Cambridge AS & A‑Level Biology (9700) – Topic 8.2
Transport of Oxygen and Carbon Dioxide

Learning Objective

Describe the role of red blood cells (RBCs) in transporting oxygen (O₂) and carbon dioxide (CO₂) with reference to:

• Haemoglobin (Hb)
• Carbonic anhydrase
• Formation of haemoglobinic acid (Hb‑H⁺)
• Formation of carbaminohaemoglobin (Hb‑CO₂)
• The Bohr shift and the chloride (Hamburger) shift

1. Red Blood Cells – The Vehicle for Gas Transport

• Shape & size: Biconcave discs (≈ 7 µm diameter) give a large surface‑to‑volume ratio, allowing rapid diffusion of gases.
• Absence of nucleus & organelles: Maximises cytoplasmic space for haemoglobin.
• Key contents:

  • ≈ 2.7 × 10⁸ haemoglobin molecules per cell (≈ 34 % of cell dry weight).
  • High concentration of carbonic anhydrase (≈ 1 g L⁻¹ cytoplasm).

• O₂ transport in blood:

  • ≈ 98 % of O₂ is carried bound to Hb (HbO₂).
  • ≈ 2 % is dissolved directly in plasma (≈ 0.3 mL O₂ dL⁻¹ blood).

2. Haemoglobin – The Primary O₂ Carrier

• Protein composed of four globin chains, each with a heme group that can bind one O₂ molecule (4 O₂ per Hb).
• Binding is reversible and co‑operative, producing a sigmoidal O₂‑dissociation curve.

2.1 Structure & Cooperative Binding

• In the oxygenated (R) state the four subunits are in a high‑affinity conformation.
• Binding of the first O₂ increases the affinity of the remaining sites – the basis of positive cooperativity.

2.2 O₂‑Dissociation Curve

2.3 Bohr Shift (Bohr Effect)

In metabolically active tissues:

1. CO₂ production → hydration (catalysed by carbonic anhydrase) → H⁺ + HCO₃⁻.
2. ↑ H⁺ binds to deoxy‑Hb forming Hb‑H⁺, stabilising the T‑state.
3. Hb’s affinity for O₂ falls → O₂ is released.

In the lungs the reverse occurs: CO₂ is expelled, H⁺ concentration falls, Hb regains high affinity and binds O₂.

3. Carbonic Anhydrase – Rapid Inter‑Conversion of CO₂

• Located in the RBC cytoplasm, catalyses:

  \$\text{CO}2 + \text{H}2\text{O} \;\xrightleftharpoons[\text{CA}]{}\; \text{H}2\text{CO}3 \;\rightleftharpoons\; \text{H}^+ + \text{HCO}_3^-\$

• Without the enzyme the reaction is ~10⁴ times slower.
• ≈ 70 % of CO₂ is transported as plasma bicarbonate (HCO₃⁻); the remaining 30 % is carried as Hb‑CO₂ (5–10 %) and dissolved CO₂ (≈ 10 %).

4. Formation of Haemoglobinic Acid (Hb‑H⁺)

• Deoxy‑Hb has a high affinity for H⁺; binding produces haemoglobinic acid:

  \$\text{Hb} + \text{H}^+ \;\rightleftharpoons\; \text{HbH}^+\$

• In the lungs O₂ binding induces a conformational change that releases H⁺ (Hb‑H⁺ → Hb), raising pH.
• In tissues the reverse occurs, contributing to the Bohr shift.

5. Formation of Carbaminohaemoglobin (Hb‑CO₂)

• CO₂ can bind directly to the terminal –NH₂ groups of the globin chains:

  \$\text{HbNH}2 + \text{CO}2 \;\rightleftharpoons\; \text{HbNHCOO}^- + \text{H}^+\$

• Accounts for 5–10 % of total CO₂ transport.
• The reaction also releases H⁺, reinforcing the Bohr effect.

6. Chloride (Hamburger) Shift

Maintains electrical neutrality as HCO₃⁻ moves between RBC and plasma.

• In tissues: HCO₃⁻ leaves the RBC → Cl⁻ enters via the Band 3 protein.
• In lungs: HCO₃⁻ re‑enters the RBC → Cl⁻ exits.

7. Overall Transport Pathway

8. Quantitative Summary (AO1)

• O₂ transport:

  • ≈ 98 % bound to Hb (≈ 1.34 mL O₂ g⁻¹ Hb).
  • ≈ 2 % dissolved in plasma.

• CO₂ transport:

  • ≈ 70 % as plasma bicarbonate (HCO₃⁻).
  • ≈ 20 % as carbamino compounds (Hb‑CO₂).
  • ≈ 10 % dissolved in plasma.

9. Sample Exam‑Style Question (AO2)

Question: The Bohr effect is illustrated in the diagram below (a typical O₂‑dissociation curve). Explain how a decrease in blood pH at active muscle tissue influences the position of the curve and the amount of O₂ released from haemoglobin.

Answer outline:

1. Active muscle metabolism produces CO₂ → carbonic anhydrase forms H⁺ → pH falls.
2. H⁺ binds to deoxy‑Hb, forming Hb‑H⁺ and stabilising the T‑state.
3. This reduces Hb’s affinity for O₂, shifting the O₂‑dissociation curve to the right.
4. A right‑shift means that at a given PO₂ a lower proportion of Hb is saturated, so more O₂ is released to the tissues.

10. Practical Investigation – Demonstrating the Bohr Effect (AO3)

Objective: Show that lowering pH reduces haemoglobin’s affinity for O₂.

Materials: Fresh sheep blood or commercial Hb solution, spectrophotometer or blood‑gas analyser, dilute HCl, dilute NaOH, temperature‑controlled water bath, sealed gas‑tight chambers.

Method (outline):

1. Divide the Hb solution into three equal aliquots; keep all at 37 °C.
2. Adjust pH:

   • Sample A (control) to pH 7.4.
   • Sample B to pH 7.0 using dilute HCl.
   • Sample C to pH 7.8 using dilute NaOH.

3. Bubble a fixed O₂/CO₂ mixture (21 % O₂, 0 % CO₂) through each sample for 5 min.
4. Measure % Hb saturation with the spectrophotometer (or blood‑gas analyser).
5. Record and compare the saturation values.

Expected result: The low‑pH sample shows a markedly lower O₂ saturation at the same PO₂ (right‑shift), whereas the high‑pH sample shows a higher saturation (left‑shift).

Evaluation points:

• Accuracy of pH measurement (use a calibrated pH meter).
• Strict temperature control (Hb affinity is temperature‑dependent).
• Prevent oxidation of Hb (add an antioxidant if necessary).
• Ensure gas mixture composition remains constant throughout the experiment.

11. Key Points to Remember

• RBCs are specialised carriers: haemoglobin for O₂, carbonic anhydrase for rapid CO₂ ↔ HCO₃⁻ inter‑conversion.
• ≈ 98 % of O₂ is transported bound to Hb; ≈ 2 % is dissolved in plasma.
• ≈ 70 % of CO₂ is carried as plasma bicarbonate, ≈ 20 % as Hb‑CO₂, and ≈ 10 % dissolved.
• Bohr shift: ↑ H⁺ / ↑ PCO₂ → right‑shift of the O₂‑dissociation curve → enhanced O₂ release.
• Formation of Hb‑H⁺ and Hb‑CO₂ both lower Hb’s O₂ affinity.
• Chloride shift (Hamburger shift) maintains electrical neutrality as HCO₃⁻ moves between RBC and plasma.
• Understanding the quantitative distribution of gases aids in answering AO1 and AO2 exam questions.