It needs to collect and release CO2 and O2 at the correct times and pressures. It needs to not be poisonous or toxic. It needs to not be quickly processed by th

It needs to collect and release CO2 and O2 at the correct times and pressures. It needs to not be poisonous or toxic. It needs to not be quickly processed by the liver or kidneys, but processed enough that it doesn’t build up and become toxic. It needs to be evenly distributable via blood. It needs to not get into places where it shouldn’t be. It needs to not stick to things like artery walls. It needs to not cause an immune or inflammatory response. It needs to not cause you to clot or bleed out. And it needs to do all this and more in disparate parts of the body in varying chemical environments. The list of requirements is very long. Out body evolved alongside hemoglobin and finding a replacement that fits the bill perfectly is very difficult. Also hemoglobin appears in other places. Free oxygen is actually toxic in large amounts and will oxidize random things. Hemoglobin is used as an antioxidant to collect excess oxygen.