Peripheral Edema

Excerpt

Fluid compartments in the human body are divided into intracellular and extracellular spaces. The extracellular space accounts for approximately one-third of total body water and is further divided into intravascular plasma volume (25%) and the extravascular interstitial space (75%). Fluid balance between these compartments is governed by hydrostatic and oncotic pressures, as described by Starling's law. Hydrostatic pressure drives fluid out of the capillaries, while oncotic pressure, primarily maintained by proteins such as albumin, pulls fluid back into the capillaries, ensuring proper fluid distribution.

Additional key factors influencing fluid balance include vessel wall permeability and the lymphatic system, which has a vital role in returning excess interstitial fluid and proteins to the circulation. Disruptions in this system—whether due to increased capillary filtration or impaired lymphatic drainage—can result in fluid accumulation, leading to edema. Edema may present as localized swelling, such as from an insect bite, or as widespread fluid retention (anasarca), as seen in conditions such as nephrotic syndrome.

In contrast to localized edema, generalized edema typically does not become clinically apparent until the interstitial volume increases by 2.5 to 3 liters. This is because the tissues in the interstitial space are highly flexible and can accommodate several liters of fluid without visible swelling. Consequently, a patient's weight may increase by nearly 10% before pitting edema is evident, indicating that the body's capacity to manage excess fluid has been exceeded.