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ICD- 10 R 60.9
ICD- 9 782.3
DiseasesDB 9148
MeSH D004487

Edema (American English), or oedema ( British English), formerly known as dropsy or hydropsy, is the increase of interstitial fluid in any organ — swelling. Generally, the amount of interstitial fluid is determined by the balance of fluid homeostasis, and increased secretion of fluid into the interstitium or impaired removal of this fluid may cause edema.

Edema has five pathophysiologic causes. It can be due to increased hydrostatic pressure, reduced oncotic pressure, lymphatic obstruction, sodium retention, or inflammation.


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.

There are two types: exudate and transudate

Organ-specific edema

Edema of specific organs ( cerebral edema, pulmonary edema, macular 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 cellulose.

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