Leg Oedema/ Swelling

prof. Usman Jaffer

This topic talks about peripheral oedema or leg swelling and the potential causes of why it happens. Leg swelling generally happens as a result of accumulation of excess fluid in the leg beneath the skin between the cells (called the interstitial space). It can be in one leg or both. Oedema can be generalised or affecting the whole body or just the legs. This article talk about oedema affecting the legs only. Leg oedema can be separated medically into 'pitting' and 'non-pitting'. The basic difference is that in 'pitting oedema' if you press continuously on a bony bit of the leg, a dip occurs where you press. This fills up again slowly when you release the pressure.

In order to understand why excess build build up in the leg, we need to understand the forces which interact to cause fluid to accumulate as well as the system normally in place to reabsorb excess fluid that accumulates (lymphatic system). Any disturbance in these forces or in the lymphatic system can cause an abnormal accumulation of fluid and result in leg swelling.

How does fluid form in the Leg tissue spaces ?

Fluid from the blood tends to leak across capillaries (which are the tiny blood vessels in us where nutrient and oxygen exchange happens in our body.

The balance of leak across the capillaries is due to a number of forces acting in either direction. The main forces are the pressure of the fluid trying to leak out into the relatively low pressure in the interstitial space between your cells. The balancing force keeping fluid in the capillaries is the 'suction' force caused by proteins in the blood pulling the fluid back in.

Lastly the relative leakiness or permeability of the capillaries also determines how much fluid will leak out of the capillaries into the interstitial space.

The blood pressure tends to favour fluid leaking out into the tissue spaces, the proteins in the blood tend to provide a force which 'sucks' back fluid into the blood vessels.

The lymphatic system - absorbs the fluid that naturally leaks out of the capillaries

There is a net slight leakiness of the capillaries to fluid normally.

Fluid that does leak out gets absorbed into the lymphatic system. These are tiny blood vessels like the capillaries but their job is to take tissue fluid/ interstitial fluid back to the circulation so it does not accumulate in the tissues and lead to swelling.