inspiration and expiration

• breathing refers to the movement of air in and out of the lungs.
• inspiration - breathing in

  expiration - breathing out
• inhalation - diaphragm contracts (moves down)

  exhalation - diaphragm relaxes (moves up)
• Role of intercostal muscles :

  • To draw air into the lungs the brain sends nerve messages to the external intercostal muscles and the diaphragm. The contraction of these muscles causes the ribcage to swing up and out whilst the diaphragm contracts and flatten.

  • As the pleural membranes are attached to the ribcage, diaphragm and lungs, movement of the ribcage and diaphragm means the lungs have to follow so the lungs are stretched.

  • This increases the internal volume of the lungs meaning that pressure is decreased below that of the atmosphere so rushes into the lungs = inspiration.
• Role of the diaphragm :

  • To carry out expiration the brain stops sending nerve messages to the ribs and diaphragm so they relax. The diaphragm recoils upwards into its dome shape and gravity makes the ribcage drop down and back.

  • This decreases the volume of the lungs so air pressure increases above the pressure of atmospheric air so air is forced out.

  • If we need to force air out more quickly we may cough which contracts the internal intercostal muscles which pulls the ribcage back down with more force.
• Role of the pleural membranes :

  • These are crucial to the lungs inflating and deflating as well as

  preventing friction when the lungs move.

  • If the space between the membranes is punctured the lungs on that side will not inflate.
• role of plural membranes

  • the pleural membranes consists of two layers of thin membrane.

  • they are moist and slippery, having a thing film of liquid between the type layers.

  • this lubricates the surface so that the two plural layers will slide over each other, allowing the lungs to move easily within the chest cavity.

  • this means that they move with the chest wall as breathing occurs.
• Respiratory system


  Delivers oxygen into the body by breathing in (inspiration) and removes the waste of carbon dioxide gas by breathing out (expiration)
• Breathing in (inspiration)


  1. Intercostal muscles pull the ribcage upwards and outwards

  2. Diaphragm flattens inwards

  3. Increase in volume

  4. Decrease in pressure

  5. Air forced into the lungs, causing them to inflate
• Breathing out (expiration)


  1. Diaphragm lifts back into a dome shape

  2. Intercostal muscles pull the ribcage inwards and downwards

  3. Carbon dioxide forced out of the lungs, causing them to deflate
• Diaphragm


  • Flattens inwards during inspiration
  • Lifts back into a dome shape during expiration
• Intercostal muscles


  • Pull the ribcage upwards and outwards during inspiration
  • Pull the ribcage inwards and downwards during expiration