2020 Nov 20. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan–.
ABSTRACT
Fluid compartments in the human body are divided between the intracellular and extracellular spaces. The extracellular space constitutes about one-third of total body water, which is further divided into intravascular plasma volume (25%) and the extravascular interstitial space (75%). The fluid balance between these compartments is maintained by hydrostatic pressures and oncotic pressures described by Starling. The other two factors that play an important role in fluid balance is vessel wall permeability and lymphatic system. The lymphatic system collects fluid and filtered proteins from the interstitial space and returns that back to the vasculature. Any disturbance in this delicate homeostasis that results in net filtration out of the vascular space or impaired return of fluid by lymphatics leads to the accumulation of fluid in the interstitial space that is called edema. Edema can affect any part of the body and ranges from local swelling to full-blown anasarca, depending on the underlying pathology. A classic example of local swelling is an insect bite. An example of anasarca can be seen in nephrotic syndrome.
Edema, other than localized edema, does not become clinically apparent until the interstitial volume has increased by 2.5 to 3 liters because the tissues constituting the interstitium can easily accommodate several liters of fluid. Therefore, a patient's weight may increase by nearly 10% before pitting edema is evident.