Biology · Breathing and Exchange of Gases · NEET
NCERT is exact here. About 70% of CO2 is carried as bicarbonate (HCO3-) in the plasma. About 20-25% is carried by RBCs as carbamino-haemoglobin. About 7% is carried dissolved in the plasma. For NEET, remember the order: bicarbonate is the biggest share, dissolved is the smallest.
Carbonic anhydrase is an enzyme present in very high concentration inside RBCs (and in tiny amounts in plasma). It speeds up the reaction CO2 + H2O -> H2CO3 (carbonic acid). Without the enzyme this reaction is very slow — only about 200 molecules of H2CO3 form per hour. The enzyme makes it happen almost instantly, so blood can pick up and release CO2 fast enough.
Carbonic anhydrase does not decide direction; the direction depends on the partial pressure of CO2 (pCO2). At the tissues pCO2 is high, so the reaction goes forward: CO2 + H2O -> H2CO3 -> H+ + HCO3-. At the alveoli pCO2 is low, so it runs in reverse: HCO3- + H+ -> H2CO3 -> CO2 + H2O, and CO2 is breathed out. The enzyme simply speeds up whichever way the conditions push.
When HCO3- forms inside the RBC, a lot of it moves out into the plasma. To keep electrical balance, chloride ions (Cl-) move into the RBC. This inward movement of Cl- to replace the outgoing HCO3- is called the chloride shift or Hamburger's phenomenon. Note: the chloride shift is NOT written in the NCERT Class 11 text as a named term, but it is a classic NEET add-on that examiners include, so learn it.
Bicarbonate transport = CO2 changed chemically into HCO3- ions carried in plasma (about 70%), and it depends on carbonic anhydrase and pCO2. Carbamino-haemoglobin = CO2 bound directly to the globin part of haemoglobin (about 20-25%), and it depends on both pCO2 and pO2. Both release CO2 at the alveoli where pCO2 is low.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
In very high concentration inside RBCs, and in minute quantities in the plasma.
CO2 diffuses into blood, carbonic anhydrase makes CO2 + H2O -> H2CO3, which dissociates into H+ and HCO3-. The HCO3- moves to plasma and Cl- moves into the RBC (chloride shift).
At the alveoli, where pCO2 is low. HCO3- and H+ recombine to form CO2 and H2O, and the CO2 is breathed out.
The named term chloride shift is not printed in the Class 11 NCERT chapter, but the idea is important for NEET; many previous questions expect you to know it.
It explains how CO2 is loaded and unloaded quickly, and it links to the 70% bicarbonate figure — a common one-liner and assertion-reason source in the exam.
