Definition of Edema

Figure 1 - Distribution of body fluid. Edema is defined as an increase in interstitial fluid that is clinically evident. Ordinarily, two thirds of the body water is in cells and one third is in the extracellular space. As shown here, about one quarter of the extracellular space consists of plasma volume and the remaining three quarters is composed of interstitial volume. For edema to be detected, the interstitial volume is usually expanded by several liters. Two processes must be present for edema formation: an alteration of the forces governing fluid exchange between the plasma and interstitial spaces favoring the movement of fluid into the interstitium sodium retention by the kidneys leading to a net positive sodium balance