Why do we feel dizzy sometimes when we stand up too fast.

I believe the answer posted above needs some
clarification. It is true that when we suddenly get up from a resting or lying down
position there is a sudden rush of blood in lower limbs and less blood in upper parts of
the body including the brain. And the dizziness we may sometimes feel when we stand up
too fast, is because of the reduced blood supply to the brain. But I would like to
clarify that this is more because of action of gravity rather than because of heart
pumping more blood to lower limbs as compared to
brain.

When we suddenly stand up from lying down position,
more blood tends to collect in lower limbs due to gravitational pressure, and the return
flow of blood to the heart is reduced. This causes a short term reduction of blood
pumped by the heart. This in turn leads to reduced blood supply to brain. However the
body quickly adjusts to the changed situation by increasing blood pressure so that
adequate supply of blood to brain is restored. In this connection it is worthwhile
noting that general blood pressure level while lying down is lower than that while in
sitting up or standing position. This is because of the need to overcome greater
gravitational pressure in the sitting up and standing position.