Gas exchange in Lungs

• Henry's law


  The amount of a gas that dissolves in a liquid is directly proportional to the partial pressure of that gas in equilibrium

  
• Partial pressure


  Pressure exerted by a single gas
• Dalton's law


  Sum of partial pressures = total pressure

  
• Partial pressures


  • Pgas1 = 0.10 x 200 = 20mmHg
  • Pgas2 = 0.50 x 200 = 100mmHg
  • Pgas3 = 0.40 x 200 = 80mmHg

  
• Total pressure = 200mmHg
• Partial pressures in alveoli: 160mmHg O2, 600mmHg H2O, 40mmHg CO2

  
• Inspired air is humidified
• Alveolar air is slowly replaced
• Alveolar air composition: 79% N2, 21% O2, 0.04% CO2

  
• Factors that determine alveolar PO2 & PCO2: rate of alveolar ventilation and perfusion, PO2 of atmospheric air, rate of cellular oxygen consumption and carbon dioxide production

  
• Hyperventilation


  ↑ alveolar ventilation, ↑ PO2, ↓ PCO2

  
• Hypoventilation


  ↓ alveolar ventilation, ↓ PO2, ↑ PCO2

  
• Effect of elevation on gas partial pressures

  • Sea level (760mmHg): N2 600mmHg, O2 160mmHg, CO2 0.3mmHg
  • Mt Cook (480mmHg): N2 380mmHg, O2 100mmHg, CO2 0.2mmHg
  • Mt Everest (230mmHg): N2 181mmHg, O2 48mmHg, CO2 0.1mmHg

  
• Reinhold Messner: '"I am nothing more than a single narrow gasping lung, floating over the mists and summits..."'
• Edmund Hillary and Tenzing Norgay summited Mt Everest in 1953
• Reinhold Messner and Peter Habeler summited Mt Everest without supplementary O2 in 1978