8.2.4—Oxygen dissociation curve

The oxygen dissociation curve shows haemoglobin saturation against the partial pressure of oxygen (pO2). Its sigmoidal shape reflects changing haemoglobin affinity as oxygen binds and dissociates.

1. Read the axes: x-axis = oxygen partial pressure; y-axis = percentage saturation of haemoglobin with oxygen. The graph is not a time graph and does not show the total oxygen content of all blood.
2. Explain the low-pO2 region: at low pO2, the first oxygen binds less easily, so affinity and saturation are relatively low; dissociation of the final bound oxygen also becomes slower when few binding sites remain occupied.
3. Explain the steep region: after one oxygen binds, haemoglobin changes conformation → the next oxygen molecules bind more easily → a modest pO2 change can produce a relatively large change in saturation. This is cooperative binding.
4. Explain the high-pO2 plateau: as haemoglobin approaches saturation, few binding sites remain → further pO2 increases produce only a small additional saturation change.
5. Apply the shape: high pO2 in the lungs supports loading and keeps haemoglobin near the plateau; the lower pO2 conditions of respiring tissues lie in the unloading region, where oxygen can be released for cellular respiration.
6. Make a cautious read-off: identify the relevant pO2, project to the curve, then read the approximate saturation; report an estimate rather than inventing precision between graph marks.

Boundary: this card explains the curve and cooperative shape. Card 4609 handles the detailed partial-pressure gradient and loading/unloading application; card 4610 handles the Bohr shift.

A right-hand plateau does not mean no oxygen can be released, and a left/steep region does not mean haemoglobin is absent. Saturation is a percentage at a specified pO2. No image generated or bound.