Functional Residual Capacity (FRC)

• A spirometer of known volume is filled with an air-helium mixture of known concentration.
• At the end of a normal quiet expiration (i.e., at the FRC level), the subject is connected to the spirometer and begins rebreathing.
• Helium equilibrates between the spirometer and the lung gas, becoming diluted in proportion to the FRC.
• FRC is then calculated from the degree of helium dilution using the formula FRC = (C₁ x V_spirometer / C₂) - V_spirometer, where C₁ and C₂ are the initial and final helium concentrations.

1. Buffer for alveolar gas composition - Since FRC (~2300 mL) is large compared with tidal volume (~500 mL), each fresh breath mixes with a large volume of residual air already in the lungs. This prevents marked breath-to-breath fluctuations in alveolar PO2 and PCO2, keeping arterial blood gas levels relatively stable between inspirations rather than swinging with each breath.
2. Continuous gas exchange during expiration - Because some air always remains in the lungs, alveoli never completely empty or collapse during normal breathing, allowing continuous blood oxygenation and CO2 removal even during the expiratory phase, not just during inspiration.
3. Prevents alveolar collapse (atelectasis) - FRC keeps the alveoli partially inflated at end-expiration, opposing the tendency of alveolar surface tension to cause collapse, and helps maintain a favorable ventilation-perfusion relationship.
4. Relation to closing capacity - If FRC falls below the closing capacity (the lung volume at which small airways in dependent lung zones begin to close), airway closure occurs during normal tidal breathing, leading to ventilation-perfusion mismatch and hypoxemia. This relationship is clinically important in anesthesia, obesity, pregnancy, and old age.
5. Diagnostic and clinical relevance - FRC changes characteristically in disease and is used to assess pulmonary status:
   • Increased in obstructive diseases such as emphysema, chronic bronchitis, and acute asthma, due to air trapping and loss of elastic recoil.
   • Decreased in restrictive conditions such as pulmonary fibrosis, ARDS, obesity, pregnancy, supine posture, and general anesthesia, predisposing to atelectasis and hypoxemia.
6. Determines resting lung volume - FRC represents the equilibrium point at which the inward elastic recoil of the lungs is exactly balanced by the outward recoil of the chest wall, making it the natural resting position of the respiratory system between breaths.