Pathogenesis:

Oedema is result of increase in forces that tends to move fluids from intravascular compartment to interstitial spaces. It is usually due to disturbances in fluid exchange.

Normally fluids are maintained at balance by two opposing forces; vascular hydrostatic pressure and plasma osmotic pressure

Fluids usually moves from intravascular compartment to interstitial space at arteriolar end of microcirculation under influence of hydrostatic pressure

Fluid returns to intravascular compartment at venular end under influence of osmotic pressure

Balance between these forces results in exchange of small amount of fluid in interstitial space which is usually cleared through lymphatics and no edema occurs

Edema results when there is increased hydrostatic pressure or fall in plasma osmotic pressure or obstruction to blood flow