Lung Volumes & Capacities

• Lung volumes and lung capacities

  The volume of air in the lungs at different phases of the respiratory cycle

  
• The average total lung capacity of an adult human male is about 6 liters of air

  
• Determination of lung volumes

  Used to assess the efficiency of the respiratory system and to diagnose respiratory diseases

  
• Lung volume


  Measured directly, by a spirometer or by a gas dilution technique

  
• Lung capacity


  The sum of two or more lung volumes; it is therefore a derived value

  
• Lung volumes


  • Tidal volume (VT = 500 mL)
  • Inspiratory reserve volume (IRV = 2500 mL)
  • Expiratory reserve volume (ERV = 1500 mL)
  • Residual volume (RV = 1500 mL)

  
• Lung capacities


  • Functional residual capacity (FRC = RV + ERV = 3000 mL)
  • Vital capacity (VC = ERV + VT + IRV = 4500 mL)
  • Inspiratory capacity (IC = VT + IRV = 3000 mL)
  • Total lung capacity (TLC = RV + ERV + VT + IRV = 6000 mL)

  
• All lung volumes, with the exception of RV, can be measured with a spirometer

  
• RV cannot be directly measured as it is the volume of gas that remains in the lungs at the end of maximal expiration

  
• Capacities that include RV, such as TLC and FRC, cannot be directly measured by spirometry
• FRC can be calculated by

  • Gas dilution
  • Body plethysmography
  • Multiple-breath N2 washout
• Factors affecting lung volumes

  • Larger volumes: taller people, who live at higher altitudes, fit
  • Smaller volumes: shorter people, people who live at lower altitudes, obese