Control of breathing, chemoreceptors and lung reflexes !
- Regulation
- Exam priority: high
The brainstem sets the rhythm of breathing. Blood CO₂ tunes it most; low O₂ only matters once it falls a lot.
Updated 2026-11-01
Also called: Respiratory centre, Central and peripheral chemoreceptors, Hering–Breuer reflex
See it in 3D: Medulla oblongata · Pons · Carotid bifurcation · Phrenic nerves · Aortic arch · Intercostal nerves
What it is
Groups of neurons in the medulla generate the breathing rhythm and the pons fine-tunes it. Central chemoreceptors sense the H⁺ that CO₂ produces in cerebrospinal fluid; peripheral chemoreceptors (carotid and aortic bodies) sense mainly low PaO₂, plus CO₂ and H⁺. Lung receptors and the cortex adjust the pattern.
How it works, step by step
- Arterial CO₂ crosses the blood–brain barrier easily; H⁺ and HCO₃⁻ cross poorly.
- In CSF, CO₂ forms H⁺, which stimulates chemoreceptors on the ventral surface of the medulla.
- The respiratory centre raises rate and depth, alveolar ventilation rises and PaCO₂ falls.
- If PaO₂ falls below ≈ 60 mmHg, the carotid body fires and adds a strong ventilatory drive.
- As the lung distends, stretch receptors signal via the vagus and cut inspiration short (Hering–Breuer).
- Over days at altitude the kidney excretes bicarbonate, pH normalises and ventilation can stay high (acclimatization).
Regulation
- +High PaCO₂ / low CSF pH: Strongly increases ventilation (central chemoreceptors).
- +PaO₂ < ≈ 60 mmHg: Activates the carotid body and increases ventilation.
- +Metabolic acidosis (arterial H⁺): Stimulates peripheral chemoreceptors: Kussmaul breathing.
- +Exercise, pain, anxiety, fever: Cortical and proprioceptive inputs raise ventilation.
- −Opioids, sedatives, alcohol: Depress the respiratory centre and the CO₂ response.
- −Lung distension (Hering–Breuer): Shortens inspiration; important in infants and at large volumes.
Sensors of respiratory control
| Sensor | Where | What it senses |
|---|---|---|
| Central chemoreceptors | Ventral surface of the medulla | CSF H⁺ (reflects PaCO₂) |
| Carotid bodies | Carotid bifurcation (CN IX) | Low PaO₂, high PaCO₂, high H⁺ |
| Aortic bodies | Aortic arch (CN X) | Low PaO₂, high PaCO₂ |
| Stretch receptors | Airway smooth muscle (CN X) | Lung distension |
Clinical notes
- Opioid overdose: bradypnea, pinpoint pupils, hypercapnia; naloxone restores breathing.
- COPD with chronic CO₂ retention: excessive supplemental O₂ can push PaCO₂ higher (V/Q, Haldane and less hypoxic drive).
- Obstructive sleep apnea: the airway collapses while breathing effort continues; rising CO₂ wakes the patient.
- Arriving in Bogotá from the coast: hyperventilation and mild respiratory alkalosis in the first days.
Test yourself
Think of the answer, then tap the question to see it.
Why do central chemoreceptors respond to CO₂ rather than to blood H⁺?
CO₂ crosses the blood–brain barrier and forms H⁺ in CSF; blood H⁺ and bicarbonate cross very poorly.
Which chemoreceptors explain Kussmaul breathing in ketoacidosis?
The peripheral ones, mainly the carotid bodies, which sense arterial H⁺; central receptors respond later.
Why is the initial hyperventilation at altitude held back in the first hours?
Falling PaCO₂ raises CSF pH, which brakes the central chemoreceptors; over days CSF and plasma bicarbonate fall and ventilation keeps rising.
Which nerve carries carotid body signals to the brainstem?
The glossopharyngeal nerve (CN IX), through its carotid sinus branch.
What does morphine do to the ventilatory response curve to CO₂?
It flattens it and shifts it right: more CO₂ is needed for the same ventilation.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Abnormal breathing patterns and sleep apnea
- High altitude and acclimatization
- CO₂ transport and the Haldane effect
- Respiratory acidosis and alkalosis
- Mechanics of breathing: pressures and muscles
Related conditions
- Chronic obstructive pulmonary disease (COPD)
- Obstructive sleep apnoea
- Respiratory alkalosis
- Respiratory acidosis
- Diabetic ketoacidosis
In Pharmacology
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 22.3 The Process of Breathing (OpenStax (Rice University), 2022); Neuroanatomy, Medulla Oblongata (StatPearls (NCBI Bookshelf), 2024); Obstructive Sleep Apnea (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 42). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.