Effects of Pulmonary Receptors on Ventilation

The lungs contain various types of receptors that influence the brain stem respiratory control centers via sensory fibers in the vagus nerves. Unmyelinated C fibers are sensory neurons in the lungs that can be stimulated by capsaicin, the chemical in hot peppers that creates the burning sensation. These receptors produce an initial apnea, followed by rapid, shallow breathing when a person eats these peppers. The unmyelinated C fibers are also stimulated by histamine and bradykinin, which are released in response to noxious agents. Irritant receptors in the wall of the larynx, and receptors in the lungs identified as rapidly adapting receptors, can cause a person to cough in response to

Table 16.6

Sensitivity of Chemoreceptors to Changes in Blood Gases and pH

Stimulus

Chemoreceptor

Comments

îpco2

Medullary chemoreceptors; aortic and carotid bodies

Medullary chemoreceptors are sensitive to the pH of cerebrospinal fluid (CSF). Diffusion of CO2 from the blood into the CSF lowers the pH of CSF by forming carbonic acid. Similarly, the aortic and carotid bodies are stimulated by a fall in blood pH induced by increases in blood CO2.

ipH

Aortic and carotid bodies

Peripheral chemoreceptors are stimulated by decreased blood pH independent of the effect of blood CO2. Chemoreceptors in the medulla are not affected by changes in blood pH because H+ cannot cross the blood-brain barrier.

iPo2

Carotid bodies

Low blood P02 (hypoxemia) augments the chemoreceptor response to increases in blood Pco2 and can stimulate ventilation directly when the Po2 falls below 50 mmHg.

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