Functional residual capacity (FRC) is the

volume Volume is a measure of regions in 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) ...

of air present in the

lungs The lungs are the primary organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the backbone on either side of the heart. Their function in the respiratory syste ...

at the end of passive

expiration Expiration or expiration date may refer to: Expiration Expiration may refer to: *Death *Exhalation of breath, breathing out * Expiration (options), the legal termination of an option to take an action *Shelf life, or the time after which a product ...

.Hopkins E, Sharma S. Physiology, Functional Residual Capacity. pdated 2022 Jan 4 In: StatPearls nternet Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK500007/ At FRC, the opposing elastic recoil forces of the lungs and

chest wall The thoracic wall or chest wall is the boundary of the thoracic cavity. Structure The bony skeletal part of the thoracic wall is the rib cage, and the rest is made up of muscle, skin, and fasciae. The chest wall has 10 layers, namely (from ...

are in equilibrium and there is no exertion by the diaphragm or other respiratory muscles.

Measurement

FRC is the sum of expiratory reserve volume (ERV) and residual volume (RV)Levitzky M.G. (2021). Alveolar ventilation. Levitzky M, & McDonough K, & Kaye A, & Hall S(Eds.), Clinical Physiology in Anesthetic Practice. McGraw Hill. https://accessanesthesiology.mhmedical.com/content.aspx?bookid=2979§ionid=249590087 and measures approximately 3000 mL in a 70 kg, average-sized male. It cannot be estimated through

spirometry Spirometry (meaning ''the measuring of breath'') is the most common of the pulmonary function tests (PFTs). It measures lung function, specifically the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled. Spirometry is h ...

, since it includes the residual volume. In order to measure RV precisely, one would need to perform a test such as

nitrogen washout Nitrogen washout (or Fowler's method) is a test for measuring anatomic dead space in the lung during a respiratory cycle, as well as some parameters related to the closure of airways. Procedure A nitrogen washout can be performed with a single n ...

, helium dilution or

body plethysmography A plethysmograph is an instrument for measuring changes in volume within an organ or whole body (usually resulting from fluctuations in the amount of blood or air it contains). The word is derived from the Greek "plethysmos" (increasing, enla ...

. 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.

Clinical significance

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, ...

. In restrictive diseases, the decreased total lung capacity leads to a lower FRC. In turn in obstructive diseases, the FRC is increased. For instance, in

emphysema Emphysema is any air-filled enlargement in the body's tissues. Most commonly emphysema refers to the permanent enlargement of air spaces (alveoli) in the lungs, and is also known as pulmonary emphysema. Emphysema is a lower respiratory tract di ...

, 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 Lung volumes and lung capacities are measures of the volume of air in the lungs at different phases of the respiratory cycle. The average total lung capacity of an adult human male is about 6 litres of air. Tidal breathing is normal, resting b ...

also increases, largely as a result of increased functional residual capacity. Obese and pregnant patients will have a lower FRC in the supine position due to the added tissue weight opposing the outward recoil of the chest wall thus reducing chest wall compliance. In pregnancy, this starts at about the fifth month and reaches 10-20% decrease at term. FRC tends to increase with aging due to changes in the static recoil 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. The LOINC code is 19843-2 and the

SNOMED CT SNOMED CT or SNOMED Clinical Terms is a systematically organized computer-processable collection of medical terms providing codes, terms, synonyms and definitions used in clinical documentation and reporting. SNOMED CT is considered to be the mo ...

concept is 65825000.

References

{{Respiratory physiology Respiratory physiology Pulmonary function testing Pulmonology