The cardiovascular system is affected by three types of pressure. These are:
- Heamodynamic - caused by the contraction of the heart, which would give the view of pressure being higher closer to the aorta as in Kevin's answer and my first thought too.
- Kinetic - caused by the action of skeletal muscles in movement in squeezing primarily veins to return blood to the heart.
- Hydrostatic - the combination of fluid density and gravity that leads to pressure on the vessel endothelium. The pressure at a given point is proportional to the volume of fluid above it. This means that pressure is highest at the bottom of the vessel.
Arteries can of course be treated as one long tube due to their lack of valves, therefore the hydrostatic pressure could be significant in contributing to total blood pressure. If hydrostatic pressure is more significant than heamodynamic pressure then this would explain why pressure would be higher the more distal the artery is - when standing there is simply more blood pushing down on it.
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