Haemoglobin shows positive cooperativity with oxygen. When an oxygen atom binds to one of hemoglobin's four binding sites, the affinity to oxygen of the three remaining available binding sites increases; i.e. oxygen is more likely to bind to a hemoglobin bound to one oxygen than to an unbound hemoglobin. This is referred to as cooperative binding.
Haemoglobin reaches 50% saturation when the partial pressure of oxygen reaches about 28 mm Hg. Now partial pressure had to be increased by about 70 mm Hg for haemoglobin to reach maximum saturation. If haemoglobin is a cooperative molecule, once haemoglobin reaches 50% saturation it should be easier to attain ~100% saturation. But it is taking a difference of 70 mm Hg. Whereas 50% saturation requires only 28 mm Hg. Hope this makes sense. Please answer this and correct my understanding. Thank you