The central chemosensitive area responds to changes in:

The most direct stimulus (and the only stimulus) for central chemoreceptor is the H+ ions. However, these hydrogen ions are not coming from aerial blood; they are formed locally in the brain/CSF. (Note: H+ can not cross BBB easily.) Aerial CO2 crosses BBB easily. It enters brain or CSF; combines water in the interstitial fluid of brain or water in CSF. This forms H2CO3, which then splits into hydrogen and bicarbonate ions. These hydrogen ions then stimulate the central chemoreceptor, thereby stimulating the respiratory center. Thus, basically the respiratory center is responding to changes in aerial CO2, which forms the H+ and stimulates respiration.