Respiratory system and mechanics of ventilation

• Functions of the thorax

  • To protect the heart, lungs, and some abdominal organs
  • To provide attachment for muscles of the neck, thorax, upper limb, abdomen, and back
  • To allow thoracic expansion in order that breathing may occur
  • To allow greater rotation of the trunk, and therefore greater coordination of movement
• Protection

  • Limited mobility in the upper thorax – ribs 1-7 are attached directly to both sternum and spine
  • Protection of the contents of the mediastinum: heart in its pericardium sac; vessels = (great vein, aorta and pulmonary arteries, thoracic duct); nerves (phrenic nerves, X vagus nerve); oesophagus; trachea

  
• Attachments

  • Less flexible region – the upper limbs are suspended from the thorax via the arrangement of clavicle and scapula
  • The only connecting joint = sternoclavicular joint
• Muscles of respiration

  • Quiet inspiration (normal breathing, not thinking about it) = diaphragm and external intercostals
  • Accessory muscles of inspiration (join in when necessary) = scalenes, sternocleidomastoid, pectoralis major and minor, serratus anterior, trapezius, erector spinae
  • Accessory muscles of expiration = internal intercostals, transverse thoracic, internal and external obliques, rectus abdominis, latissimus dorsi, quadratus lumborum
• Actions of the diaphragm

  • Major muscle of inspiration
  • Controlling intra-thoracic pressures (important for functions like coughing – can also tense to increase abdominal pressure which is important for fluid and bowel movements) and intra-abdominal pressures
  • Micturition, defecation, and parturition
  • Supporting lumbar spine during lifting
  • Preventing gastric reflex
  • Assisting venous return and lymphatic damage
• Contraction of the diaphragm

  1. Ribs fixed – central tendon lowers 1.5cm (vertical diameter)

  2. Central tendon fixed, reversed origin and insertion – lower ribs moved up and out (bucket handle) (transverse/lateral diameter)

  3. Sternum raised by movement of upper ribs (pump handle) (AP diameter)

  4. Expiration by controlled relaxation against elastic recoil of lungs
• Thoracic expansion

  • Pump handle and bucket handle
  • Diaphragmatic excursion
  • To a lesser dome – not flat
  • A flattened diaphragm suggests hyperinflation