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....