Oxygen is transported in the blood in two ways:

1.A small amount of O2(1.5 percent) is carried in the plasma as a dissolved gas.

2.Most oxygen (98.5 percent) carried in the blood is bound to the protein hemoglobin in red blood cells. Afully saturated oxyhemoglobin (HbO2) has four O2molecules attached. Without oxygen, the molecule isreferred to as deoxyhemoglobin (Hb).

Carbon dioxide is transported in the blood in the following ways:

1.A small amount of CO2(5 percent) is carried in the plasma as a dissolved gas.

2.Some CO2(10 percent) binds to Hb in red blood cells, forming carbaminohemoglobin (HbCO2). (The CO2binds to the amino acid portion of hemoglobin instead of to the iron portion.)

3.Most CO2(85 percent) is transported as dissolved bicarbonate ions (HCO3–) in the plasma. The formation of HCO3–, however, occurs in the red blood cells, where the formation of carbonic acid (H2CO3) is catalyzed by the enzyme carbonic anhydrase, as follows:

CO2+ H2O ← → H2CO3← → H++ HCO3–

Following their formation in the red blood cells, most H+bind to hemoglobin molecules (causing the Bohr effect) while the remaining H+diffuse back into the plasma, slightly decreasing the pH of the plasma. The HCO3–ions diffuse back into the plasma as well

OxygenTransport. In the lungs, oxygen diffuses from the air in alveoli into the blood of surrounding capillaries. Most of the oxygen enters RBCs and combines with the heme portions of hemoglobin (Hb) to form oxyhemoglobin (HbO2). About 98.5% of the oxygen istransportedas oxyhemoglobin.