Hypoosmotic

Hypoosmotic describes a solution that has a lower osmolarity (solute concentration) than another solution with which it is compared. Osmolarity measures the total concentration of dissolved particles. When two solutions of different osmolarities are separated by a semipermeable membrane, water tends to move from the hypoosmotic side to the more concentrated, or hyperosmotic, side, potentially changing cell volume.

In physiology, hypoosmotic is a relative term. An environment or solution is hypoosmotic to a living organism’s body fluids if its osmolarity is lower than that of the organism’s internal fluids. For example, freshwater is hypoosmotic relative to the blood plasma of many aquatic animals; these organisms experience a net influx of water and must actively excrete dilute urine and uptake salts to maintain homeostasis. By contrast, seawater is hyperosmotic relative to most marine animals’ body fluids, leading to different osmoregulatory challenges.

Cellular implications follow the osmotic gradient. A cell placed in a hypoosmotic solution tends to take on water and swell, which can lead to lysis if the effect is extreme. Conversely, placing a cell in a hyperosmotic solution causes water to leave the cell, resulting in shrinkage. In medical contexts, hypoosmotic or hypotonic fluids (such as certain intravenous solutions) can cause cellular swelling and hyponatremia if administered inappropriately.

Overall, hypoosmotic is a relative descriptor used across biology and medicine to denote lower solute concentration compared with another solution, with important implications for water movement, cell volume, and organismal osmoregulation.