Page 13, hemodynamics, Dr. D. Penney

Effects of Orthostasis (continued..)

Perfusion pressure on the other hand is unchanged, i.e. still 90 mmHg, the same as when the individual was lying down. The same thing has occurred at the level of the head, however in an opposite manner. Pressure is now subtracted from the pressure generated by the heart. For example, on the arterial side the pressure is now only 51 mmHg because 44 mmHg was subtracted from 100. On the venous side 44 mmHg is also subtracted, from 5, yielding -39 mmHg. Therefore the pressure at the head level is actually negative on the venous side. This has the potential for collapsing flexible vessels, especially those such as the jugular vein, which are not protected by the bony cranium.

This can also occur in other situations, as when you raise your arm above your head to screw in a light bulb. Numbness and pain commonly result. Why does this happen? It is said that the heart can't raise the blood to that level, but this is untrue. Since the path is a complete circuit, the blood is pulled up by the blood going down, as in a common siphon. The reason has to do with the fact that thin-walled blood vessels in the hand tend to collapse as transmural pressure falls, raising the resistance to flow. Thus there is a subtle but important distinction.

Finally, since the circulatory system is a closed loop the heart does not do work against gravity. Instead it does work against friction, i.e. dissipates energy against resistance within the circulation, due to the blood components bumping against one another and the walls of vessels. [SEE MISCONCEPTIONS 13 - 14]

See Summary of gravity effects and non-effects.