Functional residual capacity (FRC) is the

volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). Th ...

of air present in the

lungs The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of ...

at the end of passive expiration. At FRC, the opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the

diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...

or other respiratory muscles. FRC is the sum of expiratory reserve volume (ERV) and residual volume (RV) and measures approximately 2500 mL in a 70 kg, average-sized male (or approximately 30ml/kg). It cannot be estimated through spirometry, since it includes the residual volume. In order to measure RV precisely, one would need to perform a test such as nitrogen washout, helium dilution or body plethysmography. A lowered or elevated FRC is often an indication of some form of

respiratory disease Respiratory diseases, or lung diseases, are pathological conditions affecting the organs and tissues that make gas exchange difficult in air-breathing animals. They include conditions of the respiratory tract including the trachea, bronchi, ...

. For instance, in

emphysema Emphysema, or pulmonary emphysema, is a lower respiratory tract disease, characterised by air-filled spaces ( pneumatoses) in the lungs, that can vary in size and may be very large. The spaces are caused by the breakdown of the walls of the a ...

, FRC is increased, because the lungs are more compliant and the equilibrium between the inward recoil of the lungs and outward recoil of the chest wall is disturbed. As such, patients with emphysema often have noticeably broader chests due to the relatively unopposed outward recoil of the chest wall. Total lung capacity also increases, largely as a result of increased functional residual capacity. In healthy humans, FRC changes with body posture. Obese patients will have a lower FRC in the supine position due to the added tissue weight opposing the outward recoil of the chest wall. Positioning plays a significant role in altering FRC. It is highest when in an upright position and decreases as one moves from upright to supine/prone or Trendelenburg position. The greatest decrease in FRC occurs when going from 60° to totally supine at 0°. There is no significant change in FRC as position changes from 0° to Trendelenburg of up to −30°. However, beyond −30°, the drop in FRC is considerable. The helium dilution technique and pulmonary plethysmograph are two common ways of measuring the functional residual capacity of the lungs. The predicted value of FRC was measured for large populations and published in several references. FRC was found to vary by a patient's age, height, and sex. Functional residual capacity is directly proportional to height and indirectly proportional with obesity. It is reduced in the setting of obesity primarily due to a reduction in chest wall compliance. The LOINC code is 19843-2 and the SNOMED CT concept is 65825000.

References

{{Respiratory physiology Respiratory physiology Pulmonary function testing Pulmonology