How do red blood cells carry oxygen?

"We' have said that red cells carry oxygen to the cells. How is this done? The red cells contain a substance called hemoglobin which traps oxygen from the air we breathe in the lungs. The oxygen is transported by the red cell through the arteries to each living cell where hemoglobin releases the oxygen so that the cells can have the fuel needed form life. Hemoglobin is a specialized type of substance which is found in the centre of each red blood cell. It is the presence of this protein that gives the Oxygen-carrying capacity of the red cells. Hemoglobin has two different types of chains, called alpha (α) and beta (β).  These chains surround another substance, called heme which is where the metal, iron, is integrated into its structure. Iron has a very special capacity: It easily binds and loses oxygen, making it the perfect means of picking-up and delivering oxygen. It is exactly because of the presence of -the iron- that Hb has the capacity to transport oxygen around the body."