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Notes
      3. Old stuff
          3.2. Old physio stuff (around 2005)
              3.2.3. Physiology
                  3.2.3.13. Respiratory
                      3.2.3.13.2. Ventilation and perfusion
                          3.2.3.13.2.2. Ventilation
                              3.2.3.13.2.2.2. Lung volumes
 3.2.3.13.2.2.2.2. Functional residual capacity 

Functional residual capacity (FRC)

FRC is essentially the balance point between the tendency of the chest wall to spring outwards and the tendency of the lung to recoil
--> Intrapleural pressure is negative

Factors affecting FRC

FRC increases with

  • Increasing height
  • Erect position (30% higher)
  • Reduced lung recoil

 

FRC decreases with

  • Decreased height
  • Supine position
  • Increased lung recoil
  • Obesity
  • Muscle paralysis
  • Pregnancy
  • Anaesthesia

FRC does not change with age

 

Functions of FRC

  • Oxygen store
  • Buffer to maintain a steady PaO2
  • Prevent atelectasis
  • Minimise work of breathing
  • Minimise pulmonary vascular resistance
  • Minimise V/Q mismatch
    * By keeping lung volume above closing capacity
  • Keep airway resistance low (though not minimal)
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