A patient with obstructive sleep apnea has recurrent nocturnal hypoxia. The peripheral chemoreceptors that primarily detect acute falls in PaO2 are located in the carotid bodies. At which partial pressure of O2 does their firing rate increase most steeply?
- A Below 60 mmHg PaO2, where the oxyhemoglobin dissociation curve is steep ✓
- B Below 100 mmHg PaO2, rising linearly with falling PO2
- C Below 40 mmHg PaO2, which represents the venous PO2 threshold
- D Below 80 mmHg PaO2, independently of the oxyhemoglobin saturation curve
Correct answer: A. Below 60 mmHg PaO2, where the oxyhemoglobin dissociation curve is steep
Explanation
The carotid body glomus cells contain O2-sensitive K+ channels (primarily TASK and TREK subtypes). Their discharge rate increases modestly as PaO2 falls from 100 to 60 mmHg but rises steeply below 60 mmHg, corresponding to the steep portion of the oxyhemoglobin dissociation curve where small PO2 changes cause large drops in saturation. This is physiologically important because the body has minimal respiratory reserve above 60 mmHg. Central chemoreceptors (medulla) respond primarily to CO2/H+, not O2.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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