Respiratory system: carbon dioxide transport: 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: carbon dioxide transport

Sample item

In systemic tissue capillaries, which event occurs inside the red cell as CO2 diffuses in from the tissues?

ACarbonic anhydrase forms carbonic acid, and HCO3- leaves in exchange for Cl-BCarbonic anhydrase in plasma converts most of the CO2 to bicarbonateCHCO3- enters the cell in exchange for Cl- moving outDMost of the CO2 binds haemoglobin directly as carbaminohaemoglobin

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

The answer and the reasoning

A. Carbonic anhydrase forms carbonic acid, and HCO3- leaves in exchange for Cl-

About 70% of CO2 is carried as bicarbonate. CO2 enters the red cell, where carbonic anhydrase catalyses CO2 + H2O to H2CO3, which dissociates to H+ and HCO3-. H+ is buffered by deoxyhaemoglobin and HCO3- exits through the band 3 exchanger in exchange for Cl- (the chloride or Hamburger shift).

Option by option

A. Carbonic anhydrase forms carbonic acid, and HCO3- leaves in exchange for Cl- (correct)
Bicarbonate exits and chloride enters through band 3, maintaining electroneutrality.
B. Carbonic anhydrase in plasma converts most of the CO2 to bicarbonate
Carbonic anhydrase is present in the red cell, not the plasma, so conversion occurs inside the cell.
C. HCO3- enters the cell in exchange for Cl- moving out
Bicarbonate entering the cell in exchange for chloride moving out is the reverse process, in the lungs.
D. Most of the CO2 binds haemoglobin directly as carbaminohaemoglobin
Carbamino compounds carry only about 20 to 25% of CO2, and bicarbonate carries most.

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.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is A, Carbonic anhydrase forms carbonic acid, and HCO3- leaves in exchange for Cl-. About 70% of CO2 is carried as bicarbonate.

Why is "Carbonic anhydrase in plasma converts most of the CO2 to bicarbonate" not the answer?

Carbonic anhydrase is present in the red cell, not the plasma, so conversion occurs inside the cell.

Why is "HCO3- enters the cell in exchange for Cl- moving out" not the answer?

Bicarbonate entering the cell in exchange for chloride moving out is the reverse process, in the lungs.

Why is "Most of the CO2 binds haemoglobin directly as carbaminohaemoglobin" not the answer?

Carbamino compounds carry only about 20 to 25% of CO2, and bicarbonate carries most.

What concept does this question test?

It tests Respiratory system: carbon dioxide transport in Physiology. CO2 enters the red cell, where carbonic anhydrase catalyses CO2 + H2O to H2CO3, which dissociates to H+ and HCO3-. H+ is buffered by deoxyhaemoglobin and HCO3- exits through the band 3 exchanger in exchange for Cl- (the chloride or Hamburger shift).

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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: carbon dioxide transport?

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.