The fluid levels in the body are tightly regulated through homeostatic mechanisms involving the kidneys and other parts of the body.

The production of interstitial fluid is regulated by the Starling equation forces. Hydrostatic pressure building up within the blood vessels causes water to flow out of the blood vessels and into the surrounding tissue. This increases the concentration of protein in the plasma, which causes more fluid to be drawn back into the vessels from the tissue.

According to the Starling equation, the rate at which fluid flows to and from the blood vessel walls depends on two important factors – the filtering forces and the permeability of the vessel wall to water.

The majority of leakage occurs in the capillaries or post capillary venules, which have membranes that are semipermeable and therefore allow water to pass more easily than protein. Gaps between the cells of the vessel wall open up and the permeability to water is increased. As the gaps increase in size, however, the membrane also becomes permeable to protein. If the forces in the Starling equation are altered, edema is more likely to occur.

Factors that contribute to the development of edema include:

• An increase in the hydrostatic pressure
• A decrease in the blood vessel oncotic pressure
• An increase in tissue oncotic pressure
• An increase in vessel wall permeability.
• Obstructed lymphatic drainage
• Water retention in tissues. For example, increased hydrostatic pressure often indicates sodium and water retention in the kidney.