Explain how the structure of the alveoli is adapted to maximise the rate of gas exchange in the lungs.

IB DP Biology Standard Level (2023 syllabus) — B3.1 Gas exchange · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Alveoli have a very large total surface area, increasing the area available for diffusion of oxygen and carbon dioxide.
The alveolar walls are one cell thick (squamous epithelium), giving a very short diffusion distance/pathway for gases.
Alveoli are surrounded by a dense network of capillaries, maintaining a steep concentration gradient by continuously removing oxygen and delivering carbon dioxide.
The moist lining of the alveoli allows gases to dissolve, enabling diffusion across the membrane.

Examiner tips

Use the word "adapted" to link structure to function. Mention the large surface area and thin walls explicitly. Show the role of capillaries in maintaining gradients. Include the importance of the moist lining for gas solubility.

Common mistakes

Failing to mention the thinness of the alveolar wall. Omitting the role of capillaries in sustaining concentration gradients. Using vague terms like "big" instead of "large surface area". Not linking the moist lining to gas solubility.

Mark scheme (4 marks)

Alveoli have a very large total surface area, increasing the area available for diffusion of oxygen and carbon dioxide.
The alveolar walls are one cell thick (squamous epithelium), giving a very short diffusion distance/pathway for gases.
Alveoli are surrounded by a dense network of capillaries, maintaining a steep concentration gradient by continuously removing oxygen and delivering carbon dioxide.
The moist lining of the alveoli allows gases to dissolve, enabling diffusion across the membrane.

Key terms in this question

alveoli