Why does a running person start breathing more intensely?

When you do more physical activity, such as cycling, running or climbing, your breathing becomes faster. In addition to bringing oxygen into the body faster, it speeds up the process of getting rid of carbon dioxide. Your brain – or more specifically, the nucleus in the brainstem – sends instructions to your body to increase your breathing rate. This causes excess carbon dioxide to be removed from the blood faster through the lungs. Once the normal level is reached, your breathing returns to normal.