an Increased amount of co2 produced due to respiration diffuses into red blood cells where it reacts with water to form carbonic acid, catalysed by carbonic anhydrase. Carbonic acid dissociates into hydrogen carbonate ions and hydrogen ions which Lowers blood ph, So haemoglobin acts as a buffer by binding to hydrogen ions forming haemoglobinic acid which prevents a further fall in pH This causes haemoglobins shape to change slightly Reducing hbs affinity for oxygen Resulting in faster unloading of oxygen to respiring tissues