Fainting (syncope) is lost consciousness suddenly, usually only a few seconds or minutes, because your brain is not getting enough oxygen.
The brain has several parts, including the two hemispheres, cerebellum, and brain stem. The brain requires blood flow to provide oxygen and glucose into the cells. For your body to remain conscious, an area known as the reticular activating system located in the brainstem should live, and at least one hemisphere of the brain must function. Fainting occurs when the reticular activating system or both hemispheres of the brain is deprived of blood, oxygen, or glucose.