mechanisms of breathing

• Lungs and chest wall


  Elastic structures
• Intrapleural space


  Thin layer of fluid present between the lungs and the chest wall
• Intrapleural pressure

  Subatmospheric pressure in the space between the lungs and chest wall
• Lungs are stretched when they expand at birth, and at the end of quiet expiration their tendency to recoil from the chest wall is just balanced by the tendency of the chest wall to recoil in the opposite direction
• If the chest wall is opened, the lungs collapse; and if the lungs lose their elasticity, the chest expands and becomes barrel shaped
• Pulmonary ventilation

  Comprises two major steps: inspiration and expiration
• Inspiration


  Process that causes air to enter the lungs
• Expiration


  Process that causes air to leave the lungs
• Fall in air pressure


  Alveolar spaces fall and air enters the lungs (inspiration)
• Pressure of the alveoli exceeds atmospheric pressure


  Air is blown from the lungs (expiration)
• Flow rate of air

  Proportional to the magnitude of the pressure difference
• Breathing


  Physical process of inhaling oxygen and exhaling carbon dioxide
• Respiration


  Biochemical process that occurs at the cellular level and produces energy
• Inspiration


  1. Active process

  2. Contraction of inspiratory muscles increases intrathoracic volume

  3. Intrapleural pressure decreases

  4. Lungs pulled into more expanded position

  5. Pressure in airway becomes slightly negative, air flows into lungs
• Expiration


  1. Passive process

  2. Lung recoil pulls chest back to expiratory position

  3. Pressure in airway becomes slightly positive, air flows out of lungs
• Diaphragm


  • Most important muscle of inspiration
  • Responsible for 75% of inspiratory effort
  • Thin dome-shaped muscle attached to lower ribs, xiphoid process, lumbar vertebra
  • Innervated by Phrenic nerve (Cervical segments 3,4,5)
• External intercostal muscles

  • Responsible for 25% of inspiratory effort
  • Connect to adjacent ribs
  • Motor neurons originate in respiratory centers of brainstem, travel down spinal cord via intercostal nerves
• Accessory inspiratory muscles

  • Sternocleidomastoid
  • Scalene
  • Anterior serrati
  • Elevators of scapulae
  • Pectorals
• Active expiration


  1. Requires abdominal & internal intercostals muscle contraction

  2. Rectus abdominus/abdominal oblique muscles contract to raise intra-abdominal pressure and move diaphragm upward

  3. Internal intercostals muscles assist expiration by pulling ribs downward & inward, decreasing thoracic volume
• Gaseous exchange


  Occurs by diffusion in the alveoli, depends on pressure differences between blood, tissues, and atmospheric air
• Both inspiration and expiration are important in the process of respiration in the body