Sometimes that's 30 seconds into a breath hold, sometimes a minute, sometimes multiple

minutes if the conditions are just right.

Everyone's breakpoints are different, and individually, multiple factors influence our

time on the stopwatch.

But how does your body know that you're at your breaking point?

When do you know you've reached your absolute limit you know, before death?

As it turns out, it has to do with what's in our blood.

In this video, we'll go over the physiology that monitors your blood chemistry and keeps

you breathing right.

Quick disclaimer, I'm going to talk about intentional breath holding in this video,

not choking or airway obstruction which are emergencies.

Also, some young infants and toddlers experience involuntary breath holding spells, but this

video is for curiosity and education, not medical advice.

So please, please use common sense.

If you think something is wrong, get professional medical help.

Okay, now we can get into the fun stuff.

The whole point of breathing is so that our tissues can consume oxygen and glucose and

turn them into energy, leaving water and carbon dioxide as waste products.

This process called cellular respiration is essential to anything that breathes oxygen.

And if our tissues don't get enough oxygen, or experience hypoxia, they can start to die

off or see other problems.

There are a bunch of reasons that a tissue might not get enough oxygen — like an iron

deficiency might cause anemia, which means that less oxygen will be able to ride on each

red blood cell and oxygenate your tissues.

Hypoxia can also happen if there's not enough blood flow to a tissue, like when an artery

is too narrow and doesn't deliver as much blood to its target tissue.

Then there's high altitude where oxygen isn't as easily available, but we'll come

back to that later.

Now, in a breath holding situation, it's pretty easy to see why you'd become hypoxic.

You're not breathing so you're not taking in any of that sweet sweet oxygen.

But we have to keep another gas in mind during breath holding — carbon dioxide.

High CO2 levels in our blood, or hypercapnia, can cause symptoms like headaches and dizziness,

but also more severe symptoms like paranoia, irregular heartbeats, and seizures.

Hypercapnia can happen without breath holding too, like if you're in a submarine or just

a stuffy room with the windows closed.

Since neither hypoxia or hypercapnia are ideal, our bodies are constantly measuring and reacting

to oxygen and carbon dioxide in the blood.

That's where the carotid bodies come in, receptors embedded in the carotid artery in

your neck that are triggered by certain chemicals.

Since they measure our blood's chemistry, we call them chemoreceptors. 

Their positioning in the neck lets them sample arterial blood before it gets into the brain,

which consumes a bunch of oxygen.

These bodies take information about CO2, pH, temperature and oxygen to give your body an

idea of what's happening in your blood.

And their structure at the tissue level supports that.

The carotid body itself is made of glial cells to give it some shape, which are hooked up

to neurons that communicate information about the blood to the brain.

When they detect hypoxia or hypercapnia, they'll kick off some kind of response in the cardiorespiratory system.

And they work fast.

They can detect hypoxia in just a few seconds. 

When the carotid bodies detect hypoxia, they communicate with parts of your body that slow

down heart rate.

The blood vessels of the skin, muscles, and most organs constrict in order to conserve

oxygen rich blood for the brain.

A few studies have looked into how competitive breath holders delay their breakpoint, the

time at which someone takes a breath after trying to hold it as long as they can.

Their carotid bodies still work just fine.

Their bodies still know they're hypoxic, but they're able to brute force their way

to a longer hold. 

Sometimes the studies will look at how to delay breakpoint with some kind of distraction

like squeezing a rubber ball.

But if these pro-breath holders are able suppress every last urge to breathe, they ultimately

pass out and reflexive breathing takes over again. 

In fact, loss of consciousness is fairly common among competitive breath-holding athletes

which, ehh, “athlete” is used loosely.

As interesting as voluntary breath holding is, other low-oxygen situations work differently.

High altitude creates hypoxia because less oxygen is available for our bodies to grab onto.

Our bodies react by breathing more and more, but after a while of living in high altitude,

our bodies acclimatize by making more red blood cells.

That's a different situation entirely from being hypoxic because you're underwater.

Breath-holding underwater causes something called the mammalian dive reflex — our heart

rate slows down even more in an effort to conserve oxygen.

That's what I find so fascinating about this phenomenon.

Hypoxia from diving is different from visiting Denver.

So while I'm definitely not telling you to hold your breath until you lose consciousness,

I'm confident knowing that our bodies can use our blood chemistry to keep us in check.

Here's a pro tip: Instead of holding your breath until the next episode of Human, just

subscribe and hit the notification bell so you never miss an upload from us.

Thanks for watching Seeker, I'll see you next week.