The oxygen affinity of hemoglobin

Hemoglobin is a protein in red blood cells (RBCs) that carries oxygen. Almost $30\mathrm{\%}$‍ of the mass of RBCs is made up by hemoglobin. This allows RBCs to carry much more oxygen than could otherwise be dissolved into blood.

A researcher studies the oxygen dissociation curves of normal adult hemoglobin (HbA), fetal hemoglobin (HbF), and myoglobin (Mb), a related compound. He dissolves samples of each protein in a cup of water, and places each cup in a sealed canister in contact with a gas. He changes the partial pressure of oxygen (${\text{P O}}_2$‍) in the gas. For each measured ${\text{P O}}_2$‍, he measures the saturation of oxygen in the proteins in the fluid. The saturation is the percentage of total binding sites on the HbA, HbF, or Mb, which are bound to oxygen. The results are shown below.