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Line graph showing percent of haemoglobin (Hb) saturated with oxygen (on a scale from 0% – 100%) on the vertical axis, and PO2 in mmHg from 0 – 100 mmHg on the horizontal axis.

The oxygen dissociation curve is shown as a red line starting from the origin for both axes, rising to give a 70% Hb saturation at PO2 of 40 mmHg; the curve then rises less steeply, almost to plateau at about 95% Hb saturation at PO2 of about 90 mm Hg.

At 30 mmHg an arrow notes that there is a low PO2 in tissue: less oxygen is attached to the Hb; at 78 mmHg an arrow notes that there is a high PO2 in the alveoli: lots of oxygen is attached to the Hb.

The student is invited to drag a marker to the point in the curve where 50% of the Hb is saturated with O2.

 4.2 Haemoglobin