Your breathing is steered mostly by carbon dioxide, not oxygen
It feels as if we breathe to get oxygen, and we do, but the brain mostly sets the pace by tracking carbon dioxide. Sensors in the medulla respond to the acidity that CO2 creates in blood and spinal fluid. Keeping CO2 steady keeps body fluids at the right pH, which is why over-breathing can feel so unpleasant.
Lungs cannot inflate themselves. They expand only when the chest cavity grows, mostly because the dome-shaped diaphragm contracts and flattens, helped by the intercostal muscles lifting the ribs. At rest, breathing out is passive: the muscles relax and the elastic lungs and chest wall spring back. During hard exercise, abdominal muscles push the diaphragm up to force more air out, yet even after the deepest exhalation some air always remains.
Inhaled air travels down a branching tree, which in humans divides about 23 times on average before reaching the tiny alveoli where gas exchange happens. The larger airways carry air without exchanging it, forming about 150 millilitres of dead space in an adult. Because 2.5 to 3 litres stay in the lungs between breaths and each breath adds relatively little, the gas in the alveoli stays fairly constant, letting the blood equilibrate against a stable mix. The nose helps too, warming and moistening incoming air and trapping particles in mucus.
Control centres in the brainstem integrate the signals. Central chemoreceptors track pH, while peripheral sensors in the carotid and aortic bodies mainly respond to oxygen, and the phrenic nerves carry commands to the diaphragm. We can override the autopilot to speak, sing, swim or hold our breath, and the diving reflex adjusts breathing and circulation underwater to save oxygen.
The air we breathe in is about 78 percent nitrogen and 20.95 percent oxygen. What we breathe out holds 4 to 5 percent carbon dioxide, around a hundred times more than we took in, along with traces of hydrogen made by gut microbes, carbon monoxide from broken-down heme, and hundreds of volatile compounds such as acetone and isoprene, some of which can signal disease. Spacesuits supply pure oxygen, but at only about a fifth of normal atmospheric pressure.
Source: Breathing
