Respiratory response to increased activity levels

An increase in activity raises the CO2 output. It also makes the blood and cerebrospinal fluid more acidic. Chemoreceptors in blood vessels and the medulla sense the changes and signal the brain’s respiratory center, also in the brain stem.
In response, the respiratory center signals the diaphragm and intercostal  muscles. The signals call for alterations in the rate and depth of breathing. Excess CO2 is expelled, which causes the level of this gas and acidity to decline. Chemoreceptors sense the decline and signal the respiratory center, so breathing is adjusted accordingly.