IGCSE Biology / Breathing and gas exchange
Comparing inspired and expired air
Edexcel International GCSE Biology (4BI1), chapter 3: Breathing and gas exchange, specification point 2.50. Free lesson.
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Inspired air enters the lungs, while expired air leaves them after gas exchange. Comparing their composition shows which gases are exchanged, and a limewater test can compare their carbon dioxide levels.
How inspired and expired air differ
The figures are approximate. Oxygen falls because some diffuses into the blood, but expired air still contains plenty. Carbon dioxide rises because respiration in the cells produces it. Nitrogen is not used or produced, so its amount stays the same. Oxygen: about 21% in inspired air and 16% in expired air. Carbon dioxide: about 0.04% in inspired air and 4% in expired air, about a hundred times more in expired air. Nitrogen: about 78% in both inspired and expired air.
Why nitrogen stays at about the same percentage
Nitrogen is neither used nor produced by the body during respiration. So about the same amount is breathed out as was breathed in. The oxygen lost and carbon dioxide gained are nearly equal in volume. This means the total volume of air barely changes, so nitrogen’s percentage stays almost the same.
Why expired air is warmer and moister
Inspired air is at about room temperature and has a variable amount of water vapour. As it passes through the lungs, it is warmed to body temperature. The moist lining of the alveoli allows water to evaporate into the air, so expired air is saturated with water vapour.
How to compare carbon dioxide with limewater
Limewater turns milky when carbon dioxide passes through it. Comparing how quickly two samples make it turn milky shows which sample contains more carbon dioxide.
What the limewater test can show
The test shows that carbon dioxide is present in expired air, and that there is more of it than in inspired air. By itself, it does not show where the carbon dioxide came from. To explain its source, state that it is produced by respiration in the cells.