What is haemoglobin?

Haemoglobin is a molecule produced in red blood cells as they grow. Each haemoglobin molecule consists of four small proteins that are joined together to make the whole haemoglobin molecule.

Each of the four smaller proteins that make up a haemoglobin molecule contains a 'heme group'. Each heme group can carry one oxygen molecule. So each complete haemoglobin molecule can carry up to four oxygen molecules.