Edema is the excess accumulation of fluid in the intercellular (between the cells) spaces. This usually represents as a swelling.
There are several causes of edema, some of the important ones are:
Increased pressure of the fluid of body (hydrostatic pressure)
Drop in oncotic pressure (absorbing capability of fluid)
Increased permeability of the vessel wall
Obstruction of lymphatic channels
Abnormal functioning of kidneys causing excess fluid retention in body
In a normal situation the interstitial fluid (intracellular fluid) is kept within normal limits by the forces of the Starling equation. The hydrostatic pressure pushes the fluid out of the vasculature and oncotic pressure tends to retain this fluid. These two forces are balanced, with a little extra fluid being drained by the lymphatic system.
Abnormality in these forces cause excess of fluid to accumulate in interstitial space, resulting in edema.
Its treatment involves eradicating the underlying cause. Some postural changes may also be helpful, e.g. edema of feet can be reduced by making the person to lie down with feet a little higher than the level of head.