Edema is the presence of excess fluid in the intercellular space. It has many causes.
· Increased hydrostatic pressure causes edema in congestive heart failure (gen-eralized edema), portal hypertension, renal retention of salt and water, and venous thrombosis (local edema).
· Hypoalbuminemia and decreased colloid osmotic pressure cause edema inliver disease, nephrotic syndrome, and protein deficiency (e.g., kwashiorkor).
· Lymphatic obstruction (lymphedema) causes edema in tumor, following sur-gical removal of lymph node drainage, and in parasitic infestation (filariasis → elephantiasis).
· Increased endothelial permeability causes edema in inflammation, type Ihypersensitivity reactions, and with some drugs (e.g., bleomycin, heroin, etc.).
· Increased interstitial sodium causes edema when there is increased sodiumintake, primary hyperaldosteronism, and renal failure.
· Specialized forms of tissue swelling due to increased extracellular glycos-aminoglycans also occur, notably in pretibial myxedema and exophthalmos(Graves disease).
· Anasarca is severe generalized edema. Effusion is fluid within the body cavities.
· Transudate is edema fluid with low protein content.
· Exudate is edema fluid with high protein content and cells. Types of exudatesinclude purulent (pus), fibrinous, eosinophilic, and hemorrhagic.
· Lymphedema related to lymphatic obstruction leads to accumulation of pro-tein-rich fluid which produces a non-pitting edema.
· Glycosaminoglycan-rich edema fluid shows increased hyaluronic acid andchondroitin sulfate, and causes myxedema.
Active hyperemia versus congestion (passive hyperemia): an excessive amount of blood in a tissue or organ can accumulate secondary to vasodilatation (active, e.g., inflammation) or diminished venous outflow (passive, e.g., hepatic congestion).