<prompt> The primary direct stimulus for excitation of central chemoreceptors isHC02O2C02  <response>: The central chemoreceptors monitor the H concentration of cerebrospinal fluid CSF, including the brain interstitial fluid.CO2 readily penetrates membranes, including the blood-brain barrier, whereas H and HCO3- penetrate slowly. The CO2 that enters the brain and CSF is promptly hydrated. The H2CO3 dissociates so that the local H concentration rises. The H concentration in brain interstitial fluid parallels the aerial PCO2. Experimentally produced changes in the PCO2 of CSF have minor, variable effects on respiration as long as the H concentration is held constant, but any increase in spinal fluid H concentration stimulates chemoreceptorsRef: Ganongs Review of Medical Physiology Twenty-Third Edition, Page No: 629 