Cambridge A-Level Biology – Transport of Oxygen and Carbon Dioxide
Transport of Oxygen and Carbon Dioxide
Learning Objective
Describe and explain the oxygen dissociation curve of adult haemoglobin (HbA).
Key Concepts
Oxygen is carried in blood mainly bound to haemoglobin within red blood cells.
Carbon dioxide is transported in three forms: dissolved in plasma, as bicarbonate ions, and bound to haemoglobin (carbamino‑haemoglobin).
The oxygen‑haemoglobin relationship is expressed by the oxygen dissociation curve.
Oxygen Dissociation Curve
The curve plots the percentage saturation of haemoglobin (% HbSat) against the partial pressure of oxygen (pO₂). It has a sigmoidal (S‑shaped) form due to cooperative binding of O₂ to the four subunits of haemoglobin.
Cooperative binding can be expressed by the Hill equation:
where \$\theta\$ is the fractional saturation, \$n\$ is the Hill coefficient (≈2.8 for adult Hb), and \$P_{50}\$ is the pO₂ at 50 % saturation.
Typical \cdot alues for Adult Haemoglobin (HbA)
pO₂ (mm Hg)
% Hb Saturation
20
5
40
25
60
50
80
90
100
98
Factors Shifting the Curve
pH (Bohr effect): Decrease in pH (more acidic) shifts the curve to the right, reducing affinity for O₂.
Temperature: Higher temperature shifts the curve right; lower temperature shifts it left.
Partial pressure of CO₂: Increased pCO₂ shifts the curve right (Bohr effect).
2,3‑Bisphosphoglycerate (2,3‑BPG): Higher 2,3‑BPG concentration shifts the curve right, facilitating O₂ release in tissues.
Presence of fetal haemoglobin (HbF): HbF has higher affinity for O₂, shifting the curve left compared with HbA.
Physiological Significance
The sigmoidal shape ensures:
High loading of O₂ in the lungs where pO₂ is high (right‑hand side of the curve).
Efficient unloading of O₂ in metabolically active tissues where pO₂ is low (left‑hand side of the curve).
Rapid response to changes in pH, temperature, and CO₂, matching O₂ delivery to tissue demand.
Suggested diagram: Oxygen dissociation curve showing normal position, right‑shift (low affinity) and left‑shift (high affinity) conditions.
Summary
The oxygen dissociation curve of adult haemoglobin illustrates how haemoglobin’s affinity for oxygen varies with pO₂ and is modulated by physiological factors. Understanding the curve explains how oxygen is efficiently loaded in the lungs and released in tissues, and how conditions such as acidosis, fever, or increased 2,3‑BPG alter oxygen delivery.