Why do you faint when you see blood?
How vasovagal syncope works.
Honestly, I’m writing this out of pure spite. A couple of days ago, I was watching a show where one of the protagonists faints in the middle of a warzone because they saw a pool of blood. Frustrating? Yes. Gross? Probably. But dumb? Not at all.
In fact, it’s pretty normal—caused by a very common reflex called vasovagal syncope. Let’s talk about how this works.
Breaking it down
I think it’s helpful to understand the words you’re working with in medical terminology so let’s quickly break down vasovagal syncope and find meaning in the name of the reflex itself.
“Vaso-” as a prefix simply points to the involvement of some sort of vessels, typically blood vessels.
“-vagal” indicates that the reflex has something to do with the vagus nerve, shown loosely in the image above. The vagus nerve—actually the left and right vagus nerves—carry neural messages between the brain and other crucial organs like the heart, lungs, and digestive tract. So, it’s a pretty important nerve in your a) unconscious bodily functions, and b) your parasympathetic nervous system, which is responsible for getting your body functions back to resting levels after activity of some sort.
“Syncope” refers to an episode of fainting or passing out. It typically has a rapid onset and is not regularly a major health concern. Unless, of course, you happen to hurt yourself while unconscious.
Putting it together
Certain environmental triggers can cause you mental stress—in our case, that trigger could be seeing a bunch of blood, whether it's yours or someone else’s. When it does, your body reacts in certain ways, like making your heart beat rapidly and shooting your blood pressure way up. Vasovagal syncope occurs when your body decides to immediately stop that stress response and aggressively tries to calm you down instead, typically because of your vagus nerve overreacting. How dramatic.
The parasympathetic response of the vagus nerve widens your blood vessels and slows down your heart, trying to get you back to feeling normal. However, this induces a sharp drop in blood pressure, accompanied by a drop in the amount of oxygen that reaches your brain. Now your body has no idea what’s going on, your brain is oxygen-deprived, and they together decide to hit the snooze on you.
And voilà. You’ve fainted.
Fortunately, an episode of vasovagal syncope isn’t typically an emergency. It’s mostly unproblematic and doesn’t require treatment, it’s a natural process, after all. Unless you hit your head on the way down. Ouch.
More than just blood
Vasovagal syncope might be commonly observed with exposure to blood, but that’s not the only way it can happen. It can also trigger due to heavy physical exertion, fear or occurrence of injury or trauma, prolonged exposure to heat, and even sudden changes in posture.
But one trigger stood out to me most.
I was an Air Cadet for many years of my life, and for many years of my life, I’ve seen (more like heard) cadets suddenly collapsing while standing at attention for a while. Through my research for this piece, I found that standing for too long can cause vasovagal syncope, which makes a lot more sense now. When you move your legs, you contract muscles in your lower limbs that help your veins fight against gravity and push blood back up toward your heart, so standing still for so long will naturally decrease the blood return and therefore oxygen delivery throughout the body. So, stay moving kids.
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