Oxygen is transported in the blood in two ways:
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A small amount of O 2 (1.5 percent) is carried in the plasma as a dissolved gas.
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Most oxygen (98.5 percent) carried in the blood is bound to the protein hemoglobin in red blood cells. A fully saturated oxyhemoglobin (HbO 2) has four O 2 molecules attached. Without oxygen, the molecule is referred to as deoxyhemoglobin (Hb).
The ability of hemoglobin to bind to O 2 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 pO 2 increases toward 100 mm Hg, Hb saturation approaches 100 percent. The following four factors decrease the affinity, or strength of attraction, of Hb for O 2 and result in a shift of the O 2‐Hb dissociation curve to the right:
- Increase in temperature.
- Increase in partial pressure of CO 2 (pCO 2).
- Increase in acidity (decrease in pH). The decrease in affinity of Hb for O 2, 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 CO 2 (85 percent) is transported as dissolved bicarbonate ions (HCO 3 –) in the plasma. The formation of HCO 3 –, however, occurs in the red blood cells, where the formation of carbonic acid (H 2CO 3) is catalyzed by the enzyme carbonic anhydrase, as follows:
CO 2 + H 2O ← → H 2CO 3 ← → H + + HCO 3 –
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 HCO 3 – 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).