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9700 · 8.2

Transport of oxygen and carbon dioxide — practice questions

Practice and worked examples for 9700 Transport of oxygen and carbon dioxide. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

An athlete is exercising vigorously. The partial pressure of oxygen (pO2pO_2) in their lungs is 13.3 kPa, and the pO2pO_2 in their active muscle tissue drops to 2.5 kPa. Given that the oxygen-carrying capacity of their blood is 20 cm3cm^3 of O2O_2 per 100 cm3cm^3 of blood, calculate the volume of oxygen delivered to the muscles per 100 cm3cm^3 of blood. Use the following data from the oxygen dissociation curve:

  • At pO2pO_2 = 13.3 kPa, haemoglobin saturation is 98%.
  • At pO2pO_2 = 2.5 kPa, haemoglobin saturation is 25%.
Show solution outline

Step 1: Determine the oxygen saturation in the lungs and muscles.

  • Saturation in lungs (leaving alveoli) = 98%
  • Saturation in muscles (active tissue) = 25%

Step 2: Calculate the percentage of oxygen unloaded to the tissues.

  • Change in saturation = Saturation in lungs - Saturation in muscles
  • Change in saturation = 98% - 25% = 73%

Step 3: Calculate the volume of oxygen released per 100 cm3cm^3 of blood.

  • The blood's maximum oxygen-carrying capacity (at 100% saturation) is 20 cm3cm^3 per 100 cm3cm^3 of blood.
  • Volume of O2O_2 released = (Change in saturation / 100) ×\times Maximum O2O_2 capacity
  • Volume of O2O_2 released = (73 / 100) ×\times 20 cm3cm^3
  • Volume of O2O_2 released = 0.73 ×\times 20 cm3cm^3 = 14.6 cm3cm^3

Final Answer: 14.6 cm3cm^3 of oxygen is delivered to the muscles per 100 cm3cm^3 of blood.

Worked example 2

Describe the series of events that leads to the transport of carbon dioxide as hydrogencarbonate ions from a respiring tissue to the lungs, including the roles of carbonic anhydrase and the chloride shift.

Show solution outline
  1. CO₂ entry: Carbon dioxide diffuses from respiring tissue cells, across the tissue fluid, and into the red blood cells, as well as dissolving in the plasma.
  2. Carbonic acid formation: Inside the red blood cells, CO₂ rapidly combines with water (H₂O) to form carbonic acid (H₂CO₃). This reaction is catalysed by the enzyme carbonic anhydrase, significantly speeding up the process (CO₂ + H₂O → H₂CO₃).
  3. Dissociation: Carbonic acid then dissociates into hydrogen ions (H⁺) and hydrogencarbonate ions (HCO₃⁻) (H₂CO₃ → H⁺ + HCO₃⁻).
  4. Chloride Shift: The hydrogencarbonate ions (HCO₃⁻) then diffuse out of the red blood cell and into the blood plasma. To maintain electrical neutrality across the red blood cell membrane, chloride ions (Cl⁻) move from the plasma into the red blood cell. This counter-movement is known as the chloride shift.
  5. Buffering: The accumulated hydrogen ions (H⁺) within the red blood cell are buffered by binding to haemoglobin (which is now mostly deoxygenated, having released O₂). This forms haemoglobinic acid (HHb), preventing a significant drop in intracellular pH.
  6. Transport to lungs: The hydrogencarbonate ions are transported in the plasma to the lungs.
  7. Reverse reactions in lungs: In the lungs, where pO₂ is high and pCO₂ is low, the process reverses. Oxygen binds to haemoglobin, displacing H⁺. The H⁺ then combines with HCO₃⁻ (which moves back into the red blood cell as Cl⁻ moves out) to reform H₂CO₃. Carbonic anhydrase then converts H₂CO₃ back into CO₂ and H₂O. The CO₂ diffuses out of the red blood cell, into the alveoli, and is exhaled.
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