Orthostatic intolerance

From DictionGloss

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 to the brain. Astronauts often experience temporary orthostatic intolerance when remaining upright after returning from stays in space, because their blood volume is reduced in microgravity and the pull of gravity does not allow sufficient blood flow to reach their heads.