What causes some people to faint all of a sudden?

The symptoms are frightening: A rapid heartbeat, accelerated breathing, draining of color from face, nausea, sweating and – ultimately – collapse. During the first hazy moments of awakening, the victim is told that he fainted. What causes this unsettling experience?
Fainting
In order to function properly the body’s countless systems must at all times be nourished with oxygen-rich blood. If the heart is denied it, its beat grows unsteady. If the stomach is denied it, consciousness is lost. During a normal faint, these and other symptoms are all manifested as blood supplies suddenly drop in the body’s vital regions. Although any number of conditions can trigger this drastic circulatory change, overheating is one of the most common ones. Prolonged standing in the Sun, for example, causes blood vessels to slowly widen. This allows blood to pool in the lower extremities while withholding an adequate supply from the brain. If the denial continues long enough, unconsciousness will result. Psychological stress can also bring about a swoon. In the face of surprise or danger, the heart may begin beating rapidly but inefficiently while blood pressure suddenly plummets. Again, the result could be the lowered cerebral blood flow and rapid collapse.
An unusual type of fainting called carotid sinus syncope (or the ‘tight-collar syndrome’) can strike with no preliminary symptoms whatever. Sensitive nerve endings in the neck sometimes respond to even slight stimulation by directing blood vessels to widen. Lowered circulatory pressure and fainting thus can result from nothing more than shaving or wearing a shirt with a snug collar.
After recovering from a faint, a victim occasionally experiences a brief period of disorientation, a condition that lasts only until the brain regains its normal blood flow. Ordinarily, this takes very little time – and it’s a good thing, too. If the brain were to be deprived of oxygen for more than three minutes, irreversible damage would occur as brain cells expire quickly. But seldom are the consequences grave. More often than not, fainting is no more than a brief malfunction of a flustered and oxygen-starved brain.

Additional reading:
Syncope (medicine) (Wikipedia)

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