Why can't you suffocate by holding your breath?

It's basically impossible to hold your breath until you suffocate, or even until you pass out. But what's going on in your body to prevent that from happening? Why can't you suffocate by holding your breath?

There are multiple systems keeping you from holding your breath too long, each of which would take over if another failed — and that's a very good thing.

Several brain regions work to keep you breathing.

"The first one is your motor cortex … and that's sensing that you're not breathing," Anthony Bain, an associate professor in the Department of Kinesiology at the University of Windsor in Canada, told Live Science. "So that sends signals down to your breathing center, which is the base of the brain, the medulla." That controls the muscles that help you breathe, like the diaphragm, the main muscle responsible for inflating and deflating the lungs, and the intercostal muscles, which are located between the ribs and help to expand the chest with each breath.

Related: Why don't we breathe equally out of both nostrils?

The second region is a network farther down in the brainstem called the pre-Bötzinger complex, which acts as the body's respiratory rhythm generator.

"That is continually going, even when you're holding your breath," Bain said. "It's kind of like a heart rate." That is, even when you're not breathing, this respiratory pacemaker is still ticking away, trying to make you breathe.

In addition, groups of cells called chemoreceptors keep track of the levels of oxygen and carbon dioxide in the body. There are two sets of these cells: the central chemoreceptors and the peripheral chemoreflex.

The central chemoreceptors, located in the brain, "primarily just respond to carbon dioxide," Bain said. The peripheral chemoreflex is located in the neck, near the larynx. These cells "will respond to both carbon dioxide and then low levels of oxygen when you extend that breath hold for a prolonged period of time," Bain said.

Finally, there are receptors in the lungs that detect the stretch that happens when they inflate and deflate. When you hold your breath and they stop stretching, alarm bells start to sound.

When any one of these systems detects that something's wrong, it sends stress signals to the respiratory center in the brain, which does everything it can to restart the breathing process.

In studies, researchers have found ways to disable some of these systems to see how it affects the length of time participants can hold their breath. In one study, heart-lung transplant recipients whose lung receptors weren't fully connected to the central nervous system couldn't hold their breath any longer than normal participants, showing that those stretch receptors in the lungs aren't strictly necessary to keep you breathing.

Chemoreceptors seem to play a bigger role. In a different study, Bain and his team used dopamine to inhibit the carotid body, which is part of the peripheral chemoreflex — the group of cells that sense oxygen and carbon dioxide levels.

"If you do that in normal people holding their breath … then you are able to extend the breath hold time," Bain said.

Elite divers are different: through training, they can hold their breath much longer than regular people. The record for static apnea — holding your breath underwater without using oxygen during preparation — is just under 12 minutes. For these divers, inhibiting the carotid body doesn't do much. "It doesn't actually extend their breath hold time, because now the mechanisms for when they actually break are actually quite separate from a nonelite breath holder," Bain said.

In fact, elite divers prove that with enough training, it's actually possible to hold your breath until you pass out. They've trained to push past the signals sent by their chemoreceptors and get their oxygen levels much lower than an untrained person, breathing only when they sense they're about to fall unconscious. "It's that critical level of hypoxia that they have to sense, and then they start breathing again," Bain said.

One intervention that has a huge effect on breath-hold time in untrained people is paralysis of the diaphragm, the muscle that brings air into and out of the lungs. In the 1970s, scientists injected anesthetic into the phrenic nerves, which send signals from the motor cortex to the diaphragm, and found that it doubled the time participants could hold their breath. But it still didn't allow them to hold their breath until they passed out, which shows the importance of training.

Finally, one way for both trained and untrained people to extend their breath-holding time is by bringing more oxygen into their system, either by hyperventilating normally or inhaling pure oxygen. In fact, this is sometimes used in medical scans where patients need to stay still for long periods of time.

"You can give them 100% oxygen and then they can hold their breath for, you know, up to five minutes, in some cases," Bain said.

This is how the Guinness World Record for breath holding was set. In 2021, Budimir Šobat, a Croatian free diver, held his breath for 24 minutes, 37 seconds.

Extra oxygen tells those peripheral chemoreceptors that everything is fine. But even if they were disabled, the body has enough fail-safes to keep you breathing before you pass out.

"It just makes sense from an evolutionary perspective," Bain said. "Breathing is so important. It makes sense to have a redundancy in systems to tell the body to keep breathing."