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.