Blood and the respiratory system
Blood and the respiratory system

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Blood and the respiratory system

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:

cap h sub two times cap o plus cap c times cap o sub two right harpoon over left harpoon cap h sub two times cap c times cap o sub three right harpoon over left harpoon cap h super plus plus cap h times cap c times cap o sub three times super minus

In chemistry, the ⇌ arrow represents a reversible reaction, meaning it can go in the right or the left direction. In this case, adding more CO2 will push the reaction to the right and generate more H+ and HCO3 ions. H+ ions decrease the pH of a solution (make it more acidic) whereas HCO3 ions increase the pH and make it more basic.

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.

Download this video clip.Video player: Video
Skip transcript: Video 12 Bicarbonate buffering.

Transcript: Video 12 Bicarbonate buffering.

SPEAKER
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.
Video 12 Bicarbonate buffering.
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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:

cap h times b times cap o sub two plus cap h super plus right harpoon over left harpoon cap h times b times cap h super plus plus cap o sub two

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.

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

Question 10 Higher H+

In what direction will the higher concentration of H+ push the equilibrium reaction?

cap h sub two times cap o plus cap c times cap o sub two right harpoon over left harpoon cap h sub two times cap c times cap o sub three right harpoon over left harpoon cap h super plus plus cap h times cap c times cap o sub three times super minus

a. 

left, towards increased CO2 production


b. 

right, towards HCO3 production


c. 

neither, the reaction will stay in equilibrium


The correct answer is a.

Answer

The answer is left. It will help drive the diffusion of CO2 out of the blood and into the alveoli to be exhaled.

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).

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