Normally the body maintains a balance of fluid in tissues by ensuring that the same of amount of water entering the body also leaves it. The circulatory system transports fluid within the body via its network of blood vessels. Oxygenated blood is circulated under higher pressure through the arteries propelled by the contractions of the heart muscle. The fluid contains oxygen and nutrients needed by the cells and these necessary supplies are transported to the tissues through the capillaries. The venous blood on the other side of the tissue capillary is depleted of oxygen and nutrients and contains by products of cellular metabolism. It is at the level of the capillary and in the small veins where most of the liquid (water and protein) leak out into the tissues causing edema. The higher the venous pressure the more fluid will leak through the vessels into the tissues. In cases of edema, either too much fluid moves from the blood vessels into the tissues, or not enough fluid moves from the tissues back into the blood vessels. This fluid imbalance can cause mild to severe swelling in one or more parts of the body.
Lower extremity edema can be broken down into two main categories, bilateral or unilateral. Common causes of bilateral edema include systemic (body wide) conditions such as congestive heart failure, kidney failure, malnutrition, liver cirrhosis, excess salt intake and in rare cases, thyroid disease. Certain medications can lead to salt retention resulting in reversible edema when the medication is discontinued. General classes of medications contributing to edema include steroids, hormone replacements, non-steroidal anti-inflammatory drugs (NSAIDs), and some blood pressure medications. Other causes of bilateral leg edema include sitting or standing for extended periods without contracting the muscles (long airplane or car rides, wheel chair bound patients) and perhaps the most dangerous and life threatening, deep vein thrombosis.
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