How veins work
Lying Down
The legs, like any other part of the body, need a
blood supply. Blood from the heart takes oxygen and food to the tissues in blood vessels called arteries. Blood then returns to the heart through the
veins. The blood pumps the blood through the arteries at high pressure, and so it moves quickly down the leg and into the tissues. However getting blood back from the legs isn't so easy. There is no "heart" in the leg and so the body needs a different way to get blood back to the heart. After the blood has been through the tissues, delivering oxygen and food to keep the cells alive, it collects together in the veins. When you are
lying down, there is just enough pressure left in the blood to get flow back to the heart. However, as soon as you sit up or stand, things change.
Standing up
When you stand up, the heart is raised above the feet. This increases the
pressure in the blood because of the weight of the column of blood from the heart to feet. This is called gravitational or
hydrostatic pressure. This extra pressure from standing helps the blood to flow from the heart to the legs, in the arteries. But, this same gravitational or hydrostatic pressure means that there is not enough pressure in the blood in the veins to get back to the heart. This is the reason that that people who have to stand still (such as soldiers on parade) faint. They cannot get the blood out of their legs and so they cannot get it pumped to their brains, and so they faint. So, to stop this happening, blood needs to be pumped out of the legs and get back to the heart.... Credited to Mark Whiteley, The Whiteley Clinic
