Causes of Oedema

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Blood is carried from the heart in arteries and returns to it in veins. Between the arteries and the veins are networks of tiny blood vessels called capillaries. The capillary walls have microscopic gaps through which water from the blood can pass. Normally the red blood cells and the large protein molecules in the blood canâ€™t get through these gaps.

If a membrane, such as the capillary wall, through which water can pass, separates two liquids - in this case blood and tissue fluid - water will flow from the liquid with the lesser amount of substance dissolved in it into the stronger solution.

When the balance of strength is equal on the two sides, no water flows. This is called an equal osmotic pressure. Dissolved protein is one of the most important substances that increases the osmotic pressure in the blood. In healthy people there is plenty of protein in the blood.

If blood protein levels drop significantly, there will be little or nothing to draw the water back into the blood from the tissue spaces through the capillary walls. The result is an accumulation of fluid in the tissue - also called oedema.

Generalised oedema may result from:

Kidney disease, such as nephrotic syndrome where protein loss in the urine is considerable.
Acute glomerulonephritis which also causes protein loss.
Liver disease, such as cirrhosis, in which the production of protein is reduced.
Starvation, in which the intake of protein is inadequate.

Problems with osmotic pressure are not the only causes of oedema. In women who have had breast cancer surgery involving the removals of the lymph nodes in the armpit, excess tissue fluid can no longer return via the lymph system. The result can be a condition of arm oedema called lymphoedema.

Anything that increases the leakiness (permeability) of capillaries can cause oedema. This can result from physical or chemical injury, from burns or from hormonal causes, as in premenstrual syndrome. The exact mechanism in this last case is uncertain, but some experts believe it is due to increased leakage of protein into the tissue fluid.

Oedema also occurs in heart failure, when the heart canâ€™t pump blood round fast enough to clear fluid from the tissues. There is a rise in back-pressure in the veins and an accumulation of fluid.

Oedema of the ankles is quite common, especially in hot weather, at high altitude, and if movement is restricted. This swelling is usually â€˜dependentâ€™, meaning that the pressure of fluid in the legs, from prolonged standing or sitting, pushes it into the surrounding tissues. Rotating the ankles regularly is recommended, as is keeping the feet up where possible. The use of support stockings will also help.

Generalised oedema causes weight increase. In mild cases there may be no more effect than this, but more severe oedema can cause difficulty in breathing. If the lung oedema becomes severe, the situation may become life-threatening.