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How do carotid bodies detect changes in oxygen partial pressure?

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A pine grosbeak with pink and gray plumage perches on a branch, eating red berries among autumn foliage. — Chemoreception Animals, such as this pine grosbeak, use chemoreception to respond to chemicals that can be tasted and smelled in their environments. Many of these chemicals affect behaviors such as food preference and defense.

The carotid bodies are small, highly perfused organs located at the bifurcation of the carotid arteries that function as primary peripheral chemoreceptors. They detect changes in the partial pressure of oxygen in arterial blood, rather than the total oxygen content bound to hemoglobin.1

The physiological mechanism involves specialized type I cells, which contain electron-dense vesicles filled with neuroactive substances such as acetylcholine and catecholamines. When the partial pressure of oxygen falls—a state known as hypoxia—these type I cells release these substances, which then act upon adjacent sensory nerve fibers. This process causes the sensory nerve to increase its firing rate hyperbolically as oxygen levels decline. These signals are transmitted to the brain via the carotid sinus nerve, allowing for the regulation of respiratory activity. Notably, the mechanisms for sensing oxygen and carbon dioxide appear to be independent.12