Previous: Could Dinosaurs Have Been Warm-Blooded?
Next: Our Food Is Full of These 5 Chemicals



View count:992,233
Last sync:2024-01-30 16:30


Citation formatting is not guaranteed to be accurate.
MLA Full: "What Happens When You Faint?" YouTube, uploaded by SciShow, 18 August 2015,
MLA Inline: (SciShow, 2015)
APA Full: SciShow. (2015, August 18). What Happens When You Faint? [Video]. YouTube.
APA Inline: (SciShow, 2015)
Chicago Full: SciShow, "What Happens When You Faint?", August 18, 2015, YouTube, 02:26,
Why do we faint? Because sometimes, your nervous system just doesn’t know what to do with itself.

Hosted by: Hank Green
Dooblydoo thanks go to the following Patreon supporters -- we couldn't make SciShow without them! Shout out to Justin Ove, John Szymakowski, Fatima Iqbal, Justin Lentz, David Campos, and Chris Peters.
Like SciShow? Want to help support us, and also get things to put on your walls, cover your torso and hold your liquids? Check out our awesome products over at DFTBA Records:

Or help support us by becoming our patron on Patreon:
Looking for SciShow elsewhere on the internet?



Fainting isn't fun, when it's happening to you or when it's happening to someone else. It is scary and it is unpleasant and why? Why do we do that? When you faint it's because of your nervous system went haywire and your brain wasn't getting enough oxygen. 

A brain requires oxygen like a computer requires electricity and that oxygen is carried to your brain by blood. A lot of blood. About 750 milliliters of blood flowing through your head every minute. If you suddenly decrease that oxygen rich blood flow you get a temporary loss of consciousness known to doctors as syncope and to the rest of us as fainting.

There a lots of different and sometimes serious medical conditions that can make someone faint, but the kind we are talking about today, the kind that makes one drop at the sight of, blood, is called vasovagal syncope. It's named after the vagus nerve which goes from your brain all the way down through your body ending in your colon. It's part of your autonomic nervous system which regulates your body's involuntary functions. The vagus nerve in particular manages your heart rate and blood pressure.

But it's also responsible for recalibrating your body after you experience a so-called Fight-or-Flight response. When something particularly shocking or unpleasant happens your body goes into survival mode. You release stress hormones like Adrenaline and Cortisol which makes your heart beat faster and constricts your blood vessels, raising your blood pressure and sending more blood and oxygen to your muscles and brain. Heart pumping and muscles primed you're ready to either fight or run away.

But it's all very temporary. After the initial shock your vagus nerve kicks in telling your heart to slow down. It also tells your blood vessels to open back up and get your blood pressure back to normal. In some people this recalibration goes temporarily overboard like the pendulum swinging back in the other direction. So if the vagus nerve overcorrects and the heart rate drops too much blood pressure drops to low and the brains oxygen supply is suddenly interrupted.

It's like unplugging that computer, you black out, you lose muscle control and you tumble down like a Jenga set. This kind of fainting doesn't usually last long though, within a matter of seconds your vagus nerve corrects the error and returns your heartbeat and blood pressure back to normal. You regain consciousness and open your eyes and try to figure out what the heck just happened.

This question came our way from a Patreon Patron known only to us as Fan From Singaporeeeeee! If you'd like to submit a question to be answered you can go to and don't forget to go to and subscribe for more answers all the time to stuff that you're curious about, I know you are.