Edema

Overview
Edema (American English), oedema or œdema (British English), formerly known as dropsy or hydropsy, is the increase of interstitial fluid in any organ &mdash; swelling. Generally, the amount of interstitial fluid is in the balance of homeostasis. Increased secretion of fluid into the interstitium or impaired removal of this fluid may cause edema.

Generation of interstitial fluid is regulated by the Starling equation of tissue fluid which states that it depends on the balance of osmotic pressure and of hydrostatic pressure which act in opposite directions across the semipermeable capillary walls. Consequently, anything that increases oncotic pressure outside blood vessels (for example inflammation), or reduces oncotic pressure in the blood (states of low plasma osmolality, for example cirrhosis) will cause edema. Increased hydrostatic pressure inside the blood vessel (for example in heart failure) will have the same effect. If the permeability of the capillary walls increases, more fluid will tend to escape out of the capillary, as can happen when there is inflammation.

Abnormal removal of interstitial fluid is caused by obstruction of the lymphatic system, for example due to pressure from a cancer or enlarged lymph nodes, destruction of lymph vessels by radiotherapy, or infiltration of the lymphatics by infection such as elephantiasis.

Organ-specific edema
Edema of specific organs (cerebral edema, pulmonary edema, macular edema, pedal edema) may also occur, each with different specific causes to peripheral edema, but all based on the same principles. Ascites is effectively edema within the peritoneal cavity, as pleural effusions are effectively edema in the pleural cavity. Causes of edema which are generalized to the whole body can cause edema in multiple organs and peripherally. For example, severe heart failure can cause peripheral edema, pulmonary edema, pleural effusions and ascites.

Common and usually harmless appearances of cutaneous edema are observed with mosquito bites and skin contact with certain plants (urticaria).

Edema may be found in the eyes after corrective surgery or procedures of that nature.

Edema in plants
Edema in plants is the extended swelling in plant organs caused primarily by an excessive accumulation of water since the cell wall is composed of flexible cllulose.