Comparing Inspired & Expired Air (WJEC GCSE Biology: Combined Science)

• 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

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 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