Hemoglobin is the protein used in the blood of all vertebrates to transport oxygen from the lungs to the tissues in the body. A major constituent of blood is water, and since oxygen is not very soluble in water, a protein, hemoglobin, must be used.
The oxygen binds to the hemoglobin, which can release oxygen at the same time. A hemoglobin molecule consists of four polypeptide chains. The chains include one heme group, each containing one iron ion. The iron is where the oxygen binding occurs, and each iron can bind one O2 molecule, meaning each hemoglobin molecule can bind to a total of four O2 molecules.
In the average human, the concentration of hemoglobin measures 16 grams per 100 milliliters. The amount of hemoglobin determines how much oxygen can be transported. When all binding sites of the hemoglobin molecules are filled by oxygen, the blood is considered 100 percent saturated, making it unable to carry anymore oxygen. If a tissue is 70 percent saturated when the blood is 100 percent saturated, hemoglobin releases oxygen to the tissue to fill the remaining 30 percent. Ultimately, the body makes enough hemoglobin so that it automatically delivers more oxygen to the tissues that are using the more oxygen.