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Fainting is usually the result of a drop in blood pressure. As the head is at the top of the body, it is the first part to feel the decrease in circulation, which leads to a lack of oxygen in the brain and loss of consciousness. Before fainting, the low level of oxygen in the blood makes the retinal cells not work properly, leaving the vision dark or with black dots, while the sounds become more and more distant. Normally, when the person falls to the ground and the head is at the same level as the rest of the body, circulation returns to normal. When that feeling of dizziness and loss of vision arrives, it is best to prevent the fall, sit down and lower your head.
But a person can also faint from a lack of glucose in the blood. “This is common among teenagers when they skip breakfast. As they spend a lot of energy, at the end of the morning the blood glucose is very low, causing fainting. In that case, a glass of water with sugar should be given immediately”, says neurophysiologist Paulo Henrique Bertolucci, from the Federal University of São Paulo (Unifesp). Also known as syncope, fainting can still happen because of diseases, such as cardiac arrhythmia, which impair blood circulation.