Question 2

Define functional residual capacity (10% marks). Outline the functions (70% marks) of the functional residual capacity and the factors affecting it (20% marks).

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College Answer

This question was in two parts with the percentage of marks allocated an indication of the relevant time or detail expected per part. The second part of the question also contained two distinct headings which should have been used in the answer. As an outline question, dot points with a brief explanation of each point were expected. Most candidates drew diagrams, few of which added value. For a diagram to add value it should be accurate, have labelled axes, a scale with numerical values and units. As a general rule, diagrams should also be explained and help to illustrate or relate to a written point.

For factors affecting FRC, to score full marks, it should be clearly stated if the factor causes an increase or decrease in FRC. This topic is well covered in the recommended respiratory texts.


The FRC is:

  • The volume of gas present in the lung at end expiration during tidal breathing
  • It is composed of ERV and RV
  • This is usually 30-35 ml/kg, or 2100-2400ml in a normal sized person
  • It represents the point where elastic recoil force of the lung is in equilibrium with the elastic recoil of the chest wall, i.e. where the alveolar pressure equilibrates with atmospheric pressure. 

Functions of the FRC are:

  • Oxygen reservoir
    • FRC maintains an oxygen reserve which maintains oxygenation between breaths
    • This prevents rapid changes in alveolar oxygen tension and arterial oxygen content
  • Maintenance of small airway patency
    • At FRC, the small airway resistance is at its lowest. 
    • Where closing capacity is greater than the FRC, gas trapping and atelectasis can develop because of small airway collapse
  • Optimisation of pulmonary vascular resistance
    • At FRC, pulmonary vascular resistance is minimal
    • The RV afterload and pulmonary blood flow are therefore optimal
  • Optimisation of respiratory workload
    • At FRC, lung compliance is maximal
    • The work of breathing required to inflate the lung from FRC is minimum

Factors affecting the FRC are:

  • Factors that increase FRC:
    • Male sex
    • Large body size
    • Emphysema
    • PEEP or auto-PEEP
    • Open chest
    • Erect body position
  • Factors that decrease FRC:influence lung size (height and gender) - larger lung size increases FRC
    • Female sex
    • Small  stature
    • ARDS
    • increased intraabdominal pressure
    • pregnancy, obesity
    • anaesthesia and paralysis
    • supine position

It would be unreasonable to expect a massive amount of detail for these factors in a two-minute answer, but just as a reference, a longer list is offered here:

Factors which Influence the Functional Residual Capacity
Factors which increase FRC Factors which decrease FRC
Factors which influence lung size
Increased height Short stature
Male gender Female gender
Age: ratio of FRC to total lung capacity increases, but absolute FRC remains stable 
(Wahba et al, 1983)
Factors which influence lung compliance
Increased compliance due to disease, eg. emphysema Decreased lung compliance due to disease, eg. ARDS
Increased end-expiratory pressure, eg. PEEP or auto-PEEP Negative end-expiratory pressure
Factors which influence chest expansion and chest wall compliance
Open chest or mediastinum Increased intraabdominal pressure:
pregnancy, ascites, abdominal surgery
  Decreased respiratory muscle tone, eg. anaesthesia/sedation
Upright position and prone position Supine and head down position
  Circumferential burns, chest binder devices (eg. post mastectomy)


Wanger, J., et al. "Standardisation of the measurement of lung volumes." European respiratory journal 26.3 (2005): 511-522.

Lutfi, Mohamed Faisal. "The physiological basis and clinical significance of lung volume measurements." Multidisciplinary respiratory medicine 12.1 (2017): 3.

Boren, Hollis G., Ross C. Kory, and James C. Syner. "The Veterans Administration-Army cooperative study of pulmonary function: II. The lung volume and its subdivisions in normal men." The American Journal of Medicine 41.1 (1966): 96-114.

Pappenheimer, J. R., J. H. Comroe, and A. Cournand. "Standardization of definitions and symbols in respiratory physiology." Fed Proc. Vol. 9. No. 3. 1950.

Gandevia, Bryan, and P. Hugh-Jones. "Terminology for measurements of ventilatory capacity: a report to the Thoracic Society." Thorax 12.4 (1957): 290.

Chandra Selvi, E., and Kuppu KV Rao. "Should the Functional Residual Capacity be Ignored?." Journal of Clinical and Diagnostic Research: JCDR 7.1 (2013): 43.