The mechanics of breathing

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The mechanics of breathing

Similarities

  • External intercostal muscles contract
  • diaphragm contracts - becomes flatter
  • internal intercostal muscles relax
  • ribs and sternum move upwards and outwards
  • increase in chest cavity volume
  • pressure between pleural membranes is reduced
  • allows elastic pulmonary tissue to expand
  • lung volume increases
  • pulmonary air pressure falls below atmospheric pressure (outside the body)
  • hence atmospheric air is forced into the lungs
  • until lung pressure equals the pressure outside again
  • DURING EXERCISE
  • additional muscles in the chest and torso contract (scalenes, sternocleidomastoid, pectoralis major / minor).
  • chest cavity volume further increases
  • more air forced into the lungs

Differences

  • external intercostal muscles relax - a passive process
  • diaphragm relaxes - domes upward into chest cavity - a passive process
  • ribs and sternum move downwards and inwards
  • decrease in chest cavity volume
  • pressure between pleural membranes increased
  • compressing elastic pulmonary tissue
  • lung volume decreases
  • pulmonary air pressure is driven above atmospheric pressure (outside the body)
  • hence atmospheric air is forced out of the lungs via the respiratory passages
  • until lung pressure equals the pressure outside again
  • DURING EXERCISE
  • internal intercostal muscles and abdominal muscles contract powerfully, acting on ribs and body cavity
  • chest cavity volume is further reduced
  • more pulmonary air is forced out of the lungs

Overall comparison

The actual mechanism of breathing is brought about by changes in air pressure (intrapulmonary pressure) in the lungs relative to atmospheric pressure, and as a result of the muscular actions of the 11 pairs of intercostal muscles and the diaphragm.

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