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Transport of Carbon Dioxide: The Three Ways CO2 Travels in Blood

Biology · Breathing and Exchange of Gases · NEET

Carbon dioxide (CO2) travels in blood in three ways: about 70% as bicarbonate ions (HCO3-) in plasma, 20-25% as carbamino-haemoglobin bound to RBCs, and only about 7% dissolved in plasma. Memory hook: "70-20-7, plasma-RBC-plasma" — the biggest share (70%) is the invisible bicarbonate, not the part attached to haemoglobin. Every 100 mL of deoxygenated blood delivers about 4 mL of CO2 to the alveoli.
Three Ways CO2 Travels in BloodAs Bicarbonate70%HCO3- in plasma(carbonic anhydrasemakes it in RBC)Carbamino-Hb20-25%CO2 on globinof haemoglobin(in RBCs)Dissolved7%plain CO2in plasma100 mL deoxygenated blood delivers about 4 mL CO2 to the alveoli
The three transport forms of CO2 in blood with NCERT percentages: 70% as bicarbonate (plasma), 20-25% as carbamino-haemoglobin (RBCs), and 7% dissolved (plasma).

Your doubts, answered

Is CO2 mostly carried by haemoglobin or as bicarbonate?

Mostly as bicarbonate. NCERT says nearly 70% of CO2 is carried as bicarbonate ions (HCO3-) in the plasma. Only 20-25% is carried by RBCs as carbamino-haemoglobin, and about 7% is dissolved in plasma. So the largest share of CO2 is the bicarbonate form, not the haemoglobin-bound form. Students often mix this up because oxygen is mostly carried by haemoglobin (97%), but CO2 is different — bicarbonate wins.

Why is only about 7 percent of CO2 dissolved in plasma?

CO2 is more soluble than O2, but a gas dissolved directly in plasma cannot carry a large amount. To carry more, the body converts CO2 into other forms. Most CO2 is quickly changed into bicarbonate (by the enzyme carbonic anhydrase inside RBCs) and some binds to haemoglobin. These two forms carry far more CO2 than plain dissolving can, so only a small 7% stays simply dissolved.

What is carbamino-haemoglobin and how much CO2 does it carry?

Carbamino-haemoglobin is formed when CO2 binds to the amino (globin) part of haemoglobin — NOT to the iron where oxygen binds. It carries about 20-25% of the CO2. This binding depends on partial pressures: when pCO2 is high and pO2 is low (in tissues), more CO2 binds; when pCO2 is low and pO2 is high (in alveoli), the CO2 is released. That is how CO2 picked up in tissues is dropped off at the lungs.

Does CO2 attach to the same site on haemoglobin as O2?

No. Oxygen binds to the iron (Fe) in the haem group, forming oxyhaemoglobin. CO2 binds to the amino groups of the globin (protein) chains, forming carbamino-haemoglobin. Because they use different sites, both can be handled by haemoglobin. This is a common NEET trap — CO2 does NOT compete with O2 for the iron site.

Why does CO2 unload at the alveoli and load at the tissues?

It is driven by partial pressure (pCO2). In tissues pCO2 is high, so CO2 moves into blood and more binds to haemoglobin and forms bicarbonate. In the alveoli pCO2 is low, so bicarbonate turns back into CO2 and carbamino-haemoglobin releases its CO2 — this CO2 then diffuses out to be breathed away. So transport direction is set by the concentration (pressure) gradient.

⚠️ The NEET trap
✗ CO2 is transported mainly bound to haemoglobin, just like oxygen.
✓ CO2 is transported mainly as bicarbonate ions (about 70%) in plasma; only 20-25% is carried by RBCs as carbamino-haemoglobin.
🧠 Swap the majority carrier: for O2 it is haemoglobin (97%), but for CO2 it is BICARBONATE (70%). NTA loves to flip these two percentages.

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Frequently asked

What are the three forms in which CO2 is transported in blood?

As bicarbonate ions (about 70%), as carbamino-haemoglobin bound to RBCs (about 20-25%), and dissolved in plasma (about 7%).

How much CO2 does 100 mL of deoxygenated blood deliver to the alveoli?

Approximately 4 mL of CO2 per 100 mL of deoxygenated blood, according to NCERT.

Which form carries the most CO2?

Bicarbonate ions (HCO3-) carry the most CO2 — nearly 70%, transported in the plasma.

Is carbamino-haemoglobin the same as oxyhaemoglobin?

No. Carbamino-haemoglobin is CO2 bound to the globin part of haemoglobin, while oxyhaemoglobin is O2 bound to the iron of the haem group.

What controls whether CO2 binds or releases from haemoglobin?

The partial pressure of CO2 and O2. High pCO2 with low pO2 (tissues) favours binding; low pCO2 with high pO2 (alveoli) favours release.

Related concepts:
Carbonic Anhydrase, Bicarbonate and the Chloride ShiftCarbamino-Haemoglobin: CO2 Binding to HaemoglobinTransport of Oxygen: Formation of OxyhaemoglobinPartial Pressure of O2 and CO2 (Values at Alveoli, Blood, Tissues)Exchange of Gases at the Alveoli and Tissues
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Biology MCQs (free, chapter-wise)Breathing and Exchange of Gases PYQs