O-LevelBiologyRespiration5090/11 · May/June 2013 · Q191 Mark
In the human breathing system, which features maintain the carbon dioxide gradient between the alveoli and the outside air? 1 blood continually pumped to the alveoli 2 breathing in and out 3 moist alveolar surfaces 4 thin alveolar walls
A1 and 2
B1 and 4
C2 and 3
D3 and 4
✓ Correct answer
A: 1 and 2
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