Mechanism of breathing

• Gases move from an area of high pressure to an area of low pressure
• Ventilation involves the alternate increase and decrease of air pressure in the lungs
• Inspiration = air pressure in lungs decreases lower than atmospheric pressure so air moves into the lungs
• Expiration = air pressure inside lungs increases above atmospheric pressure so air moves out of the lungs
• Inspiration

  • external intercostal muscles contract
  • internal intercostal muscles relax
  • ribs and sternum move upwards and outwards
  • diaphragm contracts and flattens
  • volume of thorax is increased
  • elastic tissue of lungs is stretched
  • lungs expand reducing pressure lower than atmospheric pressure
  • air is forced into alveoli from atmosphere
• Expiration

  • external intercostal muscles relax
  • internal intercostal muscles contract
  • ribs and sternum moves downwards and inwards
  • diaphragm relaxes and returns to its dome shape
  • volume decreases
  • lung size is reduced
  • pressure inside lungs in increased above atmospheric pressure
  • air is forced out the alveoli into the atmosphere
• Pulmonary ventilation = volume of air moved into the lungs in one minute
• Residual volume = volume of air remaining in the lungs when all other volumes have been exhaled
• Ventilation rate = number of breaths taken a minute
• Tidal volume = volume of air taken in at each breath when at rest
• Pulmonary ventilation = tidal volume x ventilation rate
• Total lung capacity = total volume of air in the lung at the end of a maximal inhalation
• Vital capacity = maximum volume of air a person can expel from their lungs after a maximum inhalation