2.3.1 O₂– transport

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.

Fig. 11