CO₂ Transport

Carbon dioxide is the main waste product of aerobic respiration, and at steady state it must be removed from tissues at the same rate as it is produced. Because CO₂ is much more soluble than oxygen and reacts chemically with water, the blood transports it in three distinct ways. The majority travels as hydrogencarbonate ions (HCO₃⁻) in the plasma, with smaller contributions from dissolved CO₂ and carbaminohaemoglobin. This lesson examines each pathway, the role of the enzyme carbonic anhydrase, the ingenious chloride shift that preserves electrical neutrality in red blood cells, and the mechanism by which CO₂ carriage is linked to the Bohr effect. Content matches OCR A-Level Biology A specification 3.1.2 (l).

Key Definitions:

• Carbonic anhydrase — an enzyme in red blood cells that catalyses the reversible conversion of CO₂ + H₂O to H₂CO₃ (carbonic acid).
• Hydrogencarbonate ion (HCO₃⁻) — the main form in which CO₂ is transported in the blood.
• Carbaminohaemoglobin — haemoglobin with CO₂ bound (to free amine groups on the globin, not to the haem iron).
• Chloride shift — the exchange of HCO₃⁻ out of the red blood cell for Cl⁻ into the cell, maintaining electrical neutrality.
• Haemoglobinic acid (HHb) — haemoglobin that has accepted an H⁺ ion, acting as a buffer.

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The Three Forms of CO₂ Transport

Form	Approximate percentage	Location
Dissolved CO₂ in plasma	~5%	Plasma
Carbaminohaemoglobin	~10%	Bound to globin chains in RBCs
Hydrogencarbonate ions (HCO₃⁻)	~85%	Plasma (after being made inside RBCs)

Only a small fraction is carried as dissolved gas, because CO₂ — while more soluble than O₂ — is still not sufficiently soluble to transport large amounts at body temperature. The majority is chemically converted to hydrogencarbonate ions for transport.

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Pathway 1: Dissolved CO₂

About 5% of the CO₂ produced simply dissolves in the plasma and is carried in the free gaseous form. At the lungs, the same 5% diffuses straight back out along its gradient into the alveolar air.

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Pathway 2: Carbaminohaemoglobin

Approximately 10% of CO₂ binds reversibly to free amine (–NH₂) groups on the globin polypeptides of haemoglobin, forming carbaminohaemoglobin:

$\text{Hb--NH}_2 + \text{CO}_2 \rightleftharpoons \text{Hb--NH--COOH}$Hb–NH2​+CO2​⇌Hb–NH–COOH

Note that the CO₂ binds to the globin chains, not to the haem iron. Deoxyhaemoglobin binds CO₂ more readily than oxyhaemoglobin does, which makes sense physiologically: haemoglobin that has just unloaded its oxygen in the tissues is in the best state to pick up CO₂ for the return journey. This is a key reason why venous blood carries more CO₂ than arterial blood.

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Pathway 3: Hydrogencarbonate Ions (the Majority)

About 85% of the CO₂ carried in the blood travels as hydrogencarbonate ions (HCO₃⁻), produced inside the red blood cells and then exported into the plasma. The full sequence is:

Step 1: CO₂ Enters the RBC

CO₂ diffuses from respiring tissues → plasma → through the RBC membrane into the cytoplasm.

Step 2: Conversion to Carbonic Acid

Inside the RBC, carbonic anhydrase catalyses the hydration of CO₂:

$\text{CO}_2 + \text{H}_2\text{O} \xrightleftharpoons{\text{carbonic anhydrase}} \text{H}_2\text{CO}_3$CO2​+H2​Ocarbonic anhydrase​H2​CO3​

Without this enzyme the reaction would be far too slow; carbonic anhydrase is one of the fastest enzymes known, with a turnover number of up to 600,000 reactions per second.

Step 3: Dissociation

Carbonic acid is a weak acid and dissociates spontaneously:

$\text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$H2​CO3​⇌H++HCO3−​

This produces hydrogen ions (H⁺) and hydrogencarbonate ions (HCO₃⁻) inside the RBC.

Step 4: The H⁺ is Buffered by Haemoglobin

The hydrogen ions would rapidly lower the pH of the cytoplasm and disrupt haemoglobin function if left free. Instead, haemoglobin buffers the H⁺ by accepting them, forming haemoglobinic acid (HHb):

$\text{HbO}_2 + \text{H}^+ \rightleftharpoons \text{HHb} + \text{O}_2$HbO2​+H+⇌HHb+O2​

Crucially, binding H⁺ promotes the release of oxygen from haemoglobin — this is the molecular basis of the Bohr effect. The more CO₂ the tissues produce, the more H⁺ is generated inside the RBC, and the more oxygen is released to those same tissues.