Since I can't post an image in a comment I've created this post.

I've seen a fair bit written about digital oxymeters (pulse ox) and percentage figures but no mention of the underlying physiology to increase understanding of the significance of those figures.

Haemoglobin (in red blood cells) takes up oxygen in the lungs to then carry around the body.

Oxygen disassociates from haemoglobin (Hb) when the partial pressure of O2 in surrounding tissues is reduced according to this graph:

Note the steep drop in oxygen saturation (the % given by an oximeter) when PO2 (level of oxygen in surrounding tissues) is low i.e. a lot of oxygen leaves red blood cells quickly at those levels so there isn't enough oxygen in the blood to get round everywhere.

SOURCE: (and much more info.)