6.4  Gas Exchange

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Essential Idea:

The lungs are actively ventilated to ensure

that gas exchange can occur passively


  • Ventilation maintains concentration gradients of oxygen and carbon dioxide between air in alveoli and blood flowing in adjacent capillaries
  • Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles
  • Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas exchange
  • Type II pneumocytes secrete a solution containing surfactant that creates a moist surface inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing surface tension
  • Muscle contractions cause the pressure changes inside the thorax that force air in and out of the lungs to ventilate them
  • Different muscles are required for inspiration and expiration because muscles only do work when they contract


  • Causes and consequences of lung cancer
  • Causes and consequences of emphysema
  • External and internal intercostal muscles, and diaphragm and abdominal muscles as examples of antagonistic muscle action


  • Monitoring of ventilation in humans at rest and after mild and vigorous exercise