Transport of oxygen and carbon dioxide

Transport of gases

In plain words: haemoglobin in red blood cells is a fleet of taxis that pick up oxygen where it is plentiful (the lungs) and drop it where it is scarce (the tissues). Carbon dioxide mostly travels a different way: an enzyme in the red cells turns it into bicarbonate, which rides in the blood and is turned back into CO_₂ in the lungs.

The CO_₂ shares are approximate: as printed, they add to 97–102 per cent. For NCERT and NEET questions, use NCERT’s values. Beyond the NCERT text: the actual shares depend on conditions, and they are fractions of the gas carried, not haemoglobin saturation (our reading).

Transport of oxygen

• Haemoglobin is a red, iron-containing pigment in the RBCs. O_₂ binds to it reversibly, forming oxyhaemoglobin (NCERT Section 14.4.1, page 189).
• Each haemoglobin molecule can carry a maximum of four molecules of O_₂ (page 189); it has four subunits (see structure of proteins).
• Binding depends mainly on the partial pressure of O_₂. The partial pressure of CO_₂, the hydrogen ion concentration and temperature can interfere with it (page 189).
• Every 100 mL of oxygenated blood can deliver around 5 mL of O_₂ to the tissues under normal physiological conditions (page 189). Beyond the NCERT text: this is the amount the tissues take up, not the blood’s whole oxygen content (our reading).

(NCERT page 189.) So O_₂ is bound to haemoglobin at the lung surface and released in the tissues.

Oxygen dissociation curve

• When the percentage saturation of haemoglobin with O_₂ is plotted against pO_₂, a sigmoid curve is obtained: the oxygen dissociation curve (NCERT page 189, Figure 14.5).
• It is highly useful for studying how factors like pCO_₂ and H^⁺ concentration affect the binding of O_₂ to haemoglobin (page 189).
• Beyond the NCERT text, the S shape comes from the cooperative binding of O_₂ to haemoglobin. The position of the curve changes with pH, pCO_₂ and temperature; more CO_₂ shifts it to the right, lowering haemoglobin’s affinity for O_₂ (Valsecchi and colleagues, Frontiers in Medicine, 2026).
• This effect of CO_₂ and H^⁺ is called the Bohr effect: more CO_₂ and more H^⁺ reduce haemoglobin’s affinity for O_₂ and promote its release in active tissues (beyond the NCERT text; Valsecchi and colleagues, Frontiers in Medicine, 2026). In the tissues, low pO_₂, high pCO_₂, high H^⁺ concentration and higher temperature are favourable for the dissociation of oxygen from oxyhaemoglobin (NCERT page 189).

Transport of carbon dioxide

• About 20–25 per cent of CO_₂ is carried by haemoglobin as carbamino-haemoglobin (NCERT Section 14.4.2, page 189).
• This binding is related to the partial pressure of CO_₂, and pO_₂ is a major factor that affects it (page 189):
  • in the tissues, where pCO_₂ is high and pO_₂ low, more CO_₂ binds;
  • in the alveoli, where pCO_₂ is low and pO_₂ high, CO_₂ dissociates from carbamino-haemoglobin.
• So CO_₂ bound to haemoglobin at the tissues is delivered at the alveoli (pages 189–190).
• Beyond the NCERT text, this effect of pO_₂ on CO_₂ binding is called the Haldane effect: deoxygenated haemoglobin binds CO_₂ more effectively, which helps carry CO_₂ from the tissues to the lungs (Valsecchi and colleagues, Frontiers in Medicine, 2026); in the alveoli, where pO_₂ is high, the CO_₂ is released (NCERT pages 189–190).

Beyond the NCERT text: in carbamino-haemoglobin, CO_₂ binds to amino groups of the globin chains, at their ends and on some side chains (Wikipedia, “Carbaminohemoglobin”, facts in our words), not to the haem sites that carry O_₂ (our reading).

Bicarbonate and carbonic anhydrase

• RBCs contain a very high concentration of the enzyme carbonic anhydrase, and minute quantities are present in the plasma too (NCERT page 190). Beyond the NCERT text: the enzyme catalyses the reversible hydration of CO_₂, and the dissociation of carbonic acid that follows is a separate equilibrium (our reading). The overall sequence can proceed in both directions (see how fast carbonic anhydrase works):

CO_₂ + H_₂O ⇌ H_₂CO_₃

H_₂CO_₃ ⇌ HCO_₃^⁻ + H^⁺

• At the tissues, where pCO_₂ is high because of catabolism, CO_₂ diffuses into the blood (RBCs and plasma) and forms HCO_₃^⁻ and H^⁺ (page 190).
• At the alveoli, where pCO_₂ is low, the reaction runs the other way, forming CO_₂ and H_₂O. So CO_₂ trapped as bicarbonate at the tissues is carried to the alveoli and released as CO_₂ (page 190). (NCERT’s text here points to Figure 14.4, the section of an alveolus; the carriage of CO_₂ from tissues to alveoli is drawn in NCERT Figure 14.3, page 188.)
• Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO_₂ to the alveoli (page 190).

