How Veins Work (Part 2)
Standing up
When you stand up, the heart is raised above the feet.
This increases the pressure in the blood at the ankles because of the weight of the column of blood from the heart to feet. This is called "Gravitational" or "Hydrostatic" pressure.
This extra pressure when standing helps the blood flow from the heart to the legs through the arteries. This is why people with poor circulation to their feet feel better when they sit up, put their legs down (out of bed) or stand up.
However, this same "Gravitational" or "Hydrostatic" pressure acts AGAINST the small amount of pressure in blood in the veins.
This means that although there is enough pressure to get the blood back to the heart when lying down, there isn't enough pressure to pump the blood up hill against gravity.
Therefore, when standing still, blood cannot get back from the lower legs and feet 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. Unless they learn some tricks to pump the blood back to the heart they will faint everytime they have to stand up and still for long periods of time.
So, to stop this happening, blood needs to be pumped out of the legs and get back to the heart. This explained on the next page. The Muscle Pump »
