Fainting, or ‘syncope’, is a temporary loss of consciousness due to a lack of oxygen to the brain. It is usually preceded by dizziness, nausea, sweating and blurred vision.
The most common cause is overstimulation of the body’s vagus nerve. Possible triggers of this include intense stress and pain, standing up for long periods or exposure to something unpleasant. Severe coughing, exercise and even urinating can sometimes produce a similar response. Overstimulation of the vagus nerve results in dilation of the body’s blood vessels and a reduction of the heart rate. These two changes together mean that the body struggles to pump blood up to the brain against gravity. A lack of blood flow to the brain means there is not enough oxygen for it to function properly and a fainting episode occurs. However, shortly after fainting, the blood flow to the brain is restored (usually from simply lying down as a result of the fainting) and the brain functions normally again.
Heavy bleeding, diabetes, drugs and low blood pressure can similarly reduce blood flow to the brain causing a lack of oxygen and fainting.
Josh Moore, Science Museum