Mechanism of breathing

• What is breathing?

  To maintain diffusion of gases across the alveolar epithelium, air is constantly moved in and out the lungs
• What is inspiration (inhalation)?


  When the air pressure of the atmosphere is greater than air pressure inside of the lungs, the air is forced into the lungs
• What is expiration?

  When air pressure in lungs is greater than the atmosphere, air is forced out of the lungs.
• Pressure changes within the lungs brought about by the movement of 3 sets of muscles:

  • Diaphragm = a sheet of muscle that separates thorax from the abdomen
  • Intercostal muscles lie between the ribs
  • Internal intercostal muscles = contraction leads to expiration
  • External intercostal muscles = contraction leads to inspiration.
• Stages of inspiration (inhalation) active process

  1. External intercostal muscles contract and internal intercostal muscles relax
  2. Ribs are pulled upwards and outwards, increasing volume of thorax
  3. Diaphragm muscles contract causing it to flatten, also increasing volume of thorax
  4. Increased volume of thorax results in reduction of pressure in the lungs.
  5. Atmospheric pressure is now greater than pulmonary pressure and so air is forced into the lungs.
• Stages of expiration (exhalation) passive process

  1. Internal intercostal muscles contract and external intercostal muscles relax
  2. Ribs move downwards and inwards  decreasing volume of thorax
  3. Diaphragm muscles relax so pushed up again by contents of abdomen that were compressed during inspiration, volume of thorax decreases
  4. Decreased volume of thorax results in increased pressure in lungs
  5. Pulmonary pressure is now greater than in the atmosphere, air is forced out.