Active tissues, such as exercising muscle, have a higher O₂ consumption and changes to the Hb-O₂ carriage help to provide this.

As tissue metabolism increases, more CO₂ is produced leading to local acidosis. This environment moves the Hb-O₂ curve to the right, so that at a given PO₂ the Hb is less saturated, giving up more O₂ to the tissues. This is known as the Bohr effect (Fig 1).

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The small rise in temperature locally during exercise also contributes to the right shift of the curve.

The opposite occurs in the lungs, with CO₂ removal leading to a left shift of the curve, increasing the affinity of Hb for O₂ to aid loading.