Original: Hemoglobin has an oxygen binding capacity of 1.34 ml O2 per gram of hemoglobin, which increases the total blood oxygen capacity seventyfold compared to dissolved oxygen in blood. The mammalian hemoglobin molecule can bind (carry) up to four oxygen molecules.
Simplified: Hemoglobin (also spelled hemoglobin) is a protein in the blood which contains Iron and is used to transport oxygen around the body.