concept

Oxygen dissociation curve

The plot of how saturated a respiratory pigment is at each oxygen pressure around it, and it is S-shaped rather than a straight line. The reason is that the molecule's subunits talk to each other: the first oxygen to bind makes the next one easier. The consequence is a very steep middle section - falling exactly across the pressure range that separates a lung from a working muscle - so saturation swings from nearly full to nearly empty within that narrow band. The whole curve can slide left or right, and that sliding is how biology tunes the handover.