If I understand correctly, arterial blood is usually saturated to ~95% with oxygen, the oxygen being transported in combination with haemoglobin in a ratio of 4:1.
Venous blood is saturated at around 75% implying that only around one of the four bound oxygen molecules was used.
When then is it dangerous to have SpO2 levels lower than 88%? Cannot the other bound oxygen molecules be released under low oxygen conditions?
There is currently no biological mechanism that would increase a cell's absorption of oxygen in conditions of low SpO2 (unless you're a mutant). The amount taken by the cell is based on the amount of oxygen rich blood cells that reach the outer walls of the capillary to be permeated through and deliver oxygen. So this means that the cells have no way to just grab more oxygen when they don't have enough. The oxygen they get is what is available to them. Now the next part I'm not sure about, but I can give you a theory. When SpO2 goes below 88%, it may cause a failure in which the lungs can no longer support themselves with oxygen because they themselves don't have any to work (to be more exact, the diaphragm). Therefore making the oxygen levels continue to fall, which is not good if you don't like death.
