CAIE A-Level Biology AS 9.1.7 Gas Exchange Between Alveolar Air and Blood Questions

Explain gas exchange between alveolar air and capillary blood using diffusion gradients, barrier pathways and evidence of how ventilation or lung damage alters oxygen uptake.

Exam points

Use same-gas partial pressures, with kPa units where required, to explain or mark oxygen diffusion into blood and carbon dioxide diffusion into alveolar air.
Trace a respiratory gas across the alveolar and capillary barriers to the stated endpoint, distinguishing cell membranes from phospholipid layers and any route through pores.
Calculate the diffusion distance to haemoglobin by adding the relevant membrane, cytoplasm and intervening tissue thicknesses, using the stated assumptions and nanometre units.
Explain how ventilation, continuous blood flow and oxygen uptake by haemoglobin maintain steep oxygen gradients between alveoli and blood.
Select changes that increase gas diffusion, linking a steeper concentration gradient, a shorter air–blood barrier and a larger exchange area to faster transfer.
Use lung images or stated changes to explain reduced gas transfer when alveolar walls are lost, fewer capillaries remain, the barrier thickens or alveolar air is refreshed less effectively.

[Maximum number: 1]

Exchange of carbon dioxide and oxygen occurs between air in the alveoli and blood in the capillaries of the lung.

Which partial pressures of the gases will allow gaseous exchange to occur?

$\mathrm{CO}_{2}$ in alveolar air /kPa

$\mathrm{CO}_{2}$ in blood /kPa

$\mathrm{O}_{2}$ in alveolar air /kPa

$\mathrm{O}_{2}$ in blood /kPa

5.3

6.0

13.3

13.9

5.3

6.0

13.9

5.3

6.0

5.3

13.9

5.3

6.0

5.3

13.3

13.9

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CAIE A-Level Biology AS 9.1.7 Gas Exchange Between Alveolar Air and Blood Questions question 1