Respiratory system

notes on breathing and lungs etc

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Air composition

Inhaled air:

  • Nitrogen 79%
  • Oxygen 21%
  • Carbon Dioxide 0.04%

Exhaled air:

  • Nitrogen 79%
  • Oxygen 16%
  • Carbon dioxide 4%


glucose + oxygen = energy + carbon dioxide + water

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How do we breathe?

1) air enters through nose and mouth

2) nasal passages moisten, filter and warm the air

3) air passes through larynx (voice box) on its way to the trachea (windpipe)

4) trachea divides into 2 bronchi and each bronchus branches out into smaller tubes called bronchioles

5) the bronchioles are split up into alveoli (thin-walled spongy air sacs). The alveoli fill with air when we breathe, and then empty as we breathe out.

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What happens when we breathe?

Breathing in (inspiration):

  • intercostal muscles contract, lifting ribs upwards and outwards. our chest expands.
  • diaphragm contracts, pulling down and flattening out the floor of our rib cage. our chest expands further.
  • lungs increase in size as our chest expands. this is because their outside surface is stuck to the chest wall.
  • the pressure inside our lungs falls as they expand.

Breathing out (expiration):

  • intercostal muscles relax and ribs move downwards and inwards. our chest gets smaller
  • diaphragm relaxes, and is pushed back into a domed position by the organs underneath it. chest gets even smaller
  • lungs decrease in size as chest gets smaller. they are squeezed by ribs and diaphragm.
  • pressure inside lungs increases as they get smaller
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