Hypotonic (osmotic solution)

A solution with a lower concentration of osmotically active solutes than a comparison solution. Across a semipermeable membrane, water moves from it toward the more concentrated solution.

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Osmosis: Net movement of water across a selectively permeable membrane toward the side with higher effective solute concentration. It is the water movement a hypotonic solution drives across a semipermeable membrane.
Solute: A substance dissolved in a solvent to form a solution. Hypotonicity compares the concentrations of dissolved particles across a membrane.
Cell swelling: An increase in cell volume caused by net water entry. Water entering cells exposed to a hypotonic extracellular solution can enlarge them.
Hypertonic solution: A solution that causes a cell to lose water and shrink relative to its surroundings. It reverses the usual direction of cell-volume change associated with a hypotonic exterior.
Osmotic pressure: The pressure required to stop net solvent movement across a semipermeable membrane. It quantifies the force associated with the concentration difference between solutions.
Molar concentration: The amount of a substance in moles per unit volume of solution. It is one way to express the solute concentration used in osmotic comparisons.
Hemolysis: The rupture of red blood cells and release of their contents into surrounding fluid. Red blood cells can rupture when water influx in a sufficiently hypotonic medium exceeds membrane capacity.
Hypo-osmotic solution: A solution with lower total osmolarity than a comparison solution, regardless of solute permeability. It describes total particle concentration, which may not predict lasting cell-volume change.
Water potential: The potential energy of water relative to a reference state, determining the direction water moves. Water moves from higher to lower water potential when solutions are separated by a suitable membrane.
Tonicity: The lasting effect of a solution on cell volume, determined by nonpenetrating solutes. It distinguishes cell-volume effects from osmolarity when solutes cross membranes.