Osmolarity and partitioning of fluids

The cell membrane is highly permeable to water, but less permeable to solutes. The osmotic activity of fluid surrounding cells influences water movement across the cell membrane and therefore determines the relative volumes of cellular and extracellular fluid. Electrolytes are principally responsible for the osmotic pressure of body fluids and their distribution across the cell membrane is dependent (in part) on non-diffusible anions within the cell. In systemic capillaries, the osmotic pressure exerted by plasma albumin acts in concert with hydrostatic forces to regulate water flow across the capillary wall and this process influences plasma volume. Oedema and ascites are examples of conditions that are caused by an imbalance of the forces that control transcapillary fluid movement. The osmotic pressure and volume of body fluids are remarkably constant from day to day, homeostasis being achieved by processes operating at cell and systems levels. This contribution explains the concepts of osmosis, osmotic pressure and tonicity and discusses their importance in controlling water flow across cell membranes. An outline of the processes that maintain the volume of body fluid compartments and their osmotic pressure within narrow limits is also presented.