The peripheral chemoreceptors (carotid and aortic bodies) respond to all of the following stimuli EXCEPT:
- A Carbon monoxide poisoning (normal PaO2, reduced oxygen content) ✓
- B Hypoxaemia — reduced PaO2 below 60 mmHg
- C Elevated PaCO2 — hypercapnia
- D Reduced blood pH — acidaemia
Explanation
Peripheral chemoreceptors (carotid bodies, primarily) respond to reduced PaO2 (not reduced oxygen content per se), elevated PaCO2, and reduced arterial pH. In carbon monoxide (CO) poisoning, PaO2 is normal (CO does not reduce dissolved O2 tension), so peripheral O2 sensors detect no hypoxaemia even though haemoglobin is saturated with CO and oxygen content is severely reduced. This explains why CO poisoning causes little subjective dyspnoea and is insidious. The sensors measure O2 partial pressure (tension), not oxygen content or saturation. Options B, C, and D are all genuine stimuli for peripheral chemoreceptors.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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