When the intensity of blood directed to the brain decreases, a loss of consciousness and thus fainting may occur.
Fainting, mechanically, is manifested by a sudden loss of muscle tone, which causes a fall towards the ground, with loss of facial colouring. Before fainting, one may experience weakness, nausea and problems with vision and hearing. Usually the loss of consciousness lasts about two minutes.
The causes of fainting can be as varied as the symptoms preceding it. (Source: Humanitas Hospital Milan)