Some Cardiac Physiology
Blood flows preferentially downwards so at every given point in time -- due to the effects of gravity -- thus, a certain amount of our blood is always un-oxygenated. The upper body towards the head has less blood supply.
Postural hypertension is a condition where individuals lose consciousness briefly after changing position, specifically after standing up because of a sudden drop in blood pressure (and shortage of blood supply to the brain) —> That is why upon fainting individuals tend to recover after blood reaches the brain again.