Peripheral Chemoreceptors

Location and Characteristics

• Location: Most are in the carotid bodies, located bilaterally at the bifurcations of the common carotid arteries. A few are also present in the aortic bodies along the arch of the aorta, and a very few are found elsewhere along thoracic and abdominal arteries.
• Afferent pathway: Carotid body fibers pass through Hering's nerves to the glossopharyngeal nerves, reaching the dorsal respiratory area of the medulla. Aortic body fibers pass through the vagi to the same area.
• Blood supply: Each body has its own minute nutrient artery with an extremely high blood flow (about 20 times the weight of the body per minute), so almost no O2 is extracted from the blood passing through. This means the chemoreceptors are exposed to arterial, not venous, PO2 at all times.
• Cellular basis: The bodies contain glandular-like glomus cells that act as the actual O2 sensors. When PO2 falls below about 60 mm Hg, K+ channels in glomus cells close, causing depolarization, opening of voltage-gated Ca2+ channels, rise in intracellular Ca2+, and release of a transmitter (likely ATP) that stimulates afferent nerve endings.
• Sensitivity: Impulse firing rises sharply as arterial PO2 falls from 60 to 30 mm Hg - the range over which hemoglobin saturation also falls rapidly. Below a PO2 of about 100 mm Hg the receptors show little activity; they become powerfully activated mainly when PO2 drops below ~70 mm Hg.
• Peripheral chemoreceptors also respond to rising CO2 and H+, but this effect is far weaker (about one-seventh as powerful) than the direct central effect of CO2 on the respiratory center. However, peripheral stimulation by CO2 occurs about five times faster than the central response.

Functions

1. Respiratory control (primary role): They form the peripheral chemoreceptor system, the only mechanism by which hypoxia can drive ventilation. When arterial PO2 falls below ~70-100 mm Hg, alveolar ventilation increases progressively - up to 5-fold at very low PO2 - even when PCO2 and pH are held constant.
2. Rapid response component: Because their stimulation by CO2/H+ is faster than the central chemoreceptors, they add speed to the ventilatory response, which is especially useful at the onset of exercise.
3. Role in acclimatization to altitude: They allow ventilation to increase markedly (400-500%) during chronic hypoxia once central sensitivity to CO2 falls, helping deliver more O2 at high altitude.
4. Cardiovascular (chemoreceptor) reflex: When arterial pressure falls below a critical level (mainly below 80 mm Hg), reduced blood flow through the chemoreceptor bodies leads to local O2 deficiency and CO2/H+ accumulation, exciting the chemoreceptors. This excites the vasomotor center and helps raise arterial pressure back toward normal - a reflex that becomes important mainly at low blood pressures, acting alongside the baroreceptor reflex.
5. Clinical relevance: Chemoreceptor activation contributes to hypertension in conditions such as severe obesity and obstructive sleep apnea, where repetitive hypoxic episodes stimulate this pathway.