasma is typically about 290 to 300 mOsm/L, and solutions with about this osmolarity are described as isosmotic relative to plasma.

Isosmotic does not always imply that there will be no movement of water across a membrane, because whether water moves depends on the membrane’s permeability to solutes. If the membrane is permeable to many solutes, an isosmotic solution may still alter cell volume. Conversely, two solutions can be isosmotic but not isotonic if one contains permeant solutes that equilibrate across the membrane.

The term is commonly used when selecting intravenous fluids, dialysis fluids, or experimental buffers to match the osmolarity of body fluids, minimizing osmotic stress on cells. In clinical practice, the term isotonic is often used to describe fluids that do not cause net water movement across cell membranes, which largely overlaps with the concept of isosmotic but is not identical.

See also: Osmolarity, Osmolality, Isotonic solution, Hyperosmotic solution, Hypoosmotic solution.