Alveolus Gas Exchange Diagram: Structure and Diffusion
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key diagram
An alveolus is a tiny air sac in the lungs with walls one cell thick, surrounded by capillaries. Oxygen diffuses from the alveolar air into the blood, and carbon dioxide diffuses from the blood into the alveolus, both down their concentration gradients.
The alveolus diagram is a core gas exchange question that combines structure with the process of diffusion. Students must draw the alveolus wall, the adjacent capillary, and arrows showing gas movement in opposite directions.
Drawing the alveolus and capillary
Draw a rounded sac shape for the alveolus. Along one side, draw a narrow tube curving around it for the capillary. The wall of the alveolus and the wall of the capillary should both be drawn as very thin lines: each is one cell thick. Label the alveolar air space inside the sac and the blood inside the capillary.
Show a red blood cell inside the capillary to emphasise the short diffusion distance.
Gas exchange by diffusion
Draw an arrow from the alveolar air into the capillary blood labelled oxygen (O₂). Draw an arrow in the opposite direction labelled carbon dioxide (CO₂). Both gases move by diffusion down their concentration gradients. Oxygen concentration is higher in the alveolar air than in the deoxygenated blood arriving in the capillary. Carbon dioxide concentration is higher in the blood than in the alveolar air.
Adaptations for efficient gas exchange
The alveoli have several adaptations that maximise the rate of diffusion. (1) Large surface area: millions of alveoli provide a huge total surface area. (2) Thin walls: the alveolus wall and capillary wall are each one cell thick, giving a very short diffusion distance. (3) Rich blood supply: an extensive capillary network maintains a steep concentration gradient by carrying gases away quickly. (4) Moist lining: gases dissolve in the thin film of moisture before diffusing across.
Ventilation maintains the gradient
Breathing in brings fresh air with high oxygen concentration into the alveoli. Breathing out removes air with higher carbon dioxide concentration. This constant replacement of air maintains the concentration gradient across the alveolar wall, so diffusion continues. Without ventilation, the gradient would equalise and gas exchange would stop.
Label checklist
Check these labels are present and correctly positioned before finishing your drawing.
✓Alveolus (air sac)
✓Alveolar wall (one cell thick)
✓Capillary
✓Capillary wall (one cell thick)
✓Red blood cell
✓Oxygen arrow (alveolus to blood)
✓Carbon dioxide arrow (blood to alveolus)
✓Moist lining
Try a diagram question (4 marks)
Explain two features of the alveoli that make gas exchange efficient. [4]
Show the answer and marking guidance
The walls are one cell thick, providing a short diffusion distance so gases cross quickly.
There are millions of alveoli, giving a large total surface area for gas exchange.
Original Cambridge-style practice written for this site.
Common diagram mistakes
Check these against your own drawing before moving on.
Drawing the alveolar wall as thick: it must be clearly one cell thick.
Putting both gas arrows in the same direction.
Writing "breathing" instead of "diffusion" for the process of gas exchange.
Forgetting the moist lining of the alveolus.
Not mentioning the concentration gradient when explaining diffusion.