Gas Transport in Blood Washington DC

Oxygen is transported in the blood in two ways:

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

• Most oxygen (98.5 percent) carried in the blood is bound to the protein hemoglobin in red blood cells. A fully saturated oxyhemoglobin (HbO₂) has four O₂ molecules attached. Without oxygen, the molecule is referred to as deoxygemoglobin (Hb).

The ability of hemoglobin to bind to O₂ is influenced by the partial pressure of oxygen. The greater the partial pressure of oxygen in the blood, the more readily oxygen binds to Hb. The oxygen-hemoglobin dissociation curve, shown in Figure 1 , shows that as pO₂ increases toward 100 mm Hg, Hb saturation approaches 100%. The following four factors decrease the affinity, or strength of attraction, of Hb for O₂ and result in a shift of the O₂-Hb dissociation curve to the right:

Figure 1 The oxygen-hemoglobin dissociation curve.

• Increase in temperature.

• Increase in partial pressure of CO₂ (pCO₂).

• Increase in acidity (decrease in pH). The decrease in affinity of Hb for O₂, called the Bohr effect, results when H+ binds to Hb.

• Increase in BPG in red blood cells. BPG (bisphosphoglycerate) is generated in red blood cells when they produce energy from glucose.

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

• A small amount of CO₂ (8 percent) is carried in the plasma as a dissolved gas.

• Some CO₂ (25 percent) binds to Hb in red blood cells forming carbaminohemoglobin (HbCO₂). (The CO₂ binds to a place different from that of O₂.)

• Most CO₂ (65 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.

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. To balance the overall increase in negative charges entering the plasma, chloride ions diffuse in the opposite direction, from the plasma to the red blood cells (chloride shift).

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