Comparing Inspired & Expired Air (WJEC GCSE Biology)

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Cara Head

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Cara Head

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Comparing Inspired & Expired Air

  • Inspired and expired air has different amounts of gases in due to exchanges that take place in the alveoli and respiring cells of the body

Composition of air table

Gas Inspired air Expired air Reason for the difference
Oxygen 21% 16% Oxygen is removed from the blood during cellular respiration so blood returning to the lungs to be expired has a lower oxygen concentration
Carbon dioxide 0.04% 4% Carbon dioxide is diffused into the blood during cellular respiration so blood returning to the lungs to be expired has a higher carbon dioxide concentration
Nitrogen  78% 78% Nitrogen is an inert gas and is not used by the body so the same concentration is inspired and expired
Water vapour Varies Saturated with water vapour Water evaporates from the moist alveolar lining into expired air as a result of the warmth from the body

The 'huff and puff' test

  • A simple test using lime water can detect the presence of carbon dioxide 
  • It is used to compare the carbon dioxide content of inspired and expired air

Carbon dioxide test diagram

The limewater test

The limewater test for carbon dioxide

  • When we breathe in, the air is drawn through boiling tube A
  • When we breathe out, the air is blown into boiling tube B
  • Lime water is colourless but becomes cloudy (or milky) when carbon dioxide is bubbled through it
  • The lime water in boiling tube A will remain clear, but the limewater in boiling tube B will become cloudy
  • This shows us that the percentage of carbon dioxide in exhaled air is higher than in inhaled air

The alveoli and gas exchange

  • Gas exchange occurs by the process of diffusion
    • The air entering the alveoli has a high concentration of oxygen
    • The surrounding capillaries contain blood with a low concentration of oxygen: deoxygenated blood is brought to the lungs
    • The oxygen diffuses from a region of  high concentration (within the alveoli) across the walls of the alveoli and capillaries and into the red blood cells where there is a low concentration of oxygen; this oxygenated blood is then taken to the heart to be pumped all around the body
    • The opposite can be said of carbon dioxide: deoxygenated blood is brought to the lungs which contains a high concentration of carbon dioxide
    • The alveoli contain a low concentration of carbon dioxide 
    • Carbon dioxide diffuses from a region of high concentration (the blood) into the alveoli where the is a low concentration of carbon dioxide
  • The alveoli are highly specialised for gas exchange
    • Alveoli (and the capillaries around them) have thin, single layers of cells to minimise diffusion distance
    • Ventilation maintains high levels of oxygen and low levels of carbon dioxide in the alveolar air space
    • A good blood supply ensures a constant supply of blood high in carbon dioxide and low in oxygen
    • A layer of moisture on the surface of the alveoli helps diffusion as gases dissolve
  • Additionally, there are many rounded alveolar sacs within the lungs which give a very large surface area to volume ratio
  • All of these adaptations maximise the rate of diffusion of oxygen and carbon dioxide

Adaptations of alveoli diagram

role-of-the-alveoli

Alveoli are specifically adapted to maximise gas exchange

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding