Why you breathe more when you exercise

You start running and you breathe deeper and more often. That has a name: hyperpnea, which is exactly that, raising the depth and the rate of breathing to meet a greater demand for oxygen.

Here is the odd part. You would think you breathe more because your blood oxygen dropped, and that is not the case: during exercise, the oxygen and carbon dioxide in the blood hardly move. What goes up is the air moving in and out, not the numbers for those gases.

And there is something even more striking: breathing seems to speed up before the muscle's oxygen drops. So the signal does not appear to come from a shortage. Two nerve signals have been proposed: the same motor command that sets the muscles moving, and input from proprioceptors, sensors in muscles, joints and tendons that detect movement and stretch.

That said, the exact mechanism is still not well understood and some explanations remain under debate. And it is important not to confuse this with hyperventilating, which is a different thing: in that case blood carbon dioxide does drop.