Short answer
Fainting occurs when blood pressure suddenly drops very low. As a result, too little blood temporarily reaches the brain, causing a brief loss of consciousness. Fainting is very common and can be triggered by things like standing for a long time, pain, or fear. The fainting itself is not dangerous (blood pressure always recovers on its own), but people can get injured from the fall.
Longer answer
Many people have either seen someone faint or have fainted themselves. It can look frightening; people often collapse limply to the ground, become completely stiff for a moment, or experience jerks of the arms and/or legs. Fainting happens because blood pressure suddenly drops very low. At that moment, not enough blood reaches the brain which causes unconsciousness.
How can someone’s blood pressure suddenly drop so low?
The sudden drop in blood pressure is caused by a reflex that makes the blood pressure become very low. This reflex can be triggered in people who are sensitive to it by, for example, standing for a long time, fear, or pain. We are also increasingly understanding what causes the blood pressure to drop when the reflex occurs: the blood vessels in the body widen and the heart rate slows down. You can compare it to a garden hose: the force with which the water comes out of the hose decreases if you widen the nozzle (so wider blood vessels mean lower pressure) or if you turn the tap down a bit (so a lower heart rate means lower pressure).
Is fainting dangerous?
Fainting itself is not dangerous. People regain consciousness within a few seconds to a minute. However, if someone falls hard or lands awkwardly, fainting can lead to injuries.
I feel like I’m about to faint—what now?
People usually feel fainting coming on a few minutes in advance. They may then feel lightheaded, but can also become nauseous, feel warm, and/or start sweating. If you notice this in yourself, sit or lie down, preferably with your legs raised. It can also help to tense your leg and gluteal muscles forcefully. If you see someone else experiencing these symptoms, help them to sit or lie down calmly. This can help prevent fainting, but also avoids a hard fall if the person does faint. If someone has fainted, make sure they lie flat on the ground with their legs raised. This allows blood to flow back to the brain as quickly as possible, helping the person regain consciousness sooner.
Read More?
Fainting: What It Feels Like and What Causes It – More elaborate explanation of fainting