Oxyhaemoglobin dissociation curve

The curve consists of an X axis representing the percentage saturation of haemoglobin and a Y axis representing pO2. The sigmoid shape of the curve comprises: The loading region: oxygen-free molecules (deoxyhaemoglobin) are reluctant to take up the first oxygen molecule but their appetite for oxygen grows with the eating (Perutz).

Figure 8.1 Oxyhaemoglobin dissociation curve. (Hb, haemoglobin.)

The flat region or plateau is observed above pO2 of about 10 kPa. The non-linearity of the dissociation curve allows optimisation of maximum oxygen diffusion in the lungs. Maximum arterial oxygen saturation is associated with maintenance of a nearly constant paO2 despite fluctuations in alveolar O2 tension. This provides a margin of safety as a fall in paO2 does not result in profound desaturation. The unloading region. The steep portion of the curve below a pO2 of about 10 kPa. Small falls in pO2 cause a large transfer of O2 to the tissues. This allows ready release of oxygen if the tissue oxygen consumption increases. The more pronounced the sigmoid shape of the equilibrium curve the greater the fraction of oxygen in the red cells that can be unloaded to the tissues. The sigmoid curve favours oxygen transport, allowing the saturation of haemoglobin in the lungs and deoxygenation in the capillaries.

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