A Vasovagal reaction/episode can result in fainting (Syncope), when a person encounters a variety of situations such as pain, high altitude, prolonged standing, dehydration, stress, sight of blood, Intense laughter, ... the pathophysiology has something to due with the vagus nerve and parasympathetic nervous system which lowers heart rate (bradycardia) and results in low blood pressure and light headiness.
See: Vasovagal Syncope and Vasovagal episode
Vegas nerve in Neuroanatomy

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