It is mainly caused by the temporary insufficient blood supply of the cerebrum. When one crouches, his lower limbs buck and the blood vessels of the lower limbs is pressed so that the blood cannot flow to the lower limb, causing the shortage of blood of the lower limb. When you stand up suddenly, the blood vessels of the lower limb is unblocked again and a large quantity of blood moves downward, causing the inadequate blood flow of the upper part of the body. Without abundant oxygen supply, the brain and eyes break down, causing dizziness and stars before eyes. If one is in poor health, the situation will be worse. However, when such situation happens, do not be panic and there is no need to go to hospital. When the head cannot supply sufficient blood, the heart will speed up work and deliver blood upward, so the body will recover soon.
It is normal to have the dizzy symptom when standing up too fast. You will feel dizzy and see stars because of the rushed blood in the eyes parts. After you have stand for a while, you will find that this symptom will get release. Next time, you'd better not stand up too fast because the dizzy eye sight will be so uncomfortable.
It is a common condition in most cases, when you are sitting, your brain would consider it as a state of rest, thus your blood pressure is low. When you suddenly stand up, your heart has to pump blood at a higher pressure for oxygen-filled blood to reach your brain. And your action can be supported and keep overall return to normal state. In general, your heart would adjust the blood pressure and you would see starts or feel dizzy for only a few seconds. However, if you always see stars when you stand up, maybe you would need a health check for identify the exact reason.