oedema

oedema is an excess of fluid in the interstices of body tissues. It can happen anywhere, but swelling of the ankles is the commonest example, although not all ‘thick’ ankles are oedematous. To qualify as true oedema, the swelling must be capable of indentation by pressure from the fingers, known as ‘pitting’. Also, oedema of the ankles, because it is enhanced by gravity, improves when the you ‘put your feet up’.

This ‘waterlogging’ of the tissues comes about when the balance of forces tending to move water to and fro across capillary walls is disturbed. Blood capillaries in most sites are normally leaky. Because the hydrostatic pressure is greater inside them than outside, some water from the plasma, with its dissolved particles, moves out into the tissue (interstitial) fluid which everywhere bathes the cells and fills any gaps among them. But because plasma protein molecules (of which albumin is the most important in this context) are too large to escape readily from the blood, there is also an osmotic force tending to pull water back in. Usually there is a net loss into the tissues, and this is drained away by lymph vessels to re-enter the blood stream along with the rest of the lymph from the whole body.

It follows that tissue fluid may accumulate excessively if (i) capillaries become more leaky; (ii) pressure is abnormally high inside the capillaries, or low outside them; (iii) plasma proteins are deficient or diluted; (iv) lymphatic drainage is blocked.

Leaky capillaries are caused by chemicals released in response to tissue damage — hence, for example, the swelling around a wound or sting.

Pressure inside capillaries is raised by back pressure in the veins which drain them. In the legs this can be a complication of varicose veins, when damaged valves fail to prevent distension of the veins below them. Even with normal veins, prolonged standing or sitting makes upward movement of the blood sluggish, and can result in ankle oedema. When the right side of the heart, which receives venous blood from the whole body, is not pumping effectively, pressure in all veins is raised but oedema is most evident in the dependent limbs because of the effect of gravity; likewise someone with right heart failure lying in bed will have oedema on the back and buttocks.

It is not only in the outward and visible parts of the body that oedema occurs. When venous pressure is high, fluid collects easily in the potential space of the peritoneal cavity, causing ascites: the equivalent of oedema fluid in tighter tissues.

In the lungs, a serious problem can arise if the left side of the heart stops pumping effectively, causing back pressure and excessive leakage from the lung capillaries, which can interfere with the transfer of oxygen to the blood. This pulmonary oedema can also occur at high altitude, because the hypoxia has the effect of raising the blood pressure in the lungs.

Pressure is lowered outside capillaries when the ambient pressure is lower than normal, enhancing a tendency to oedema for example during a prolonged flight (see flying, G and G suits).

The concentration of proteins in the plasma falls when they are lost in the urine in kidney disease, or if they fail to be produced by the liver. They can be diluted by excessive saline infusions. Since their contribution to the osmolarity of the blood plasma is the component which normally keeps it higher than that of tissue fluid, a decrease in plasma proteins reduces the ‘pull’ of water into the blood from the tissues. This results in generalized oedema — or ‘dropsical effusion’ as it was called in the 1820s by Richard Bright, in his classic description of chronic nephritis (Bright's disease).

Lymphatics can be blocked by cancerous or parasitic invasion or interrupted by surgery: clearing the armpit of affected tissue when breast cancer has spread to the lymph glands can result in oedema of the arm.

Sheila Jennett

See also body fluids; circulation of the blood; lymphatic system; plasma.