Cambridge A-Level Biology – Transport of Oxygen and Carbon Dioxide
Transport of Oxygen and Carbon Dioxide
Overview
In the human circulatory system, oxygen (O₂) is carried from the lungs to tissues, while carbon dioxide (CO₂) is transported from tissues back to the lungs for exhalation. The primary carrier for both gases is haemoglobin (Hb) within red blood cells.
The Bohr Shift
The Bohr shift (or Bohr effect) describes the influence of pH and carbon dioxide concentration on the affinity of haemoglobin for oxygen.
Definition
When the partial pressure of CO₂ (\$p{\mathrm{CO2}}\$) rises or the pH falls (more acidic), haemoglobin’s affinity for O₂ decreases, causing the oxygen‑haemoglobin dissociation curve to shift to the right.
Mechanism
Metabolically active tissues produce CO₂ as a waste product.
CO₂ diffuses into red blood cells and reacts with water:
The increase in \$H^+\$ (decrease in pH) protonates specific amino‑acid residues on the haemoglobin molecule.
Protonation stabilises the “tense” (T) conformation of haemoglobin, which has a lower affinity for O₂.
Consequently, O₂ is released more readily at the tissue level.
Factors Influencing the Bohr Shift
Factor
Effect on Haemoglobin Affinity for O₂
Direction of Curve Shift
Increased \$p{\mathrm{CO2}}\$
Decreases affinity
Right
Decreased pH (more H⁺)
Decreases affinity
Right
Increased temperature
Decreases affinity
Right
Increased 2,3‑BPG (2,3‑bisphosphoglycerate)
Decreases affinity
Right
Decreased \$p{\mathrm{CO2}}\$
Increases affinity
Left
Increased pH (more alkaline)
Increases affinity
Left
Importance of the Bohr Shift
Efficient O₂ delivery: Active tissues generate CO₂ and H⁺, triggering the Bohr shift so that haemoglobin releases O₂ exactly where it is needed.
Facilitates CO₂ transport: As O₂ is released, haemoglobin’s capacity to bind CO₂ (as carbamate) and H⁺ (as a buffer) increases, enhancing CO₂ removal.
Regulation of blood pH: By promoting CO₂ release in the lungs (where \$p{\mathrm{CO2}}\$ falls and pH rises), the Bohr shift assists in maintaining systemic acid‑base balance.
Adaptation to exercise: During intense activity, the combined rise in temperature, \$p{\mathrm{CO2}}\$, and H⁺ produces a pronounced right‑shift, matching the heightened metabolic demand.
Summary of Gas Transport
Both O₂ and CO₂ are transported in three principal forms:
Dissolved in plasma: Approximately 1.5 % of O₂ and 5–10 % of CO₂.
Bound to haemoglobin:
O₂ binds to the iron atom in the heme groups (≈ 98 % of O₂ transport).
CO₂ binds to the amino groups of the globin chains forming carbamino‑haemoglobin (≈ 20–30 % of CO₂ transport).
As bicarbonate ion (HCO₃⁻): The majority of CO₂ is converted by carbonic anhydrase:
Suggested diagram: Oxygen‑haemoglobin dissociation curves illustrating the right‑shift (Bohr effect) under high CO₂ / low pH conditions versus the left‑shift under low CO₂ / high pH conditions.