IS Question

Let's see at the molecular level.

The oxyhemoglobin dissociation curve describes the relation between the partial pressure of oxygen (x axis) and the oxygen saturation (y axis). Hemoglobin's affinity for oxygen increases as successive molecules of oxygen bind. More molecules bind as the oxygen partial pressure increases until the maximum amount that can be bound is reached. As this limit is approached, very little additional binding occurs and the curve levels out as the hemoglobin becomes saturated with oxygen. Hence the curve has a sigmoidal or S-shape.

As long as enough oxygen is absorbed in the lung, it is not a matter to have the breathed air that still contains excess oxygen, unless there is more oxygen deficient haemoglobin by blood transfusion which can uptake more oxygen from air.