Orthostatic intolerance is a condition of lightheadedness, possibly leading to fainting, that occurs when an upright position is assumed. It is caused by low blood pressure and an inadequate supply of blood reaching the brain. Astronauts often experience temporary orthostatic intolerance when remaining upright after returning from stays in space, since their blood volume is reduced in weightlessness and the pull of gravity does not allow blood to reach their heads.
Orthostatic intolerance is the failure of the body to properly adjust to an upright position, especially with respect to blood flow, heart rate, and blood pressure.