Clinical meaning
Edema is the abnormal accumulation of fluid in the interstitial spaces (the tissue compartment outside of blood vessels and cells), resulting in visible and palpable swelling. Understanding edema formation requires knowledge of Starling forces, the four pressures that govern fluid movement across capillary membranes. Capillary hydrostatic pressure pushes fluid OUT of capillaries into the interstitium (normally 32 mmHg at the arteriolar end and 15 mmHg at the venular end). Interstitial hydrostatic pressure pushes fluid from the interstitium back INTO capillaries (normally slightly negative, approximately -3 mmHg). Plasma oncotic pressure (also called colloid osmotic pressure) pulls fluid INTO capillaries from the interstitium (normally 25 mmHg); this is primarily determined by plasma albumin concentration. Interstitial oncotic pressure pulls fluid OUT of capillaries into the interstitium (normally 8 mmHg). Edema develops when there is an imbalance in these forces: increased capillary hydrostatic pressure (as in heart failure or venous obstruction), decreased plasma oncotic pressure (as in hypoalbuminemia from liver disease, nephrotic syndrome, or malnutrition), increased capillary permeability (as in inflammation, burns, allergic reactions, or sepsis), or lymphatic obstruction (as in lymphedema from surgery,...