by Tarek Musslimani

Created on: June 30, 2010   Last Updated: July 04, 2010

Edema is an accumulation of fluid in the body cavities and the skin such as the pleural cavity of the lungs in which it is called hydrothorax and pleural effusion.  Also fluid can accumulate in the peritoneal cavity causing a condition which is called ascites.  In addition, fluid can accumulate in the pericardial cavity in the heart causing pericardial effusion or accumulation of fluid in the heart covering layers.

An edema which affects the whole body is called anasarca.  Fluid can also accumulate in the skin which can be characterized by the production of depression in the skin after the application of pressure to the skin surface by a finger. 

The blood in the arteries is maintained there by two opposing forces that each works in a different direction.  The first is the hydrostatic pressure which the weight of the bloods exerts on the walls of the blood vessels tend to force the blood out of the arteries while the plasma colloid osmotic pressure tend to force the blood in the opposite direction inside the arteries. 

Th forces usually cancel each other under normal conditions.  Edema can result due to increased hydrostatic pressure as occurs in increased blood pressure or hypertension due to the accumulation of fluids in the arteries or it can occur due to high blood pressure that can result due to narrowing of the arteries such as occurs in the medical condition of pheochromocytoma. 

In pheochromocytoma chronic secretion of the hormones epinephrine and nrepinephrine causes sustained elevation in blood pressure.  Edema can be localized to a special tissue or it can be generalized  in the whole body.  Example of localized edema occurs in skin inflammation in which the edema occurs due to secretion of vasodilating compounds such as histamine by the inflammatory cells causing local formation of fluid accumulation at the site of inflammation. 

Also allergic reactions such as occurs in the allergic condition of anaphylaxis.  An edema in this condition occurs in the form of skin hives or wheals as a result to the dilatation of blood vessels by the action of vasoactive amines that are released by mast cells at the site of inflammation. 

Also due to the dilatation of the blood vessels a condition of low blood pressure such as shock can occur.  This condition can be corrected by the administration of epinephrine and norepinephrine which both function

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