Why do you faint?

Answer 1

In an evolutionary sense, it doesn't seem beneficial that you should faint when you lack sufficient oxygen. In the wild, lying unconscious isn't the best option and you risk predators, a fall, bump your head, or do other bodily injury. However, those negative effects of fainting seemed to have been outweighed by the benefits.

The reason you faint is because your brain isn't getting enough oxygen and risks damage. This is when your brain kicks in and decides that whatever the body is doing is not working and temporarily knocks you out. Having lost the ability to stand, you are now horizontal on the ground. Because blood is not flowing against the force of gravity, your heart doesn't have to work as hard in order to pump oxygenated blood to the brain. The problem with human bipedilism is that the brain takes up the most oxygen, yet is situated so blood has to fight against gravity. By becoming horizontal, the factor of gravity is removed and oxygen gets to the brain.

Other factors associated with fainting include stress, blood sugar level, pregnancy, eating disorders, or cardiac problems. All affect the circulatory system and blood pressure.