Fainting, frequently called syncope ( ) in a medical context, is a sudden, and generally momentary, loss of consciousness, or blacking out caused by the Central Ischaemic Response, because of a lack of sufficient blood and oxygen in the brain. The first symptoms a person feels before fainting are dizziness; a dimming of vision, or brownout; tinnitus; and feeling hot. Moments later, the person's vision turns black, and he or she drops to the floor (or slumps if seated in a chair).

Central Ischaemic Response

The Central Ischaemic Response is the brain's response to a chronic lack of blood available to the brain. The brain attempts to prioritise its own needs above any others by commandeering all available blood flow at the expense of all other bodily functions and increasing the rate of blood oxygenation.

It diverts blood to itself by reducing the supply to most of the rest of the body through vasoconstriction and increasing the pulse rate, tachycardia. It attempts to increase oxygenation of the blood by accelerating the breathing rate hyperventilation. This gives rise to the typical symptoms of fainting: pale skin, notably a white face, rapid breathing and weakness of the limbs, particularly the legs, progressing to collapse and is subjectively felt as weakness and nausea.

Weakness of the legs tends to cause victims to lay themselves down or fall down involuntarily thereby countering low blood pressure, hypotension, in the brain. It is unclear whether this is an evolved response or merely a serendipitous result of collapsing.

Other causes

Factors that influence fainting are taking in too little food and fluids, low blood pressure, hypoglycemia, growth spurts, physical exercise in excess of the energy reserve of the body, emotional distress, and lack of sleep. Orthostatic hypotension caused by standing up too quickly or being in a very hot room can also cause fainting.

More serious causes of fainting include cardiac (heart-related) causes such as an abnormal heart rhythm (an arrhythmia), where the heart beats too slowly, too rapidly or too irregularly to pump enough blood to the brain. Some arrhythmias can be life-threatening. Other important cardio-vascular conditions that can be manifested by syncope include subclavian steal syndrome and aortic stenosis.

Fainting can also occur following hyperventilation prior to a breath-hold dive in shallow water or on ascent from a breath-hold dive in deep water.

Vasovagal syncope

main: Vasovagal syncope Vasovagal (situational) syncope, one of the most common types, may occur in scary, embarrassing, or uneasy situations or during blood drawing, coughing, or urinating. Other types include postural syncope (caused by a changing in body posture), cardiac syncope (due to heart-related conditions), and neurological syncope (due to neurological conditions). There are many other causes of syncope including low blood sugar levels and lung disease such as emphysema and a pulmonary embolus. The cause of the fainting can be determined by a doctor using a complete history, physical, and various diagnostic tests.