Edema is the abnormal accumulation of fluid in body tissues, causing swelling and discomfort. To understand edema, we must first examine normal fluid exchange between blood vessels and tissues. In healthy conditions, fluid moves between capillaries and tissue spaces in a balanced manner, maintaining proper tissue hydration.
Normal fluid balance is maintained by Starling forces. At the arterial end of capillaries, high hydrostatic pressure promotes filtration of fluid into tissues. At the venous end, lower hydrostatic pressure combined with oncotic pressure from plasma proteins promotes reabsorption back into the bloodstream. This creates a dynamic equilibrium.
Increased capillary permeability is a major mechanism of edema formation. When capillary walls are damaged by inflammation, infection, or injury, the tight junctions between endothelial cells become loose. This allows proteins to leak from blood into tissue spaces, increasing tissue oncotic pressure and drawing more fluid out of vessels, leading to tissue swelling.
Increased hydrostatic pressure is another major cause of edema. When pressure inside capillaries rises above normal, it overwhelms the opposing oncotic pressure, forcing excessive fluid filtration into tissues. This commonly occurs in heart failure where blood backs up in vessels, venous obstruction that impedes drainage, or conditions causing fluid overload in the circulatory system.