Apply the oxygen curve to lungs and tissues
Explain how the adult haemoglobin curve supports loading in lungs and unloading in respiring tissues.
Section 8.2 · Transport of oxygen and carbon dioxide
AS Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; microscope recognition, biological drawing and supervised practical competence require practical work, while human subject-teacher review remains pending.
Before you begin
- Know that mass transport moves substances between exchange surfaces and cells, and that diffusion occurs down a concentration or partial-pressure gradient.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which region of the curve is associated with high haemoglobin saturation in the lungs?
What you will learn
- Use lung and tissue oxygen partial pressures to explain loading and unloading
- Interpret the plateau and steep regions in transport contexts
Build the explanation
In the lungs, oxygen partial pressure is high, so haemoglobin reaches a high saturation on the plateau region of the adult oxygen dissociation curve. This supports reliable oxygen loading even if oxygen partial pressure varies modestly. In respiring tissues, oxygen partial pressure is lower; the operating point lies on the steep region, where a modest fall in partial pressure can produce substantial oxygen unloading. Oxygen then diffuses from blood into tissues down its partial-pressure gradient.
Apply it carefully
Name the place, compare partial pressures and connect the operating point to saturation and diffusion. The curve describes haemoglobin binding; the oxygen gradient across a tissue capillary describes diffusion into cells. Do not claim that haemoglobin actively pumps oxygen or that a lower tissue partial pressure alone proves a Bohr shift.
Pause and explain
Why can oxygen unload substantially in active tissues?
Worked example
Why does the lung operating point favour oxygen loading?
Show the worked solution
- Compare lung oxygen partial pressure with tissue partial pressure.
- High lung partial pressure corresponds to high saturation on the plateau.
Answer The high oxygen partial pressure in the lungs favours haemoglobin loading and produces a high saturation on the plateau of the curve.
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Explain how the curve supports oxygen loading in the lungs and unloading in tissues.
Give me a hint
Compare oxygen partial pressure and curve region in each place.
Compare my reasoning
- Lungs: high oxygen partial pressure, high saturation on plateau.
- Tissues: lower partial pressure, steep region supports unloading; oxygen diffuses into cells.
High oxygen partial pressure in the lungs favours haemoglobin loading and high saturation on the plateau. Respiring tissues have lower oxygen partial pressure, so the operating point lies on the steep region and haemoglobin unloads oxygen, which diffuses into tissues down its partial-pressure gradient.
Look for these in your work
- Lung loading explained
- Tissue unloading explained
- Diffusion gradient included
Blood saturation falls from 96% to 72% as it passes through an active tissue. What does this indicate, and what should not be inferred from saturation alone?
Give me a hint
Interpret unloading, then distinguish curve position from curve shift.
Compare my reasoning
- Haemoglobin released oxygen to the tissue.
- The change alone does not prove an affinity shift; partial pressure and other conditions are needed.
Haemoglobin has unloaded oxygen as blood passed the tissue. The saturation fall alone does not show that the curve shifted; it may represent movement along the same curve as oxygen partial pressure fell.
Look for these in your work
- Unloading interpreted
- Shift inference limited
A patient-like teaching graph has a higher saturation at high lung oxygen partial pressure but little change on the plateau. Explain why the plateau is useful for loading.
Give me a hint
Consider how saturation responds to small changes in lung partial pressure.
Compare my reasoning
- The plateau means haemoglobin remains highly saturated across a range of high partial pressures.
- This makes oxygen loading less sensitive to modest variation in lung partial pressure.
The plateau represents high haemoglobin saturation at high oxygen partial pressure, so modest changes in lung partial pressure cause little change in loading. This supports reliable oxygen loading under those conditions.
Look for these in your work
- Plateau interpreted
- Loading significance linked to robustness
- No clinical inference
Common mistakes
- Using tissue oxygen content instead of partial pressure when reading the curve
- Confusing unloading with an automatic curve shift
- Saying oxygen diffuses from cells into blood in active tissues
What can you explain now?
Which region of the curve is associated with high haemoglobin saturation in the lungs?
Compare with the explanation
The plateau at high oxygen partial pressure
High oxygen partial pressure supports loading and the plateau limits the saturation change for modest variation.
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Make it stick
Where do lungs and respiring tissues operate on the curve, and what does that mean for oxygen movement?
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