Yeah, I didn't do the carbonic acid, then there's the increased bicarb buffering around the pleura, couple other facts.

https://www.ncbi.nlm.nih.gov/books/NBK539815/

Upon entrance into red blood cells, carbon dioxide is quickly converted to carbonic acid by the enzyme carbonic anhydrase. Carbonic acid immediately dissociates into bicarbonate and hydrogen ions. As previously stated, an increase in hydrogen ions stabilize the hemoglobin in the T-state and induces oxygen unloading which leads to shifting of the dissociation curve to the right.[6]

Thus the acidity causes o2 release. Temperature (lungs tend to be very cold in the body) is important too.

Oxygen unloading is favored at higher temperatures which will cause a rightward shift. On the other hand, lower temperatures will cause a leftward shift in the dissociation curve. A notable example of this is exercise, where the temperature of muscle increases secondary to its utilization, thus shifting the curve to the right and allowing oxygen to be more easily unloaded from hemoglobin and deliver to tissues in need.

It's amazing how subtly it works to gently increase efficiency where we need it. Otherwise it's just a very weak oxygen bond (which is hard enough given oxygen is extremely non-polar and all you have are the valence pairs. edit: This lead me to wonder how the fuck it even bonded effectively

https://www.jbc.org/article/S0021-9258(19)63845-7/fulltext

Wow, I'm impressed, they're using spin-coupling which is a pretty dicey effect.

Thus, we can conclude that the facile binding of O2 to hemo- and myoglobin arises primarily as an effect of the topology of the binding curves for the four relevant spin states. This topology, with nearly degenerat>e and parallel curves, is caused by the near degeneracy (within 10 kJ/mol) of the triplet and quintet states of deoxyheme. Therefore, the design by nature of iron porphines having close-lying spin states of a particular symmetry and energy is a means to tune binding of small ligands and overcome the activation barriers of these spin-forbidden reactions, despite the moderate SOC of first-row transition metals. The resulting barrier height makes up most of the rate enhancement due to the exponential dependence on the rate, whereas one or two orders of magnitude may come from the increase in the transmission coefficient.

That's some fucking crazy ass engineering by nature, A weak, highly reversible bond with the molecule keyed to both pH and thermal triggers. That was a fun rabbit hole.