The oxygen dissociation curve

A graph of loading and unloading

How full of oxygen is haemoglobin at a given pO$_2$? A graph answers this. Plot the per cent saturation of haemoglobin with oxygen against the pO$_2$. You get an S-shaped, or sigmoid, curve. It is called the oxygen dissociation curve.

The curve is very useful. It shows how the other factors, such as pCO$_2$ and hydrogen ion concentration, change the binding of oxygen to haemoglobin.

Reading the normal curve

• At the high pO$_2$ of the alveoli the curve is almost flat. The haemoglobin is nearly full. A small fall in pO$_2$ costs only a little saturation.
• At the lower pO$_2$ of the tissues the curve is steep. A small fall in pO$_2$ gives up much oxygen.

So blood leaves the lungs about 97 per cent saturated and returns from the tissues still about 75 per cent saturated. A standard equation for the human curve, by Severinghaus, gives about 98 per cent at 104 mm Hg. It gives 75 per cent at 40 mm Hg. So 97 and 75 are round values. The blood gives up the difference, about one fifth of its oxygen, in each round. This matches the 5 mL for each 100 mL that you met earlier, because blood that is fully saturated holds about 20 mL of oxygen in each 100 mL.

The curves in the figures of this course are drawn from a model, the Hill equation with a steepness of 2.7. It is a good fit to the normal curve, but it is a model. The curves to the right and to the left are the same equation with other values of $P_{50}$.

Why an S shape?

The first oxygen molecule that binds to haemoglobin makes it easier for the next ones to bind. So the curve is low at the start, rises steeply in the middle and flattens when the haemoglobin is full. This explanation is beyond the textbook, but it is useful.

What moves the curve?

Four conditions of the tissues push the curve to the right.

• To the right: more carbon dioxide, more hydrogen ions (a lower pH) or a higher temperature. At the same pO$_2$ the haemoglobin is less saturated, so oxygen is released more easily. A working muscle makes exactly these changes.
• To the left: less carbon dioxide, fewer hydrogen ions or a lower temperature. Haemoglobin holds on to oxygen more tightly. The alveoli have these conditions.

Many books call the effect of carbon dioxide and hydrogen ions on the curve the Bohr effect. The textbook describes the effect without using the name.

A useful number is $P_{50}$. It is the pO$_2$ at which haemoglobin is half saturated. The normal value is about 27 mm Hg, or 26.8 mm Hg for blood at 37 °C, pH 7.4 and pCO$_2$ 40 mm Hg. A shift to the right makes $P_{50}$ larger. A shift to the left makes it smaller. This number is also beyond the textbook.

You should find that a larger $P_{50}$ lowers the saturation at the tissues a lot and at the lungs only a little. So the blood gives up more oxygen.

Try it in Python

The program uses a model of the curve to find how much oxygen the blood gives up, for a normal curve and for a working muscle.

Practise with new numbers

Common mistakes

• Drawing the curve as a straight line. It is S-shaped.
• Saying that a shift to the right means that haemoglobin holds oxygen more tightly. It means that haemoglobin lets go of oxygen more easily.
• Saying that the blood from the tissues carries no oxygen. It is still about 75 per cent saturated at rest.
• Forgetting that the curve is flat at the lungs. A small drop in alveolar pO$_2$ costs only a little saturation.
• Mixing up the conditions. Higher carbon dioxide, more hydrogen ions and higher temperature all shift the curve to the right.

Remember these

• The oxygen dissociation curve is a sigmoid graph of per cent saturation against pO$_2$.
• At 104 mm Hg haemoglobin is about 97 per cent saturated. At 40 mm Hg it is about 75 per cent saturated.
• Higher pCO$_2$, more hydrogen ions and a higher temperature shift the curve to the right, so oxygen is released more easily.
• Lower pCO$_2$, fewer hydrogen ions and a lower temperature shift it to the left, so oxygen is held more tightly.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is the shape of the oxygen dissociation curve?

2. What two things are plotted?

3. Name three changes in a working muscle that shift the curve to the right.

4. What does a shift to the right do for the release of oxygen?

5. Why does the blood leaving the lungs stay nearly full even if the alveolar pO$_2$ falls a little?

References

• NCERT (2023). Biology, Textbook for Class XI, chapter 14, Breathing and Exchange of Gases.
• Severinghaus J. W. (1979). Simple, accurate equations for human blood O2 dissociation computations. Journal of Applied Physiology, 46(3), 599-602. DOI 10.1152/jappl.1979.46.3.599.
• West J. B. and Luks A. M. (2021). West's Respiratory Physiology: The Essentials, 11th edition, Wolters Kluwer, chapter Gas Transport by the Blood.