The cell membranes are more or less semipermeable membranes, since the lipid layer is less permeable to charged molecules such as ions and proteins. The osmotic pressure of the extracellular fluid depends on its content of protein and salts and corresponds approximately to that of a 0.9% solution of NaCl. Such a physiological salt solution is isotonic (that is, it is in osmotic equilibrium with the cell). Consequently, cells bathed in hypertonic (more concentrated) solutions lose water and shrink, while in hypotonic (less concentrated) solutions they take up water and swell. The organism therefore endeavors by special regulatory mecha nisms to keep the osmotic pressure of the extracellular fluid as constant as possible. Because of the good permeability of cell membranes to water, these mechanisms lead to a more or less constant osmotic pressure in the cell interior.
Thetermcolloidosmoticpressureisusedwhen,for instance, proteins for which the capillary wall is impermeable are dissolved in the blood plasma and not in the interstitial fluid. They create an osmotic pressure difference of about 25mmHg (3.3 kPa) between the interstitial fluid and the capillary space. Thiswould lead to amovement of fluid into the vessels if the hydrostatic blood pressure active inside the blood vessels were not opposed to it. Since the blood pressure at the beginning of the capillaries (37 mmHg)is greaterthanthe colloidosmotic pressure, fluid is actuallyfiltered into the interstitial space.
Development of osmotic pressure in a semipermeable membrane.