When blood is being pumped to your feet, gravity helps take it on its way. To get blood to your head your body has evolved to pump harder.
But while there are muscles that help get the blood back from the legs, the brain doesn't have these same muscles. So it doesn't take long before blood begins to pool in places where it shouldn't, like the lungs and head.
"The major concern is about the control of the vascular system," says Professor Ashley Grossman, of Barts Medical School, part of Queen Mary, University of London