Why do we pass out: The science behind fainting

The sight of blood, getting a fright, distressing news and standing in the hot sun can all make you feel faint.

According to the American Heart Association (AHA), one-third of the global population will faint at least once during their lifetime for any number of reasons.

Representational image. Pixabay

But why do we lose consciousness? Here’s the science behind fainting.

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Fainting by a medical name

Syncope or loss of consciousness for a short time occurs when the brain does not get an adequate supply of blood. Thankfully, the brain has a cool trick to get its adequate supply even if the body’s overall blood pressure (BP) drops: cerebrovascular autoregulation.

Research shows that in a young, healthy adult, even if the systolic BP (the top number) falls to 70 mm Hg, the brain gets the blood it needs. The normal top number for BP is in the range of 130-139.

Then, why do we faint?

The nerve

When we faint after getting distressing news or, say, at the sight of blood, usually it is because of vasovagal reflex - in which the vagus nerve goes into overdrive.

The vagus nerve is part of a crucial network that connects the brain to the neck, heart, lungs and abdomen. Ordinarily, it helps the body to relax with every breath, to digest food and perform a host of other functions. However, when we are scared or distressed, it can slow the heart down. This, in turn, reduces the flow of blood to the brain.

Before passing out, we may feel light-headedness, sweating, nausea and blurry vision.

A leg to stand on

Another common cause of fainting is standing up too quickly. The science behind it is a condition known as orthostatic hypotension (hypotension is low BP).

This can happen when we are dehydrated, or when it’s hot outside, or in people who are suffering from a disease or taking medication that causes their BP to drop.

Diseases like diabetes and Parkinson’s disease are commonly associated with blood pooling in the legs. When a significant amount of blood accumulates in the legs, the brain gets less blood to work with. This can lead to fainting.

Orthostatic hypotension can also stimulate the vagus nerve, which in turn can lead to syncope.

According to the AHA, abnormalities in the cardiovascular system like arrhythmia (irregular heartbeat), extremely slow heart rate (bradycardia) or extremely fast heart rate (tachycardia) can also be potential causes for fainting.

A sudden drop in BP is universal with syncope or fainting, but the pathways of fainting might be different in different situations. For example, even though a massive drop in BP doesn’t cause a healthy person to faint, even a small drop in BP for someone who has had hypertension for years may result in a fainting spell.

Whatever the reason, the brain doesn’t like to be without blood and oxygen, even for a little while. It’s a good idea to get a physical check-up if one experiences more than the occasional fainting spell.

Health articles in Firstpost are written by myUpchar.com, India’s first and biggest resource for verified medical information. At myUpchar, researchers and journalists work with doctors to bring you information on all things health. For more on this topic, please read our in-depth article on Fainting.