Did you know that our bodies were not originally made to stand up? Primitive man has evolved to us standing upright. Technically, by evolutions standards, it has not really been that long that mankind has been in the upright (erect) position. We don't think about it, but walking on two legs means that our brains are much further away from our centers of gravity.

When we stand up - gravity pulls about 1/3 of our blood to the lower part of our body. Then our autonomic system responds and immediately tells our brain we are standing up and to do three things: (1) make the heart beat faster - (2) to increase the force of the heart's contractions and (3) tighten up the blood vessels in the lower part of the body to about three times it's previous tightness.

The effect of all of this is to force blood from the lower half of our body into the upper half. Then, our blood pressure regulates properly and we go about things without even knowing all this just happened with split second timing.