Edema (cont.)
What causes pitting edema?
Edema is caused by either systemic diseases, that is, diseases that affect the various organ systems of the body, or by local conditions involving just the affected extremities. The most common systemic diseases associated with edema involve the heart, liver, and kidneys. In these diseases, edema occurs primarily because of the body's retention of too much salt (sodium chloride). The excess salt causes the body to retain water. This water then leaks into the interstitial tissue spaces, where it appears as edema.
The most common local conditions that cause edema are varicose veins and thrombophlebitis (inflammation of the veins) of the deep veins of the legs. These conditions can cause inadequate pumping of the blood by the veins (venous insufficiency). The resulting increased back-pressure in the veins forces fluid stay in the extremities (especially the ankles and feet). The excess fluid then leaks into the interstitial tissue spaces, causing edema.
How does salt intake affect edema?
The body's balance of salt is usually well-regulated. A normal person can
consume small or large quantities of salt in the diet without concern for
developing salt depletion or retention. The intake of salt is determined by
dietary patterns and the removal of salt from the body is accomplished by the
kidneys. The kidneys have a great capacity to control the amount of salt in the
body by changing the amount of salt eliminated (excreted) in the urine. The
amount of salt excreted by the kidneys is regulated by hormonal and physical
factors that signal whether retention or removal of salt by the kidneys is
necessary.
If the blood flow to the kidneys is decreased by an underlying condition such as heart failure, the kidneys react by retaining salt. This salt retention occurs because the kidneys perceive that the body needs more fluid to compensate for the decreased blood flow. If the patient has a
kidney disease that impairs the function of the kidneys, the ability to excrete salt in the urine is limited. In both conditions, the amount of salt in the body increases, which causes the patient to retain water and develop edema.
Patients experiencing a disturbance in their ability to normally excrete salt may need to either be placed on a diet limited in salt and/or given diuretic medications (water pills). In the past, patients with diseases associated with edema were placed on diets very restricted in salt intake. With the development of new and very potent diuretic agents, this marked restriction in dietary salt intake is generally no longer necessary. These diuretics work by blocking the reabsorption and retention of salt by the kidneys, thereby increasing the amount of salt and water that is eliminated in the urine.
Next: Why does a patient with heart disease retain fluid? »
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