The body's balance of salt is usually well-regulated. Most people can consume 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 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 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 remove salt in the urine is limited. In both conditions, the excess salt causes the body to retain more water, which then leaks into the interstitial tissue spaces, where it appears as edema.
People with edema 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 diuretics, this marked restriction in dietary salt intake is generally less strict. These diuretics work by blocking the reabsorption and retention of salt by the kidneys, thereby increasing the amount of salt and water eliminated in the urine.