Beyond the NCERT text: as bicarbonate builds up inside the red blood cells through the action of carbonic anhydrase, bicarbonate exits to plasma in exchange for chloride entering the cells; this is the chloride shift (Valsecchi and colleagues, Frontiers in Medicine, 2026). In the lungs, where NCERT’s reaction runs the other way (page 190), the exchange reverses, returning bicarbonate to the RBCs for CO_₂ release (our reasoning).

Solved examples

Common mistakes to avoid

Quick revision summary

• O_₂: about 97 per cent bound to haemoglobin in RBCs, 3 per cent dissolved in plasma. CO_₂: 70 per cent as bicarbonate, 20–25 per cent as carbamino-haemoglobin, about 7 per cent dissolved.
• Haemoglobin binds O_₂ reversibly, up to four per molecule; binding depends mainly on pO_₂, and pCO_₂, H^⁺ and temperature interfere.
• The oxygen dissociation curve (% saturation against pO_₂) is sigmoid. O_₂ loads in the alveoli and unloads in the tissues; 100 mL of blood delivers about 5 mL of O_₂.
• Carbonic anhydrase, mostly in RBCs, converts CO_₂ to HCO_₃^⁻ and H^⁺ at the tissues and back at the alveoli; 100 mL of deoxygenated blood delivers about 4 mL of CO_₂.

Questions students ask

What is the Bohr effect, and does NCERT name it?

The Bohr effect is the fall in haemoglobin’s affinity for oxygen when the pCO_₂ rises or the blood becomes more acidic (more H^⁺), which shifts the oxygen dissociation curve to the right so that more O_₂ can be released at a given pO_₂ (beyond the NCERT text; Valsecchi and colleagues, Frontiers in Medicine, 2026). NCERT does not use the name, but it describes the effect: it lists pCO_₂ and H^⁺ concentration among the factors that can interfere with the binding of O_₂ to haemoglobin, and uses the oxygen dissociation curve to study them (NCERT page 189). The NEET (UG) 2026 syllabus names “transport of gases” but not the Bohr effect, so learn NCERT’s wording first and the name as an extra.

What is the difference between a right shift and a left shift of the oxygen dissociation curve?

A right shift means haemoglobin holds O_₂ less tightly, so at the same pO_₂ it is less saturated and gives up more O_₂, while a left shift means it holds O_₂ more tightly and loads it more easily. The right shift goes with the conditions NCERT gives for the tissues, high pCO_₂, high H^⁺ concentration and higher temperature, and the left shift with those of the alveoli, low pCO_₂, less H^⁺ and lower temperature (NCERT page 189). The curve shifts when those modifying conditions change; a fall in pO_₂ alone moves the operating point along the same curve (beyond the NCERT text, which gives the conditions but not the shifts; Valsecchi and colleagues, Frontiers in Medicine, 2026).

What is the difference between the Bohr effect and the Haldane effect?

The Bohr effect is the effect of CO_₂ and H^⁺ on oxygen transport, while the Haldane effect is the effect of oxygen on CO_₂ transport: more CO_₂ and H^⁺ make haemoglobin release O_₂, and more O_₂ makes the blood give up CO_₂ (beyond the NCERT text; Valsecchi and colleagues, Frontiers in Medicine, 2026). The two work as a pair: in the tissues CO_₂ helps unload O_₂ and the low pO_₂ helps the blood take up CO_₂, and in the alveoli the reverse happens (same source). NCERT describes both without the names: pCO_₂ and H^⁺ concentration interfere with the binding of O_₂, and pO_₂ is a major factor that affects the binding of CO_₂ as carbamino-haemoglobin (NCERT page 189).

Why is carbonic anhydrase important in the transport of CO_₂?

Carbonic anhydrase is important because CO_₂ can travel as bicarbonate only if it is first turned into carbonic acid fast enough: the reaction CO_₂ + H_₂O ⇌ H_₂CO_₃ is very slow on its own, and the enzyme speeds it about 10 million times (NCERT Chapter 9). It also runs the reaction the other way at the alveoli, so the CO_₂ trapped as bicarbonate is released there (NCERT pages 189–190). The H^⁺ formed helps unload O_₂, because a high H^⁺ concentration favours the dissociation of oxygen from oxyhaemoglobin (reasoned from NCERT pages 189–190); beyond the NCERT text, haemoglobin can bind this H^⁺ as it shifts to its low-oxygen-affinity form (Valsecchi and colleagues, Frontiers in Medicine, 2026).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Name the iron-containing pigment that carries O_₂ in the blood.

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   Haemoglobin, present in the RBCs (NCERT page 189).

2. Q2. In what form is most CO_₂ transported?

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   As bicarbonate, about 70 per cent (NCERT page 189).

3. Q3. How much CO_₂ does every 100 mL of deoxygenated blood deliver to the alveoli?

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   Approximately 4 mL (NCERT page 190).