Tonicity

Tonicity is the ability of a solution to cause water movement across a semipermeable membrane, thereby changing the volume of a cell or compartment. It depends on the concentration of non-penetrating (impermeant) solutes in the solution relative to those inside the cell and across a given membrane. Because some solutes can cross the membrane, tonicity is not the same as osmolarity or osmolality, which measure total solute concentration regardless of membrane permeability.

A key concept is that tonicity is defined with respect to a specific cell membrane and its permeability to solutes. Non-penetrating solutes contribute to tonicity, while penetrating solutes may alter the overall osmolarity without necessarily changing cell volume.

Solutions are described as isotonic, hypotonic, or hypertonic relative to the intracellular fluid. Isotonic solutions have the same effective osmolality as the cell and induce little net water movement. Hypotonic solutions have lower effective osmolality, causing water to enter cells and potentially lead to swelling. Hypertonic solutions have higher effective osmolality, drawing water out of cells and causing shrinkage. Examples include isotonic saline (0.9% NaCl) as isotonic to blood plasma; hypertonic saline (e.g., 3% NaCl) to draw water from cells; and hypotonic solutions (e.g., 0.45% NaCl) that can cause cells to swell.

In clinical practice, tonicity guides fluid therapy and its effects on cells in tissues, including the brain. The concept emphasizes the functional impact of solutes that do not freely cross membranes, rather than merely measuring total solute concentration.