. Ans. a. Increased H+ The primary direct stimulus for central chemoreceptors is increased H+ concentration in the CSF and brain interstitial fluid."The primary direct stimulus for central chemoreceptors is increased H+ concentration in the CSF and brain interstitial fluid. But the cone: in the brain parallels the arterial PCO2 and not arterial H+ cone. "Even though the central chemoreceptors are directly stimulated by H+ ions: the blood H+ ions are poorly permeable through the blood brain barrier and hence are not able to regulate ventilation though the central chemoreceptors. Whereas, CO2 is highly permeable to the blood brain barrier. It enters the brain, where it is hydrated to give rise to H+ and HCO2 ions. Now these H+ ions directly stimulate the central chemoreceptors to regulate the ventilation. So the central chemoreceptors respond to changes in H+ ions in the cerebrospinal fluid and brain interstitial fluid. which are brought about by change in arterial PaCO2."Chemoreceptors Regulating Ventilation* There are 2 types of chemoreceptors regulating ventilation: central chemoreceptors (located in brain stem, mainly medulla) and the peripheral chemoreceptors (aortic and carotid bodies).* They regulate ventilation by responding to increased arterial H+ cone., increased PaCO2 and decreased PaO2.Chemoreceptors Regulating VentilationCentral ChemoreceptorsPeripheral Chemoreceptors* Located in brain stem, mainly medulla* Responds to arterial PC CL* Aortic and carotid bodies* Responds to arterial PCO2 arterial PO2 and arterial H+ cone.* Peripheral chemoreceptors are stimulated by hypoxia, hypercapnia, and academia to cause hyperventilation, whereas central chemoreceptors are stimulated only by hypercapnia.