Blood Flow in leg veins when sitting and standing

The previous pages explain the blood flow in the circulation at rest and when lying down. Of course, this explains the structure of the arterial and venous walls, and helps with much of the understanding of the circulation.

However things change in the veins when we sit or stand up.
This is particularly important when we consider sitting in an airplane. When a DVT occurs in this situation, it is called "Economy class syndrome" or "Traveller's Thrombosis".

Here we are going to concentrate on the flow problems - as this is a major cause of the problems (see Virchow's Triad later).

However you should be aware that research is suggesting that there is a difference between air travel and coach travel. It appears there is a higher risk of DVT in a flight than the same time sitting in a coach.

This is probably due to the oxygen being "thinner" during a flight (most commercial aircraft are pressurised to 7,000 - 12,000 ft - so there is less oxygen in each breath than at ground level). This is obviously not the case in a coach.

Nevertheless, as we try to stop DVT and "Economy class syndrome" or "Traveller's Thrombosis" by encouraging movement, compression socks (travel socks) and hydration, it is useful to understand the changes that occur when sitting.

As the vein system is a Low Pressure system, whenever the heart is above the ankles, blood cannot flow smoothly back. In fact, it cannot flow up the veins at all by itself as there is nothing pushing it upwards against gravity.

Instead it collects at the ankles and, if we sit still enough for long enough, the ankles will swell. This is due to the increasing pressure in the blood at the ankles as it starts collecting here, pushing outwards against the vein walls.
Eventually the pressure reaches a sufficient level that fluid is squeezed out of the blood, through the vein walls and into the tissues. This swelling due to fluid is called oedema.  

Most people do not get swelling of the ankles when they sit still for long periods of time. The way they get around this problem by using their ankles to pump blood back to the heart. By continually moving every few minutes or so, they use the muscles of the legs to pump the blood back up to the heart.