What causes you to almost faint when you stand up?

? hypotension. If you rise fairly quickly when going from lying to standing or sitting to standing, you could experience postural hypotension. Blood can pool inlower extremities due to gravity. This means less blood & oxygen getting to the brain . Hydrate well. Go slowly from lying to sitting to standing. Check with your doctor to see is you take any medictions or have any conditions that could be the cause.