Affinity
Haemoglobin has a high affinity for oxygen - affinity means attraction. When red blood cells reach the lungs, oxygen diffuses into the red blood cells, binding to haemoglobin.
Four molecules of oxygen bind to one molecule of haemoglobin. When oxygen bind to haemoglobin, oxyhaemoglobin is formed.
Quaternary structure
Haemoglobin is a protein. That means it’s made out of polypeptide chains. In this case, haemoglobin is made from 4 different polypeptide chains. The 4 chains give it a quaternary structure.