Together, blood plasma and interstitial fluid make up the extracellular fluid (ECF). Plasma constitutes 20%, while interstitial fluid constitutes 80% of the ECF. The distribution of extracellular fluid between these two compartments is determined by the balance between two opposing forces: hydrostatic pressure and osmotic pressure.
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
1. Reduced concentration of plasma proteins. When the concentration of plasma proteins drops, the osmotic potential of plasma drops, thus less interstitial fluid is absorbed into the capillaries. The rate of filtration, however, remain unchanged. Therefore, the ratio of filtration to absorption increases, leading to a build up of interstitial fluid. Any condition that would lead to a reduction in plasma proteins could potentially cause edema. Examples of conditions that reduce plasma proteins include: