In the sinus of the aortic arch and arteria carotis exists the nerve terminal, named pressure receptor, which has the capability to feel the change of the pressure. At ordinary times, because the blood of the human body distributes in each part of the body according to the active condition of each organ and the blood pressure stays in a constant level, which will not cause the excitation of the pressure receptor. But the position changes when you do a handstand, due to the increasing quantity of blood in the head, the pressure to the aortic arch and vessel walls of the arteria carotis increases, hence stimulating the pressure receptor, causing stars in your eyes. Those who suffer serious cardiopathy, cerebrovascular disease, nephritis, high blood should not do a handstand.