Carotid body

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Major arteries of the head and neck.

The carotid body is a small, ovoid, redish mass of tissue that lies posterior to the carotid bifurcation and close to the carotid sinus. It is covered by a fibrous capsule and composed of multiple lobules that are divided by septae.

Each lobule contains highly perfused cell clusters, that are innervated by sensory fibers. There are two cell types within the clusters: glomus (type I) cells and sustentacular (type II) cells.  Glomus (type I) cells store peptides and amines and contain several neurotransmitters. Sustentacular (type II) cells are found on the periphery of the carotid body and their processes reach into the organ to cover the surfaces of glomus cells that are not in contact with nerves.

Innervation is by the carotid sinus nerve, branch of the glossopharyngeal (CN IX) and the vagus nerve (CN X). The carotid body functions as a chemoreceptor that monitors blood oxygen levels. It is stimulated in response to hypercapnia, hypoxia, and increased hydrogen ion concentration (low pH). When oxygen levels are low, the carotid body initiates a reflex that increases the rate and depth of respiration, cardiac rate as well as blood pressure.