Final answer:
Central chemoreceptors that monitor cerebrospinal fluid stimulate an increase in ventilation when a) CO2 and pH levels decrease.
Step-by-step explanation:
The central chemoreceptors that monitor cerebrospinal fluid stimulate an increase in ventilation when CO2 levels decrease and pH levels decrease. The increase in carbon dioxide leads to increased levels of hydrogen ions, which decrease the pH. The increase in hydrogen ions in the brain triggers the central chemoreceptors to stimulate the respiratory centres to increase respiration rate and depth.