O2 is transported by the blood in mainly two ways (Fig. 11):
- dissolved in plasma
- bound to hemoglobin (Hb).
Only about 1.5% of O2 is transported dissolved in plasma.
Much more O2 is carried bound to Hb. Hb is a large protein molecule located inside the red blood cells (erythrocytes). Hemoglobin contains a heme group, which contains iron. These heme groups contain iron and these are the O2 binding structures. One hemoglobin molecule contains four heme groups, and can thus bind four O2 molecules, and in each red blood cell there are approximately 250 million Hb molecules. This means that a red blood cell can carry 1 billion O2 molecules. In a microliter (uL) of blood there are about 5 million red blood cells. Given the body contains 4-6 liters of blood, a lot of oxygen can be transported.