4.3 Bicarbonate

In the previous section, you saw how the affinity of Hb for O₂ decreases in the presence of elevated CO₂ and acidity. This is known as the Bohr effect. This is due to the chemical reaction that takes place between CO₂ and water (H₂O) to generate bicarbonate (HCO₃⁻) and protons (H⁺). This reaction is represented by the equation:

The reversible nature of this reaction is critical in allowing the body to transport CO₂ from the tissues and be exhaled in the lungs. This process is detailed in Video 12.

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Transcript: Video 12 Bicarbonate buffering.

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Tissue cells that are metabolically active produce carbon dioxide that diffuses into erythrocytes in the systemic capillaries. Carbon dioxide combines with water in the erythrocytes to produce a weak acid called carbonic acid. This reaction is facilitated by the enzyme carbonic anhydrase, which acts to speed up the reaction. Carbonic acid dissociates into a bicarbonate ion and a proton. The proton binds to haemoglobin, forming protonated haemoglobin, or HHb. The bicarbonate ion diffuses down its concentration gradient into the blood, taking along its negative charge.

To balance the charge in the erythrocyte, chloride ions, which are also negatively charged, move into the erythrocytes from the blood in a process known as the chloride shift. The reverse chemical reaction takes place in erythrocytes that move into the capillaries of the lungs. Bicarbonate from the blood moves into the erythrocyte and chloride leaves to balance the charge.

Haemoglobin donates a proton, which combines with bicarbonate ions to produce carbonic acid. Carbonic anhydrase catalyses the conversion of carbonic acid into carbon dioxide and water, allowing the reaction to take place quickly. Carbon dioxide then diffuses down its concentration gradient across the alveolar walls and is exhaled.

End transcript: Video 12 Bicarbonate buffering.

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Video 12 Bicarbonate buffering.

In Video 12, you saw that protons (H⁺) generated during bicarbonate buffering of CO₂ bind to Hb in the erythrocytes to form protonated haemoglobin (HbH⁺). This binding decreases the affinity of Hb for O₂, thereby facilitating O₂ diffusion into tissues, as described by the following equation:

At the same time, CO₂ that has not been converted into HCO₃⁻ (~30% of total CO₂ in the blood) binds with high affinity to deoxyhaemoglobin to form carbaminohaemoglobin (HbCO₂). This complex is then carried to the lungs (Figure 12).

In the alveoli, binding of O₂ to HbH⁺ results in the release of free H⁺ ions.

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Figure 12 Mechanisms by which CO₂ is carried in the blood.

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A schematic diagram. On the left-hand side of the diagram is a vertical row of five, pink, rectangular peripheral cells, one of which has CO₂ written within it.

A yellow arrow shows CO₂ leaving this cell; 10% of it is shown (by a blue arrow) as dissolved CO₂ in the blood and tissue fluids. The remaining CO₂ is shown as entering a stylised red blood cell. Yellow arrows within the cell show CO₂ movement.

At the top of the red cell CO₂ is shown binding to Hb (haemoglobin) to give Hb−CO₂. At the bottom of the red cell CO₂, after its combination with water and dissociation to give H⁺ and HCO₃^-, is shown as H⁺ with Hb to give protonated haemoglobin, Hb−H⁺, and HCO₃^- is shown leaving the cell, passing over a mauve circle which shows Cl^- moving in the opposite direction. Blue arrows to the right of the diagram indicate that 30% of the CO₂ is protein bound and 60% of the CO₂ is chemically modified.

Figure 12 Mechanisms by which CO₂ is carried in the blood.

In parallel, carbaminohaemoglobin loses its affinity for CO₂ as it becomes reoxygenated. Collectively, these actions increase the PCO₂ at the alveoli. The phenomenon by which O₂ influences CO₂ concentrations is known as the Haldane effect.

The capacity of the blood to carry O₂ is also greatly reduced by carbon monoxide (CO), a gas emitted by car exhausts and faulty gas appliances. CO competes with O₂ for binding to Hb. Because the affinity of Hb for CO is higher than its affinity for O₂, CO molecules will bind preferentially and irreversibly to form carboxyhaemoglobin (HbCO), which is cherry red in colour. Inhaling CO will therefore progressively reduce the amount of Hb available to bind O₂ and lead to CO poisoning. If the source of CO is not removed, death could result due to the total lack of oxygen (asphyxiation).