The major function of hemoglobin is to transport oxygen from the lungs to the body's tissues and then transport carbon dioxide out of the tissue back to the lungs. Each red blood cell contains approximately 280 million hemoglobin molecules.
A molecule of hemoglobin is made up of several smaller pieces. There are four polypeptide chains with each of those containing a single heme group. The key to processing the oxygen is the iron contained in each heme group. The iron molecules can each absorb one molecule of oxygen. This gives each hemoglobin the ability to transport four oxygen molecules.
There are two types of hemoglobin, which are the oxyhemoglobin and the deoxyhemoglobin. Oxyhemoglobin has a higher affinity for oxygen and the deoxyhemoglobin is more attracted to carbon dioxide. This means that the oxygen in the lungs binds to the oxyhemoglobin to be transported into the body and be absorbed. The deoxyhemoglobin picks up the carbon dioxide that is left after the body absorbs the oxygen and takes it back to the lungs to complete its process.
An important piece in the transportation of oxygen by hemoglobin is the molecules affinity to the oxygen. When hemoglobin picks up its first molecule of oxygen the affinity actually increases so that it picks up more oxygen. This domino effect continues until the molecule is full of oxygen.