Fig 1 shows oxygen molecules binding to haemoglobin.

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The main factor driving O₂ to bind to Hb is its partial pressure but the relationship between PO₂ and Hb-O₂ binding is not proportional.

Initial binding of O₂ is difficult, but as the first O₂ molecule binds it changes the shape of the Hb molecule slightly, making other binding sites more accessible.

Subsequent binding of the second and third O₂ molecules is therefore easier (cooperativity), but once these sites are occupied, full saturation with the fourth molecule becomes more difficult as only one free binding site remains.