Cambridge A-Level Biology – Transport of Oxygen and Carbon Dioxide
Transport of Oxygen and Carbon Dioxide
Learning Objective
Explain the importance of the oxygen dissociation curve at the partial pressures of oxygen in the lungs and in respiring tissues.
Key Concepts
Partial pressure of oxygen (\$P{O2}\$) determines how much O₂ binds to haemoglobin.
The oxygen‑haemoglobin dissociation curve relates % saturation of haemoglobin to \$P{O2}\$.
Its sigmoidal shape reflects cooperative binding of O₂ molecules.
Shift of the curve to the right or left alters O₂ loading in the lungs and unloading in tissues.
Oxygen Dissociation Curve – Typical Data
\$P{O2}\$ (mm Hg)
Haemoglobin Saturation (%)
20
35
40
65
60
85
80
95
100
98
Why the Curve Matters in the Lungs
Alveolar \$P{O2}\$ is high (≈100 mm Hg). At this pressure the curve is near its plateau, so a small increase in \$P{O2}\$ produces only a modest increase in saturation – haemoglobin is already almost fully loaded.
Because the curve is flat, the lungs can efficiently load O₂ onto haemoglobin even if \$P{O2}\$ falls slightly (e.g., during mild hypoventilation).
High \$P{O2}\$ also favours the formation of oxy‑haemoglobin, reducing the amount of free dissolved O₂ needed to meet tissue demand.
Why the Curve Matters in Respiring Tissues
In active muscle the \$P{O2}\$ may fall to 20–40 mm Hg. Here the curve is steep, so a small drop in \$P{O2}\$ causes a large fall in haemoglobin saturation, promoting O₂ release.
Factors that shift the curve to the right (↑ CO₂, ↑ H⁺, ↑ temperature, ↑ 2,3‑BPG) further enhance O₂ unloading where it is most needed.
The steep portion ensures that O₂ delivery is closely matched to metabolic demand.
Physiological Shifts of the Curve
The curve can move right or left in response to the following:
Shift
Cause
Effect on O₂ Loading/Unloading
Right
↑ CO₂, ↑ H⁺ (low pH), ↑ temperature, ↑ 2,3‑BPG
Decreases affinity → easier O₂ release in tissues.
Left
↓ CO₂, ↑ pH (alkalosis), ↓ temperature, ↓ 2,3‑BPG
Increases affinity → better O₂ loading in lungs.
Integrating the Concepts
When blood circulates from the lungs to the tissues, it moves from a region of high \$P{O2}\$ (plateau of the curve) to a region of low \$P{O2}\$ (steep part). This gradient, together with right‑shifts in the curve at the tissue level, ensures:
Maximum O₂ uptake in the lungs.
Efficient O₂ delivery to cells with high metabolic rates.
Rapid response to changes in activity, altitude, or disease states.
Suggested diagram: A combined graph showing the oxygen‑haemoglobin dissociation curve with marked points for typical alveolar \$P{O2}\$ (\overline{100} mm Hg) and exercising muscle \$P{O2}\$ (\overline{30} mm Hg), plus arrows indicating right‑shift factors.
Summary
The oxygen dissociation curve is a vital tool for understanding how haemoglobin loads O₂ in the high‑pressure environment of the lungs and unloads it in the low‑pressure, high‑demand environment of respiring tissues. Its sigmoidal shape and the ability to shift right or left allow the circulatory system to meet the varying oxygen needs of the body efficiently.