Edema fluid is formed from the liquid components of blood; it does not contain red or white blood cells because blood cells are too big to leak through the tiny pores in the walls of the small blood vessels (capillaries), which connect our arteries and our veins. There are millions of these capillaries and they regulate the amount of tissue fluid by absorbing it and sending it back up the veins to the heart.

Normal edema results after sitting for prolonged periods of time, as often happens on a long flight. The sitting position bends and constricts our veins at the knees and hips, reducing venous return and tipping the balance in favor of tissue fluid accumulation. Walking corrects this imbalance because muscle action actively pumps blood up the veins whereas lying down enhances venous return by neutralizing gravity. Most edema accumulations are lowest when we get up in the morning and highest after prolonged standing or sitting. It may take several days of lying flat with elevated legs to completely absorb massive edema.

The commonest cause of edema is vein damage, which causes the valves inside our veins to become leaky (insufficient). These valves, which point upward, normally direct the blood-flow against gravity, towards the heart. When these valves become insufficient, some of the returning blood is pulled back by gravity and swelling ensues. Pregnancy, varicose veins, leg trauma, blood clots, knee arthritis, knee or hip operations, and advancing age are common causes of this type of venous insufficiency that is caused by vein damage and valve dysfunction.

Reduced venous flow may also result from increased pressure inside the heart, chest, and abdomen. Conditions such as—liver cirrhosis with increased belly fluid, chronic lung disease, heart failure, heart sac restriction due to scarring or fluid accumulation—all impede venous return and cause swelling. Tumors that compress the chest or abdominal veins and scarring conditions that trap the veins (fibrosis) also lead to swelling.

Low blood proteins diminish the force (osmotic pressure), which holds-in our blood fluids (plasma) and prevents them from leaking out of our blood vessels into the tissues. Conditions such as chronic kidney disease or liver failure, by diminishing our blood proteins below a critical level, increase capillary leakage and cause swelling.

Damage or obstruction of the special channels that help carry protein rich, tissue fluids back to the heart (lymphatic channels) can also result in swelling even when the veins are normal. There are numerous disorders that can cause lymphatic channel dysfunction and the lymph edema that results from such disorders does not respond to diuretics or leg elevation, as does the vein edema. It requires a specific type of repetitive massage therapy, which physically propels the lymph edema fluid out of the feet and upward toward the thighs.

Vein inflammations and clots (thrombophlebitis) can be a silent disease that may not cause swelling or pain. It is important to suspect it because it may send deadly blood clots to the lungs, causing dangerous lung attacks. The causes of leg vain inflammations are numerous but trauma, general surgeries, joint replacements, prolonged bed rest, cancers, autoimmune diseases, clotting disorders, and severe medical illnesses are common predisposing factors.

If leg vein inflammation is not treated promptly with blood thinners, the clots will continue to grow, causing more and more damage to the valves inside our veins. When enough vein valve damage occurs, the swelling that ensues becomes chronic and persists for life, regardless of treatment. This chronic venous insufficiency can be partially improved by wearing knee-high support stockings from the time one leaves one’s bed till the time one returns to it.

Today, the commonest cause of leg swelling is obesity. The excess weight impedes venous return and causes fluid retention via several mechanisms, rendering the swelling resistant to diuretics and physical therapy. Medications may also result in leg swelling either by causing fluid retention or by causing the capillaries to become more leaky or by encouraging blood clots to form inside normal veins.

Leg swelling that does not improve after normal sleep or with diuretics, especially when it affects only one leg, should be promptly investigated. Blood clots to the lungs (pulmonary embolism) should be suspected whenever sudden, unprovoked shortness of breath develops, with or without cough or chest pain.