International A Level Biology / Mammalian transport systems

The oxygen dissociation curve revision notes

Edexcel International A Level Biology (YBI11), chapter 2: Mammalian transport systems, specification point 1.9. Free lesson.

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The oxygen dissociation curve shows how haemoglobin saturation changes as the partial pressure of oxygen changes. Its S shape explains how haemoglobin loads oxygen in the lungs and unloads it in respiring muscle.

What the axes show

The curve links the oxygen around the blood to how full the haemoglobin is. Partial pressure is the pressure due to oxygen alone. Saturation is the percentage of the haemoglobin’s oxygen-carrying capacity that is filled. The x axis shows partial pressure of oxygen, written pO₂, in kPa (kilopascals). The y axis shows percentage saturation of haemoglobin, from 0% to 100%. Each haemoglobin has four haem groups, each able to carry one oxygen molecule. So 100% saturation means all four sites are full.

How to read the curve

The curve is called a dissociation curve because it shows oxygen coming off haemoglobin. It is sigmoid, or S shaped: shallow at first, steep in the middle, then flatter near the top. Read a value by finding the pO₂ on the x axis, then reading the matching saturation on the y axis. In the lungs, pO₂ is about 13 kPa and haemoglobin is about 98% saturated. In resting muscle, pO₂ is about 5 kPa and saturation is about 70%. In active muscle, pO₂ is about 2 kPa and saturation is about 20%. Most of the oxygen has been unloaded. When asked to use the graph, quote the values. Describing only the S…

Why the curve is S shaped

The shape is caused by cooperative binding: when one oxygen molecule binds, haemoglobin changes shape and the next oxygen molecules bind more easily. This makes loading and unloading work well in different parts of the curve. At low pO₂, the first oxygen binds with difficulty, so the curve starts shallow. The first oxygen changes the shape of the whole haemoglobin molecule. It now has a higher affinity, meaning it binds oxygen more readily. The next two oxygen molecules bind easily, giving the steep middle section. The fourth oxygen is harder to bind because only one site remains, so…

How much oxygen is unloaded

To find the fall in saturation, read the saturation at the starting pO₂ and subtract the saturation at the muscle pO₂. Both readings are percentages, so subtract them directly.

What the steep section means

On the steep part, a small fall in pO₂ causes a large fall in saturation. This allows a lot of oxygen to be unloaded in respiring muscle, where pO₂ is low. The steepness is not a rate: the x axis shows pO₂, not time. In the lungs, at about 13 kPa, the curve is flat, so haemoglobin loads oxygen to almost 100% saturation. In respiring muscle, the lower pO₂ is on the steep part, so a small fall in pO₂ leads to a large fall in saturation. A gradient describes a rate only when time is on the x axis. Here it shows how much saturation changes for a change in pO₂.