Edema is the clinical manifestation of an accumulation of fluid within the interstitial spaces of the body. It develops when the normal balance between the flow of fluid out of capillaries and the return of fluid to the vascular space via capillary reabsorption and lymphatic flow is disrupted. Edema may be generalized, localized to a portion of the body (i.e., dependent edema), or confined to a single limb or organ (cerebral edema, pulmonary edema, etc.). Edema which accumulates in the subcutaneous space is most readily detected by patients and clinicians.
Generalized edema is almost always associated with a significant increase in body weight which is due to the retention of dietary salt and water by the kidneys. Usually, generalized edema does not become clinically detectable until body weight increases by at least 10%. Renal salt and water retention must accompany the development of generalized edema or cardiovascular collapse would occur. The acute capillary leak syndromes are examples of large translocations of fluid from the vascular into the interstitial space which occur so rapidly that renal salt retention cannot compensate. Therefore hypotension and shock are common with these disorders.
In some disorders the renal retention of salt and water is the primary process
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