Oxygen is transported through the blood and when you have low blood pressure, not enough blood goes to the brain. Remember, you need force to pump blood against gravity and when you stand gravity is against you. You need good blood flow to get enough blood to the brain to deliver oxygen.
The respiratory and circulatory systems work together to ensure that adequate oxygen is transferred from the lungs to the bloodstream and subsequently delivered to the body. Therefore, problems with the circulatory system that interfere with normal blood flow through the lungs can lead to hypoxemia.