The goal of the respiratory system is to exchange gases between your body’s cells and the atmosphere. Oxygen goes in, and carbon dioxide, a byproduct of your cells’ metabolic actions, comes out. You probably don’t pay much attention to your breathing unless you’re having trouble with it. So, how does your body know to take another breath? It comes down to a special set of cells.

In several locations throughout your body, special sensory cells essentially “taste” the blood to find out how much carbon dioxide is in the bloodstream. These cells—called chemoreceptors—are stimulated by high levels of carbon dioxide and very low levels of oxygen. When carbon dioxide builds up in the blood from metabolism, it sends signals to the respiratory center in the brain. That is when your body tells you to take another breath before your oxygen levels drop. 

These special chemoreceptors can react to low levels of oxygen in the bloodstream. But not everyone reacts the same way when they don’t get enough oxygen—it’s based on individual genetic and environmental differences. For example, if you were exposed to low levels of oxygen as an infant by being born at high altitude or if you had medical issues as a baby that caused a lack of oxygen, it is possible that your chemoreceptors are not as sensitive to low levels of oxygen. In other words, your body would not know to stimulate another breath until it is too late.

This situation may lead to a potentially life-threatening consequence for divers. Breathing deeply and blowing out carbon dioxide before diving deep into water creates the possibility that a diver’s body will not know to breathe again until the oxygen levels are too low. When this happens, the diver might pass out on the bottom of the pool or ocean. If the body is functioning normally, the high carbon dioxide levels in the blood act as a safety trigger that causes the diver to surface, take a fresh breath and save their own life.

The levels of oxygen and carbon dioxide in the blood change with many respiratory diseases. Doctors can diagnose and help treat respiratory problems by looking at these differences. Thankfully, in most situations your body monitors and automatically adjusts changes in these blood gases to help your cells survive.

Barb Goodman, PhD, FAPS, is a professor of physiology at the University of South Dakota.