Tissue fluid exchange.

The fluid of the plasma in under greater mechanical hydrostatic pressure than the interstitial pressure and therefore fluid tends to leave the capillaries. However, there are proteins in the plasma but not in the interstitial fluid; these plasma proteins exert an osmotic pressure which tends to suck fluid into the capillaries.

   At the arterial end of the capillaries the mechanical, hydrostatic pressure is greater than the osmotic pressure so the balance of the forces sends fluids out into the tissues. At the venous end the hydrostatics pressure is less; the osmotic pressure overcomes it and draws fluid back into the capillaries. Normally there is more fluid leaving the capillaries then there is fluid coming back into the them. This excess is removed by the lymphatics.

 Exchange between the extracellular and intracellular fluids is also depends on osmotic pressure, but the cell membrane too has a selective permeability, allowing some substances, such as oxygen, carbon dioxide and urea to cross freely, but pumping others either in or out to maintain different concentrations in the intra- and extracellular fluid, whilst sodium is pumped out.