The beating of the heart generates hydrostatic pressure, which, in turn, causes bulk flow of fluid from plasma to interstitial fluid through walls of the capillaries. In other words, the pressure in the system forces plasma to filter out into the interstitial compartment. The composition of the interstitial fluid and the plasma is essentially the same except that plasma also contains plasma proteins not found in the interstitial fluid. Because of the presence of plasma proteins, the plasma has a higher solute concentration than does the interstitial fluid. Consequently, osmotic pressure causes interstitial fluid to be absorbed into the plasma compartment. In other words, the plasma proteins drive the reabsorption of water back into the capillaries via osmosis.
If excess fluid cannot be returned to the blood stream then interstitial fluid builds up, leading to swelling of the tissues with fluid, this is called oedema.
Causes of Oedema
a) Kidney disease can result in the loss of plasma proteins in the urine.
b) Liver disease can decrease the synthesis of plasma proteins.
c) A protein-deficient diet will decrease plasma proteins.
d) Severe burns result in a loss of plasma proteins (albumin) at the burn site
3. Increase in venous pressure. If venous pressure is increased then blood dams up in the upstream capillary bed, resulting in excess filtration. Examples of this condition include: