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)