Respiratory system: peripheral chemoreceptors: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · Respiratory system: peripheral chemoreceptors

Sample item

A 35-year-old woman with severe iron deficiency has haemoglobin of 4 g/dL, so her arterial oxygen content is about one third of normal. At rest she is not hyperventilating, and her PaO2 is 98 mmHg. What is the best explanation?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Carotid body glomus cells sense PaO2, which is normal, and do not respond to oxygen content

Peripheral chemoreceptors in the carotid and aortic bodies respond to a fall in the arterial PO2 (dissolved oxygen), mainly below about 60 mmHg, as well as to H+ and CO2. In anaemia and carbon monoxide poisoning the oxygen content is low but PaO2 is normal, so no chemoreceptor drive is generated until tissue hypoxia causes lactic acidosis.

Option by option

  • A. Carotid body glomus cells sense PaO2, which is normal, and do not respond to oxygen content (correct)

    The receptors monitor PO2, not content, so ventilation is not stimulated at rest.

  • B. Central chemoreceptors are suppressed by chronic anaemia and so do not drive ventilation

    Central chemoreceptors respond to CO2 and H+, and anaemia does not suppress them.

  • C. A leftward-shifted dissociation curve keeps PaO2 normal, but the glomus cells sense lactate

    Anaemia does not shift the curve leftward, and the sensor responds to PO2 rather than lactate.

  • D. Peripheral chemoreceptors respond to haemoglobin concentration but are saturated in anaemia

    The glomus cells do not sense haemoglobin concentration.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is A, Carotid body glomus cells sense PaO2, which is normal, and do not respond to oxygen content. Peripheral chemoreceptors in the carotid and aortic bodies respond to a fall in the arterial PO2 (dissolved oxygen), mainly below about 60 mmHg, as well as to H+ and CO2.

Why is "Central chemoreceptors are suppressed by chronic anaemia and so do no…" not the answer?

Central chemoreceptors respond to CO2 and H+, and anaemia does not suppress them.

Why is "A leftward-shifted dissociation curve keeps PaO2 normal, but the glom…" not the answer?

Anaemia does not shift the curve leftward, and the sensor responds to PO2 rather than lactate.

Why is "Peripheral chemoreceptors respond to haemoglobin concentration but ar…" not the answer?

The glomus cells do not sense haemoglobin concentration.

What concept does this question test?

It tests Respiratory system: peripheral chemoreceptors in Physiology. In anaemia and carbon monoxide poisoning the oxygen content is low but PaO2 is normal, so no chemoreceptor drive is generated until tissue hypoxia causes lactic acidosis.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Respiratory system: peripheral chemoreceptors?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.