Holding your breath

When you hold your breath, what finally forces you to let it go is not that you are running out of air. It works the opposite way from what most of us assume: the urge to breathe comes from carbon dioxide, the waste gas your body makes all the time and normally gets rid of when you exhale. If you stop exhaling, it piles up. That deliberate pause is called apnea.

That gas changes the pH inside the central nervous system, and in the medulla oblongata, the lower part of the brain that keeps your breathing going without you having to think about it, there are sensors called chemoreceptors watching for exactly that change. The moment they pick it up, they give the order to breathe. Oxygen has almost no say in the decision: it only takes over once it has dropped very low, a situation called severe hypoxemia.

And something else happens if you hold your breath with your face in the water. The trigeminal nerve, the one that carries sensation from the face, detects the water and switches on the dive reflex: the heart slows down and the vessels in the arms and legs clamp shut. This is not an accident, it is a savings plan. The blood that is no longer going to idle muscles is saved for the brain and the heart, the two organs that cannot wait.