Fainting, "blacking out," or syncope is the temporary loss of consciousness followed by the return to full wakefulness. This loss of consciousness may be accompanied by loss of muscle tone that can result in falling or slumping over. To better understand why fainting can occur; it is helpful to explain why somebody is awake.
The brain has multiple parts, including two hemispheres, the cerebellum, and the brain stem. The brain requires blood flow to provide oxygen and glucose (sugar) to its cells to sustain life. For the body to be awake, an area known as the reticular activating system located in the brain stem needs to be turned on, and at least one brain hemisphere needs to be functioning. For fainting or syncope to occur, either the reticular activating system loses its blood supply, or both hemispheres of the brain are deprived of blood, oxygen, or glucose. If blood sugar levels are normal blood flow must be briefly disrupted to the whole brain or to the reticular activating system for fainting to occur.
Fainting is not caused by head trauma, since loss of consciousness after a head injury is considered a concussion. However, fainting can cause injury if the person falls and hurts themselves, or if the faint occurs while participating in an activity like driving a car.
Fainting is differentiated from seizure, during which patients may also also lose consciousness.