Step 1 topics → Respiratory → CO2 Transport in Blood

CO2 Transport in Blood

USMLE Step 1 trap: Underestimates bicarbonate as the dominant form of CO2 transport. Approximately 70% of CO2 is transported as bicarbonate (HCO3⁻) in plasma, with only ~10% dissolved and ~20% as carbaminohemoglobin.

CO2 transport is tested on USMLE Step 1 both as direct recall and as mechanism application — and the most common error is underestimating bicarbonate as the dominant form, which carries roughly 70% of CO2 in venous blood. CO2 has three fates: dissolved in plasma (~10%), bound to hemoglobin as carbaminohemoglobin (~20%), and converted to bicarbonate (~70%) by carbonic anhydrase inside red blood cells. The Haldane effect and chloride shift are the mechanistic angles the exam probes most often, and students who confuse them with the Bohr effect will miss those questions.

The tricky part is keeping the Bohr effect and Haldane effect straight — they involve the same players (CO2, H⁺, hemoglobin) but describe different phenomena. The Bohr effect is about O2 unloading: CO2 and H⁺ shift the oxyhemoglobin dissociation curve right, decreasing O2 affinity. The Haldane effect is about CO2 loading: deoxygenated hemoglobin has a higher affinity for CO2 and H⁺ than oxyhemoglobin does. Students who conflate these will consistently miss application questions. The chloride shift — HCO3⁻ leaving the RBC in exchange for Cl⁻ — is a detail that often doesn't make it into notes but shows up in passage-based questions on USMLE Step 1 about RBC physiology.

The prerequisite here is a solid grasp of the hemoglobin dissociation curve (resp_phys_hb_dissociation), because both the Bohr and Haldane effects make no sense without understanding what 'affinity' means in context. If you're fuzzy on why a rightward shift decreases O2 affinity, go back and nail that first.

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Common misconceptions

Common mistake
Wrong: Most CO2 is transported dissolved in plasma or bound to hemoglobin.
Right: Approximately 70% of CO2 is transported as bicarbonate (HCO3⁻) in plasma, with only ~10% dissolved and ~20% as carbaminohemoglobin.
Dissolved CO2 and carbaminohemoglobin together account for only about 30% of CO2 transport — bicarbonate dominates at roughly 70%. This happens because carbonic anhydrase inside RBCs rapidly catalyzes CO2 + H2O → H2CO3 → HCO3⁻ + H⁺, making bicarbonate formation kinetically favorable. If you were guessing dissolved or carbamino as the major form, you were underweighting the enzymatic efficiency of this pathway.
Common mistake
Wrong: The Haldane effect describes how CO2 shifts the oxyhemoglobin curve (confusing it with the Bohr effect).
Right: The Haldane effect states that deoxygenated hemoglobin binds more CO2 and H⁺ than oxyhemoglobin, facilitating CO2 loading in tissues; the Bohr effect describes how CO2/H⁺ shifts the O2 curve.
The Haldane effect and the Bohr effect are complementary but distinct: the Bohr effect describes CO2 and H⁺ acting on hemoglobin to reduce O2 affinity (rightward shift of the curve), while the Haldane effect describes O2 saturation acting on hemoglobin to alter its CO2-carrying capacity. Specifically, deoxyhemoglobin is a better buffer and better CO2 binder than oxyhemoglobin, so CO2 is loaded more efficiently in tissues after O2 is dropped off. Keep them straight by asking: 'Which molecule's transport am I talking about?' — Bohr = O2 delivery affected by CO2/H⁺; Haldane = CO2 pickup enhanced by deoxygenation.
Common mistake
Gap: Missing the chloride shift mechanism that accompanies bicarbonate formation in RBCs
As HCO3⁻ exits the RBC in exchange for Cl⁻ (chloride shift), carbonic anhydrase within the RBC catalyzes the rapid conversion of CO2 + H2O to H2CO3 then HCO3⁻.
When CO2 enters the RBC and is converted to HCO3⁻ by carbonic anhydrase, that bicarbonate needs to exit into the plasma to be transported — but the RBC membrane isn't freely permeable to anions without a transporter. The chloride shift (via the Band 3 anion exchanger) moves HCO3⁻ out of the RBC and Cl⁻ in, maintaining electrical neutrality. Without this exchange, bicarbonate would accumulate inside the RBC and inhibit further CO2 conversion. In the lungs, the process reverses: Cl⁻ leaves, HCO3⁻ re-enters, gets converted back to CO2, and CO2 is exhaled.
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What the exam tests

  1. Know the three forms of CO2 transport in blood and their approximate proportions: ~70% as bicarbonate (HCO3⁻), ~20% as carbaminohemoglobin, and ~10% dissolved in plasma — and expect the exam to test whether you correctly identify bicarbonate as the dominant form.
  2. Understand the Haldane effect mechanistically: deoxygenated hemoglobin binds more CO2 and H⁺ than oxyhemoglobin, which is why CO2 loading is enhanced in peripheral tissues where O2 has been released — and be able to distinguish this from the Bohr effect, which describes how CO2 and H⁺ reduce hemoglobin's affinity for O2.

Can you avoid these mistakes?

A patient is breathing normally at rest. Approximately what fraction of CO2 is being transported as bicarbonate in venous blood? What enzyme makes this possible, and where is it located?
In peripheral tissues, hemoglobin releases O2 to cells. How does this deoxygenation event affect CO2 transport, and what is this phenomenon called? How does it differ from the Bohr effect?
Trace a single CO2 molecule from a muscle cell to the alveolus: what form does it take in the RBC, how does it cross the RBC membrane into plasma, and how is it converted back to CO2 for exhalation?
A question stem describes a patient with a rightward shift of the oxyhemoglobin dissociation curve due to elevated tissue CO2. The question then asks which effect explains the enhanced CO2 loading seen simultaneously in those same tissues. Is the correct answer the Bohr effect or the Haldane effect, and why?

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