Chemoreceptors

The respiratory system functions to bring in O₂ and eliminate CO₂ from the body. This function is assisted by specialized receptors called chemoreceptors that monitor the levels of CO₂, O₂, and H⁺, and then send such information to the respiratory center.

These chemoreceptors are located in several loca­tions. There are central chemoreceptors found in the medulla oblongata that respond to changes in cerebro­spinal fluid H⁺ and Pco₂- Peripheral chemoreceptors include the aortic bodies and carotid bodies whose removal eliminates a respiratory response to hypoxia. The aortic bodies are a cluster of chemoreceptors in the aortic arch; the carotid bodies are oval nodules in the wall of the left and right common carotid arteries, where they bifurcate into the internal and external carotid arteries. Axons from the chemoreceptors in the aortic bodies are part of the vagus nerve (cranial nerve X), whereas those of the carotid bodies project in the glossopharyngeal nerves (cranial nerve IX).

The levels of CO₂ and H⁺ are highly correlated. Throughout the body, CO₂ is quickly converted to car­bonic acid catalyzed by the enzyme carbonic anhy­drase. Carbonic acid dissociates into HCO₃^- and H⁺. (See formula from #4 under the section "Other Fac­tors Affecting the Oxygen-Hemoglobin Dissociation Curve" previously in this chapter.) Therefore, increases in CO₂ lead to increases in H⁺, while decreases in CO₂ lead to decreases in H⁺. As a result, P_co, has a large affect on respiration, whereas P>, affects respiration only if its levels change substantially.

Increases in arterial blood CO₂, called hypercapnia, cause an increase in H⁺. This has a particularly large effect on central chemoreceptors since there is little protein within the cerebrospinal fluid to buffer the H⁺. Activation of the central chemoreceptors causes increased respiration rate, possibly causing hyperven­tilation. Conversely, low arterial blood CO₂, called hypocapnia, inhibits respiration. Large drops in arterial P_t, increase ventilation by stimulating periph­eral chemoreceptors. An increase in plasma H⁺ is also detected by the aortic and carotid bodies, which are highly responsive to changes in arterial H⁺ concentration.

Pulmonary and airway receptors

Three types of sensory receptors have been identified in the lungs, including slowly adapting stretch recep­tors, irritant receptors, and unmyelinated C fibers. The stretch receptors increase their firing rate as the lungs and larger airways inflate.