Location of the Carotid Body
The carotid body is situated at the base of the bifurcation of the common carotid artery into its external and internal branches. The division of the common carotid artery occurs at the level of the upper border of the thyroid cartilage (cartilago thyroidea), which corresponds approximately to the fourth cervical vertebra (C4).
The glomus lies near the base of the carotid bifurcation (bifurcatio carotidis) — the site where the common carotid artery divides into the external carotid artery (a. carotis externa) and internal carotid artery (a. carotis interna). Most commonly, it adheres to the posterior or medial surface of the artery and is connected to its wall by connective tissue.
Adjacent to the glomus in the region of the bifurcation is the carotid sinus (sinus caroticus), an arterial dilation containing baroreceptors.
Structure and Histogenesis
The carotid body is a rice grain-sized structure measuring 5–8 mm in length, 1.5–5 mm in width, and up to 1.5 mm in thickness. Its tissue color varies from grayish-pink to yellowish-red.
Anatomically, the glomus belongs to the paraganglia (paraganglia) — remnants of the chromaffin (adrenal) system. This tissue is embryologically linked to sympathetic ganglia of the autonomic nervous system. Chromaffin cells derive their name from their affinity for chromium salts: following appropriate staining, they turn yellow or dark brown. Functionally, paraganglion cells resemble the adrenal medulla.
Innervation and Central Connections
The carotid body receives interoceptive innervation, which conveys visceral and special sensory impulses from internal organs and reflexogenic zones.
- Peripheral Nerves: Sensory fibers travel within branches of the glossopharyngeal nerve (n. glossopharyngeus, cranial nerve IX) and vagus nerve (n. vagus, cranial nerve X). Branches to the carotid sinus and carotid body course alongside the carotid artery.
- Central Pathways: Afferent impulses from the carotid body are received by the nucleus of the solitary tract (nucleus tractus solitarii). Along the pathway of cranial nerves IX and X lie the superior ganglion (ganglion superius) and inferior ganglion (ganglion inferius), which contain the cell bodies of the first-order sensory neurons.
Physiological Role
The carotid body functions as a chemoreceptor. Unlike the adjacent carotid sinus, which contains baroreceptors, the carotid body is sensitive to the chemical composition of the blood: partial pressures of oxygen ($O_2$) and carbon dioxide ($CO_2$), as well as pH. The carotid body plays a critical role in the regulation of blood pressure and respiration.
Clinical Significance: Glomectomy
In surgical practice, the resection of the carotid body is known as a glomectomy.
- Indications: Severe, medically refractory bronchial asthma (historically).
- Surgical Approach: A collar incision in the region of the right carotid triangle at the level of the upper border of the thyroid cartilage.
- Surgical Steps: The surgeon opens the fascial sheath over the common carotid artery bifurcation, identifies the glomus caroticum on the posterior wall of the bifurcation, ligates the small vessels supplying the glomus from the bifurcation, and excises the body with scissors.
- Desensitization: Some surgeons additionally strip the adventitia of the common, external, and internal carotid arteries over a 1 cm length.