Revision notes for Biology AS Unit 1 – Lungs and the mechanism of breathing

 

INSPIRATION – breathing in

EXPIRATION – breathing out

To maintain diffusion of gases across the alveolar membrane, air must constantly move in and out of the lungs (breathing or ventilation).

When air pressure of atmosphere is greater than the air pressure in the lungs, air is forced into the alveoli – INSPIRATION

When air pressure in the lungs is greater than air pressure in the atmosphere, air is forced out of the lungs – EXPIRATION

The pressure changes in the lungs are due to the movement of 2 muscles:

Diaphragm – a sheet of muscle separating the thorax from the abdomen

Intercostals muscles – lie between the ribs

INSPIRATION (air forced into the lungs)

  • External intercostals muscles contract
  • Ribs move up and outwards, increasing volume of thorax
  • Diaphragm muscle contracts, causing it to flatten which also increases volume of thorax
  • Increased volume in thorax results in decreased pressure in the lungs
  • When air pressure of atmosphere is greater than the air pressure in the lungs, air is forced into the alveoli – INSPIRATION

 

EXPIRATION (air forced out of the lungs)

  • Internal intercostals muscle contracts
  • Ribs move down and inwards, decreasing volume of thorax
  • Diaphragm muscle relaxes, causing decreased volume of thorax
  • Decreased volume of thorax results in increased pressure in the lungs
  • When air pressure in the lungs is greater than air pressure in the atmosphere, air is forced out of the lungs – EXPIRATION
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