CAPILLARIES

Interstitial fluid forms the immediate environment of all cells. To keep this environment and the supply of nutrients constant, there is a ‘continuous movement of fluid through the arteriolar end of the semipermeable walls of capillaries into the interstitial fluid and removal of fluid into capillaries at their venular end.

Exchange of water and electrolytes is determined by forces called Starling forces i.e. sum of opposing hydrostatic and osmotic forces between CAPILLARY BLOOD and INTERSTITIAL FLUID.

These exchanges in systemic capillaries result in a continuous turnover and renewal of interstitial fluid.

Electrolytes (Crystalloids, e.g. Na⁺, Cl⁻, etc., SALT, MUMMIFICATION, “AGE”) in plasma and interstitial fluid also exert an osmotic pressure (OP) – this is huge (about 6000 mmHg). As the electrolyte concentration is the same on each side of the capillary membrane the crystalloid OP does not affect fluid movement. Protein is confined mainly to the plasma hence its OP does affect fluid movement.