Peripheral chemoreceptors (carotid and aortic bodies)
Also known as: carotid body, aortic bodies, glomus cells, hypoxic drive
Small, highly vascular sensory organs at the carotid bifurcation and aortic arch that detect falls in arterial PO2, and rises in PCO2 and H+, signalling via CN IX and X.
Overview
The carotid bodies (≈3–5 mm) lie at the bifurcation of each common carotid artery; the aortic bodies lie around the arch of the aorta. Each contains clusters of glomus (type I) cells, the chemosensors, supported by sustentacular (type II) cells and surrounded by sinusoidal capillaries. They have the highest blood flow per gram of any tissue (≈2000 mL/100 g/min), so their tiny O2 extraction means they sense arterial PO2, not O2 content — they are therefore not stimulated by anaemia or carbon monoxide poisoning.
Hypoxia closes O2-sensitive K+ channels in glomus cells, depolarising them; voltage-gated Ca2+ channels open and the cells release transmitters (ATP, dopamine, acetylcholine) onto afferent endings. Carotid body afferents run in the carotid sinus nerve (of Hering), a branch of CN IX; aortic body afferents in the vagus (CN X); both reach the nucleus of the solitary tract. Firing increases little until PaO2 falls below ≈60 mmHg (≈100 mmHg in the curve's flat part) and then rises steeply. The carotid bodies also respond rapidly to ↑PaCO2 (≈20–30% of the CO2 response) and to ↓pH — they are the main sensors driving respiratory compensation of metabolic acidosis — while the aortic bodies respond little to pH.
- Location
Carotid bodies at the common carotid bifurcation; aortic bodies around the aortic arch.
Anatomy
- Innervation
Carotid bodies: carotid sinus nerve (CN IX). Aortic bodies: vagus (CN X).
Function
- Sole sensors of arterial hypoxaemia for the ventilatory response.
- Fast detection of ↑PaCO2 and ↓pH (metabolic acidosis).
- Reflex sympathetic activation and bradycardia/vasoconstriction in hypoxia.
Clinical significance
Carotid body tumour (paraganglioma)
Slow-growing, usually benign tumour at the carotid bifurcation that splays the internal and external carotid arteries ("lyre sign" on angiography); pulsatile, mobile side-to-side but not vertically.
Loss of hypoxic drive
Bilateral carotid endarterectomy or carotid body denervation abolishes the ventilatory response to hypoxia, which can be dangerous at altitude or during sedation.
Not the carotid sinus
The carotid sinus — a dilatation of the internal carotid origin — contains baroreceptors (blood pressure), also innervated by CN IX; the carotid body is the chemoreceptor.
Test yourself
Afferent fibres from the carotid body travel to the brainstem in which cranial nerve?
- Vagus (CN X)
- Hypoglossal (CN XII)
- Glossopharyngeal (CN IX)
- Facial (CN VII)
Answer: Glossopharyngeal (CN IX)
Carotid body afferents run in the carotid sinus nerve (of Hering), a branch of CN IX, to the nucleus of the solitary tract. Aortic body afferents travel with the vagus (CN X).
Compiled from standard anatomy and physiology references — see the Respiratory System sources. For education only; not medical advice.