Fainting occurs when our brain stops functioning properly because of a sudden decrease in blood flow. Our brain is complex organ, and it requires a constant flow of blood to supply the nutrients and oxygen necessary for proper function. Anything that interrupts the normal flow of blood to the brain will result in the brain “shutting down.”
The heart is responsible for pumping blood through the blood vessels to all the other organs, including the brain. As the heart pumps it generates the pressure needed to allow the blood to travel uphill to the head. This “blood pressure” is usually measured by an inflatable cuff applied around the arm at every doctor visit. If the blood pressure decreases significantly, the blood cannot fight the force of gravity to go uphill, and so the brain stops receiving sufficient blood flow. The symptoms of reduced blood flow to the brain include confusion, lightheadedness, weakness, visual disturbances like tunnel-vision or blurry vision, decreased or muffled hearing, and nausea. Sweating may occur, and the heart may beat rapidly to try to compensate for the low blood pressure. Eventually, essential brain activity slows to the point where loss of consciousness occurs. At that point, the muscles generally becomes limp, though in some people the body can get rigid and brief jerking of the muscles can be seen. The bottom line, however, is that all true forms of fainting are caused by a loss of adequate blood flow to the brain.
When a person is lying down, the blood only has to travel sideways to get to the brain. Thus, once a person faints and they fall down, the blood doesn’t have to travel uphill any longer. Even in the face of a very low blood pressure, the blood can still get to the brain again, so the syncope victim begins to regain consciousness. It may take a minute or two for the brain to start working properly again, so people are sometimes a little "out of it" for a short time, but they generally know who and where they are ... they just might not remember how they wound up on the floor!
There are other medical conditions that resemble fainting. Sometimes, people can get dizziness that feels like a spinning, swimming, or a merry-go-round sensation. This symptom is called “vertigo,” and it can worsen with sudden changes in position. It can last only seconds, but sometimes vertigo persists for days. Although vertigo does not cause loss of consciousness, some people feel like they might pass out. Vertigo is usually due to problems with the balance centers in the middle ear or in the brain.
Other people have a brain condition known as “epilepsy,” which causes seizures. Seizures occur when the brain circuits fire very rapidly. Often, patients feel strange just before a seizure -- they may describe a particular smell or odd feeling. This is known as an "aura." The aura is then followed by sudden loss of consciousness associated with tensing or jerking of all the muscles that can last for several minutes (a "grand mal" seizure). Other types of seizures can involve uncontrollable, sometimes repetitive movements of part of the body (focal seizures), or a person may simply seem “out of touch” or start staring (referred to as "absence" seizures) for a few seconds or minutes. After a grand mal seizure, the patient is often lethargic (difficult to arouse) and may be confused for 10-20 minutes, a situation known as a "post-ictal" state (meaning "after the seizure"). Epilepsy is due to an electrical disturbance of the brain. The problem may develop from some kind of scar tissue or other physical abnormality within the brain tissue, including a stroke or a tumor. However, epilepsy is not simply caused by a lack of blood flow to the brain.
Some people appear to faint, but it may be due to emotional problems like depression or anxiety. This is sometimes referred to as “pseudosyncope,” which means false fainting. Less common causes of sudden loss of body tone include narcolepsy, a disorder that can cause a person to suddenly fall asleep.