Most leg swelling is caused by oedema, the accumulation of fluid within the interstitial space. Unilateral swelling usually indicates a localised pathology in either the venous or the lymphatic system, while bilateral oedema often represents generalised fluid overload combined with the effects of gravity. However, all causes of unilateral leg swelling may present bilaterally, and generalised fluid overload may present with asymmetrical (and therefore apparently unilateral) oedema. Fluid overload may be the result of cardiac failure, pulmonary hypertension (even in the absence of right ventricular failure), renal failure, hypoalbuminaemia or drugs (calcium channel blockers, glucocorticoids, mineralocorticoids, non-steroidal anti-inflammatory drugs (NSAIDs) and others).  Approximately 60% of lean body weight is water, two-thirds of which is intracellular. Most of the remaining water is found in extracellular compartments in the form of interstitial fluid; only 5% of the body’s water is in blood plasma. As noted earlier, edema is an accumulation of interstitial fluid within tissues. Increase in hydrostatic pressure are mainly caused by disorders that impair venous return. Local increases in intravascular pressure caused, for example, by deep venous thrombosis in the lower extremity can cause edema restricted to the distal portion of the affected leg. Reduction of plasma albumin concentrations leads to decreased colloidal osmotic pressure of the blood and loss of fluid from the circulation. Edema may result from lymphatic obstruction that compromises resorption of fluid from interstitial spaces.  Reference:Davidson’s Internal Medicine 23rd edition, page no.186