The peripheral chemoreceptors in the carotid bodies respond to changes in PaO2, PaCO2, and pH. The most potent acute stimulus for increased ventilation via carotid body chemoreceptors in a healthy person is:
- A PaO2 falling from 100 to 80 mmHg (mild hypoxemia)
- B PaCO2 rising from 40 to 50 mmHg (mild hypercapnia)
- C Simultaneous rise in PaCO2 and fall in PaO2 (both deviating together) ✓
- D pH falling from 7.40 to 7.30 (metabolic acidosis alone)
Explanation
Carotid body chemoreceptors respond to PaO2, PaCO2, and pH, with important interactions. PaCO2/H+ are the most potent individual stimuli for the central chemoreceptors, but carotid bodies respond to all three and show synergism: the ventilatory response to hypercapnia is markedly potentiated by concurrent hypoxia, and vice versa—the combination of elevated CO2 and low O2 produces a ventilatory response far exceeding either alone (multiplicative interaction). Mild hypoxemia alone (PaO2 80 mmHg) barely stimulates ventilation (the response is hyperbolic, significant only below PaO2 ~60 mmHg). Hypercapnia alone is a potent stimulus but is amplified many-fold by hypoxia. The highest real-world stimulus is combined hypoxia + hypercapnia (e.g., as in severe COPD exacerbation).
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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