peripheral chemoreceptor

Peripheral chemoreceptors are the body's low-oxygen sentries, posted in the great arteries rather than in the brain. They sample blood as it leaves the heart and sound the alarm to breathe harder when oxygen runs short.

They are found in two locations: the carotid bodies, where the carotid arteries divide in the neck, and the aortic bodies near the arch of the aorta. The carotid bodies are the more important for breathing in humans. They respond chiefly to a fall in arterial oxygen, but also to a rise in carbon dioxide and to acidity, sending signals along the glossopharyngeal and vagus nerves to the respiratory center.

Their response to low oxygen becomes strong only when arterial oxygen falls fairly low, so under everyday conditions carbon dioxide remains the main driver of breathing. They become crucial, however, at altitude, in severe lung disease, and in people whose carbon dioxide response is blunted, where they may carry much of the breathing drive.