CO2/H+ Chemoreceptors in the Respiratory Passages of Vertebrates

CO₂ and/or pH-sensitive chemoreceptors located in the respiratory passages play an important role in ventilatory control in most vertebrates. In fish, branchial chemoreceptors appear to detect changes in water CO₂ levels specifically, and are linked to the stimulation of ventilation frequency and/or amplitude, as well as to the initiation of cardiovascular responses such as bradycardia. The situation appears to be very similar for bimodal breathers (air-breathing fish and amphibian tadpoles) except that in many cases the input from these receptors is transformed with increasing levels of aquatic CO₂ such that it stimulates air breathing rather than gill ventilation. CO₂-sensitive chemoreceptors (gustatory and potentially, olfactory) are also located in the orobranchial cavity, although the roles of these chemoreceptors are less well understood. In air-breathing ectotherms, olfactory receptors often inhibit breathing and prolong breath holding when environmental CO₂ levels are high. Pulmonary CO₂/H⁺-sensitive receptors [intra-pulmonary chemoreceptors (IPC) and/or pulmonary stretch receptors (PSR)], on the other hand, regulate breathing pattern in all vertebrates (endotherms and ectotherms, including lungfish) in a manner that reduces dead space ventilation and enhances the efficiency of CO₂ excretion under conditions of environmental hypercarbia, and/or reduces CO₂ loss from hyperpnea/polypnea. This may be particularly true for IPC because of their greater CO₂ sensitivity. Because there are several different CO₂-sensitive chemoreceptor groups with different degrees of CO₂ sensitivity eliciting different reflex responses in all vertebrates, responses to hypercarbia (elevated environmental CO₂ tension) versus hypercapnia (elevated blood CO₂ tension) may differ, giving rise to what appear to be anomalous responses to environmental CO₂.