Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 292–294 of the course textbook. Image credits are on slide 15.
The curve plots percentage saturation of haemoglobin against the partial pressure of oxygen (pO2, in kPa). It is S-shaped because of cooperative binding: when one O2 binds, the subunit changes shape and the other haem groups gain affinity. So haemoglobin loads almost fully in the lungs (about 13 kPa) and unloads readily in tissues (2–5 kPa), where the curve is steepest.
Foetal haemoglobin has a higher affinity for oxygen, so its curve lies to the left. In the placenta, at the same pO2, the mother’s haemoglobin unloads while the foetus’s loads. Haemoglobin is an allosteric protein: CO2 binding at other sites lowers its affinity, and this effect is weaker in foetal haemoglobin. After birth it is replaced by the adult form.
In respiring tissue, CO2 forms carbonic acid in red blood cells, releasing H+ and lowering pH. H+ and CO2 change the shape of haemoglobin, so its affinity falls and the curve shifts right. More oxygen is released exactly where respiration is fastest; in the lungs CO2 leaves and affinity rises again.
Select a video to play it here. Select it again to close it.
Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
The self-check needs JavaScript to run.
A printable worksheet covering all three statements, from quick recall to data questions and a longer answer. It has 30 marks and takes about 30 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
You will need: Published oxygen saturation values at two carbon dioxide levels, graph paper or spreadsheet
Safety: No practical hazards.
Carry this out with your teacher's supervision and follow your school's lab rules.
In 1904 Bohr, Hasselbalch and Krogh measured how carbon dioxide alters haemoglobin's oxygen binding. Only one name stayed attached to the effect.
Discoveries are often credited to one person. Consider how this affects our picture of how science works.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
© 2026 Akash Parthi Bendran. Please ask before using or sharing any resources from this site: see permission to use.