An oxygen dissociation curve shows the percentage saturation of haemoglobin with oxygen (y-axis) at different partial pressures of oxygen, pO2 (x-axis). For haemoglobin the curve is S-shaped (sigmoid).
• once the first oxygen binds, the shape (tertiary and quaternary structure) of the haemoglobin changes, making it easier for the second and third oxygen molecules to bind. The curve becomes steep.
• it is harder for the fourth oxygen to bind, because with most sites filled there is a lower chance of an oxygen molecule meeting the last free site. The curve levels off near 100%.The shape suits oxygen transport:• in the lungs, pO2 is about 12 to 13 kPa, on the flat top of the curve. Haemoglobin becomes almost fully saturated (about 97%), and small drops in alveolar pO2 hardly reduce loading.
• in respiring tissues, pO2 is around 2 to 5 kPa, on the steep part of the curve. A small fall in pO2 causes a large fall in saturation, so a lot of oxygen is released where it is most needed.Worked example: blood leaves the lungs 97% saturated. In a muscle at 3 kPa it is 40% saturated. The haemoglobin has therefore unloaded 97 − 40 = 57% of its maximum oxygen.
• once the first oxygen binds, the shape (tertiary and quaternary structure) of the haemoglobin changes, making it easier for the second and third oxygen molecules to bind. The curve becomes steep.
• it is harder for the fourth oxygen to bind, because with most sites filled there is a lower chance of an oxygen molecule meeting the last free site. The curve levels off near 100%.The shape suits oxygen transport:• in the lungs, pO2 is about 12 to 13 kPa, on the flat top of the curve. Haemoglobin becomes almost fully saturated (about 97%), and small drops in alveolar pO2 hardly reduce loading.
• in respiring tissues, pO2 is around 2 to 5 kPa, on the steep part of the curve. A small fall in pO2 causes a large fall in saturation, so a lot of oxygen is released where it is most needed.Worked example: blood leaves the lungs 97% saturated. In a muscle at 3 kPa it is 40% saturated. The haemoglobin has therefore unloaded 97 − 40 = 57% of its maximum oxygen.