3.1.2i/j Transport of Carbon Dioxide

Question 1

What is the partial pressure of carbon dioxide

Answer 1

• Similarly to oxygen, the partial pressure of carbon dioxide (pCO2) is a measure of the concentration of CO2 in a cell.

Question 2

How does CO2 concentration affect oxygen unloading

Answer 2

1. Most of the CO2 from respiring tissues diffuses into RBCs. Here it reacts with water to form carbonic acid, catalysed by the enzyme carbonic anhydrase. (The rest of the CO2, around 10% binds directly to haemoglobin & is carried to the lungs).
2. The carbonic acid dissociates (splits up) to give hydrogen (H+) ions & hydrogencarbonate (HCO3-) ions.
3. This increase in H+ ions causes oxyhaemoglobin to unload its oxygen so that haemoglobin can take up the H+ ions. This forms a compound called haemoglobinic acid. (This process also stops the hydrogen ions from increasing the cell’s acidity)
4. The HCP3- ions diffuse out of the red blood cells & are transported in the blood plasma. To compensate for the loss of HCO3- ions from the RBCs, chloride (Cl-) ions diffuse into the RBCs. This is called the chloride shift & it maintains the balance of charge between the RBC & the plasma
5. When the blood reaches the lungs the low pCO2 causes some of the HCO3- & H+ ions to recombine into CO2 (& water)
6. The CO2 then diffuses into the alveoli & its breathed out

Question 3

What is the Bohr effect

Answer 3

When CO2 levels increase, the dissociation curve ‘shifts’ right, showing that more oxygen is released from the blood (bc the lower the saturation of Hb with O2, the more O2 is released). This is called the Bohr effect.

pg87 for graph

Question 4

How is the Dissociation curve affected by levels of CO2 in the blood

Answer 4

• The oxygen dissociation curve is affected by levels of CO2 in the blood, for eg when CO2 levels go up after exercising.
• The presence of CO2 helps the release of oxygen from Hb, so the curve shifts to the right
• This is known as the Bohr effect

Question 5

The carriage of CO2

Answer 5

• 5% in plasma
• 10-20% comhydrogencarbonategroups in Hb (which forms carbaminohaemoglobin)
• 75-85% as hydrogencarbonate ions (HCO3-) in cytoplasm of erythrocytes

Question 6

Formation of HCO3-

Answer 6

• 75-85% of the CO2 enters erythrocytes & combines with water to form carbonic acid:
  CO2 + H2O —(carbonic anhydrase)—> H2CO3
• Carbonic acid dissociates to release hydrogen ions & hydrogen carbonate ions:
  H2CO3 —> HCO3- + H+
• HCO3- diffuses out of RBC into plasma.
  Chloride ions (Cl-) diffuse into RBC to maintain charge/electrochemical balance.
  > chloride shift
• H+ ions are taken up by the Hb to prevent the RBC becoming acidic
  > haemoglobinic acid (HHb) is formed

Hb is acting as a buffer (a compound that helps maintain a constant pH)

Question 7

What is the Chloride shift

Answer 7

Hydrogen carbonate ions moving OUT of RBCs & chloride ions moving INTO RBCs.