Step by Step
70%
As bicarbonate (HCO₃⁻) — the major pathway
CO₂ enters red blood cells, where the enzyme carbonic anhydrase converts CO₂ + H₂O into H₂CO₃, which then splits into H⁺ and HCO₃⁻. The H⁺ is buffered by hemoglobin, while HCO₃⁻ exits the red blood cell in exchange for a chloride ion entering — this exchange, called the chloride shift (or Hamburger phenomenon), maintains electrical neutrality across the cell membrane.
23%
As carbaminohemoglobin
CO₂ binds directly to the amino groups of hemoglobin (not the heme groups, which are reserved for oxygen) — forming carbaminohemoglobin.
7%
Dissolved directly in plasma
The smallest fraction of CO₂ simply dissolves directly in the blood plasma without any chemical conversion.
Rev
Reversal at the lungs — and the Haldane effect
At the lungs, this whole process reverses: bicarbonate re-enters the red blood cell, carbonic anhydrase converts it back into CO₂, and the CO₂ is exhaled. The Haldane effect describes how oxygenation of hemoglobin (which happens at the lungs) reduces hemoglobin's capacity to carry CO₂, promoting CO₂ unloading exactly where it needs to be released.
Applied Walkthrough
1
In an actively metabolizing tissue, CO₂ produced by cells enters nearby red blood cells. Most of it (70%) is converted by carbonic anhydrase into bicarbonate and hydrogen ions.
2
The hydrogen ions are buffered by hemoglobin, while bicarbonate exits the red blood cell in exchange for chloride entering — the chloride shift, which keeps the cell's charge balanced.
3
Meanwhile, a smaller portion of CO₂ (23%) binds directly to hemoglobin's amino groups as carbaminohemoglobin, and a small remainder (7%) simply dissolves in the plasma.
4
When this blood reaches the lungs, the entire process reverses: bicarbonate converts back to CO₂ for exhalation, and as hemoglobin becomes oxygenated (via the Haldane effect), its CO₂-carrying capacity decreases — actively promoting the release of CO₂ exactly where it's meant to leave the body.
Exam Application
Exams test whether you know the three transport mechanisms and their approximate percentages, whether you understand the chloride shift's role in maintaining electroneutrality, and whether you can distinguish the Bohr effect (affecting O₂ release) from the Haldane effect (affecting CO₂ release).
⚠ Common Trap
The most common trap is confusing the Bohr effect with the Haldane effect — the Bohr effect describes how CO₂/H+ levels affect oxygen release from hemoglobin, while the Haldane effect describes how oxygenation of hemoglobin affects CO₂ release — they're complementary but distinct phenomena.
✓ Quick Self-Check
1. What are the three ways CO₂ is transported in blood, and their approximate percentages?
70% as bicarbonate, 23% as carbaminohemoglobin, 7% dissolved in plasma.
Tap to reveal / hide
2. What enzyme converts CO₂ + H₂O into carbonic acid inside red blood cells?
Carbonic anhydrase.
Tap to reveal / hide
3. What is the chloride shift?
The exchange of bicarbonate exiting a red blood cell for chloride entering, which maintains electroneutrality.
Tap to reveal / hide
4. What is the Haldane effect?
Oxygenation of hemoglobin (at the lungs) reduces its CO₂-carrying capacity, promoting CO₂ release.
Tap to reveal / hide
5. Where does CO₂ bind on the hemoglobin molecule when forming carbaminohemoglobin?
The amino groups, not the heme groups (which are reserved for oxygen).
Tap to reveal / hide