Describe the structure of haemoglobin and how it is suited to it's function?

Two alpha subunits and two other subinits that vary depending on th type of haemoglobin, either beta,delta, gamma or epsilon. 

Each subunit contains on heme group. Heme group contains four pyrol rings and a central iron atom. 

Central Iron has a high affinity for oxygen allowing the protein complex to efficiently transport oxygen.