CAIE A Level Biology 9700 · Gas exchange
The alveoli of the human lungs are lined with a single layer of squamous epithelial cells. Each alveolus is surrounded by a dense network of capillaries whose walls are also one cell thick.
Describe how the structure of the alveolar epithelium and its association with capillaries is adapted for efficient gas exchange between alveolar air and blood.
4 marks, so the mark scheme is looking for about 4 separate creditworthy points. Making the same point twice in different words earns it once.
Describe means: Say what happens or what something is like, in order and with detail. You are not being asked why. If there is data, quote it: name the trend, then give figures with units at the points where it changes.
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