Gas Transport

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 percent. 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:

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

Figure 1. The oxygen-hemoglobin dissociation curve.

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

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

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

• 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. 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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