Cambridge O Level Biology · Syllabus 5090 · Human Gas Exchange
Inspired Air
What is Inspired Air?
The air breathed into the lungs, which in ordinary conditions is atmospheric air. Dry atmospheric air contains approximately 78 per cent nitrogen, approximately 21 per cent oxygen, approximately 0.04 per cent carbon dioxide and approximately 1 per cent argon together with other gases present in trace amounts; the quantity of water vapour it carries is variable and depends on the humidity of the surroundings. These percentages are rounded approximations used for teaching and for reasoning about diffusion gradients, and any data supplied within an examination question takes priority over them.
This definition is part of the Human Gas Exchange chapter in Cambridge O Level Biology.
Questions students ask about Inspired Air
How does expired air differ from inspired air?
It contains less oxygen, about 16 % rather than about 21 %, because oxygen has diffused into the blood and been used in respiration. It contains much more carbon dioxide, about 4 % rather than about 0.04 %, because carbon dioxide produced by respiration has diffused out of the blood into the alveolar air. It carries more water vapour, because the air has passed over moist linings, and it is close to body temperature, because it has been warmed on the way through. The percentage of nitrogen is approximately unchanged.
Why does limewater go cloudy, and what does it prove?
Limewater turns cloudy when carbon dioxide is bubbled through it, and it turns cloudy faster when the concentration of carbon dioxide is higher. Comparing two identical tubes shows that expired air contains a higher concentration of carbon dioxide than inspired air. It does not measure how much: the test is qualitative, and the time taken cannot be converted into a percentage.