Chest Wall Diaphragm

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  • Inspiration occurs when alveolar pressure < atmospheric pressure, either by lowering alveolar pressure (negative pressure respiration), or raising atmospheric pressure (PPV)
  • Expiration occurs when alveolar pressure > atmospheric pressure
  • Contraction of inspiratory muscles enlarges the thorax, lowers intrathoracic and intrapleural pressures, enlarging the alveoli and airways and lowers the alveolar pressue below atmospheric pressure
  • Inspiratory muscles must overcome:
  • The elastic recoil of the lungs/chest wall
  • The frictional resistance caused by deformation of lungs/thorax (tissue resistance) and air flow in the conducting airways (airway resistance)


  • Diaphragm - most important muscle
  • Supplied by phrenic nerve from C3-5
  • Contracts, pushing down abdominal contents down increasing the transverse diameter of the thorax and elevates the ribs
  • Normally only moves 1cm up and down, but on forced inspiration/expiration - moves as much as 10cm
  • Unilateral paralysis doesn't reduce ventilatory capacity much, bilateral paralysis produces paradoxical upward movement on inspiration
  • In hyperinflation disease states, acts at a mechanical disadvantage due to being flattened.
  • External intercostal muscles
  • Connect ribs sloping downwards and forwards
  • On contraction, pull ribs upwards and forwards in bucket-handle fashion
  • Supplied by intercostal nerves at the same level
  • Accessory muscles of inspiration
  • Scalenes (elevate the first 2 ribs), sternomastoids (raise the sternum), trapezius and neck/back muscles
  • Not useful at rest, but vigorously used during exercise/dyspnoea


  • Usually passive while at rest - occurs via passive recoil of pulmonary and thoracic tissues
  • Abdominal muscles are the main players during active expiration - contract pushing abdominal contents inwards and upwards
  • Internal intercostal muscles - pull ribs downwards and inwards

Effects of Anaesthesia on Respiration

  • Deep anaesthesia results in decreased intercostal muscle activity before decreased diaphragmatic activity occurs, therefore producing "abdominal" or "diaphragmatic" breathing as well as a decreased ventilatory response to CO2
  • "Abdominal breathing" - usually has short, sharp inspiration
  • Expiratory muscle activity unaffected by anaesthesia