Biology 4.2 The Mechanism of Breathing

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  • Created by: Anna
  • Created on: 21-04-13 16:55

To maintain diffusion of gases across alveolar epithelium air must be constantly moved in and out of lungs = ventilation.

Air pressure of atomosphere = greater than that of lungs, air forced in = inspiration.

Air pressure in lungs greater than that of atomosphere, air forced out = expiration.

Pressure change brought about by two sets of muscles:

  • Diaphragm = sheet of muscle that seperates thorax from abdomen.
  • Intercostal muscles = lie between ribs, two sets: internal intercostal muscles (contraction leads to expiration) and external intercostal muscles (contraction leads to inspiration).


NB: Volume of blood pumped by heart in one minute = cardiac output.

Breathing = active process (uses energy).

  • External intercostal muscles contract, internal intercostal muscles relax.
  • Ribs pulled upwards and outwards…


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