Functional residual capacity (FRC), is the volume remaining in the lungs after a normal, passive exhalation. In a normal individual, this is about 3L. The FRC also represents the point of the breathing cycle where the lung tissue elastic recoil and chest wall outward expansion are balanced and equal. Thus, the FRC is unique in that it is both a volume and related directly to two respiratory structures.
FRC is the total amount of air in a person’s lungs at the lowest point of their tidal volume (TV), where the tidal volume is the volume of air a person normally inspires and expires. The FRC is a lung capacity, consisting of the sum of two or more volumes. It also cannot be measured directly using spirometry and has to be calculated. This because FRC is a combination of the expiratory reserve volume (ERV) and the residual volume (RV). The residual volume is the amount of air remaining the lung after expelling as much air from the lungs as possible.  The residual volume can never be exhaled; thus, it cannot be measured using spirometry and is the air causing the alveoli to remain open. The expiratory reserve volume (ERV) is the reserve amount of air that can be exhaled forcefully, after passive exhalation. Therefore, the FRC can be represented as the equation: FRC= RV+ERV
FRC is also the point at which two forces are at equilibrium; the inner recoil forces of the lung due to the elastic tissue of the alveoli, and the chest wall which wants to expand outwards.
The FRC is important because it is related to several factors such as airway and vascular resistance, work of breathing, compliance, oxygen reserve, closing capacity, and V/Q mismatch.
- Reduced lung volumes result in reduced FRC. Low lung volumes result in less alveolar tension pulling the lung airways open, and the airway narrowing results in increased airway resistance.
- Pulmonary vascular resistance is a combination of alveolar and extra-alveolar vessel resistances and is U-shaped. https://www.ncbi.nlm.nih.gov/books/NBK554380/ Thus, there are larger resistances at TLC and RV, and the lowest resistance is at the FRC volume.
- At FRC, the work to inflate the lungs is the lowest, as the inward and outward lung compliances are balanced.
- The compliance of the lung depends on the elastic recoil of the lung tissue. Decreases in this result in an increased FRC.
- The FRC results in an oxygen reserve, the residual air volume in the lungs allows for oxygen exchange. This oxygen reserve, and FRC, are important during the induction of anesthesia.
- A reduced FRC can result in shunts and atelectasis. This occurs when the FRC decreases below the closing capacity of the lung; the volume at which the respiratory bronchioles collapse.
The FRC is affected by conditions that affect lung compliance; a combination of the inward elastic recoil of the lung, and outward expansion of the chest wall. These include diseases or conditions with changes in lung tissue compliance (emphysema, and interstitial lung diseases), decreased chest movements (kyphoscoliosis), or decreased thoracic volume (obesity, pregnancy). Other factors affecting FRC include acute changes in positions such as lying supine, age, height, and gender.
FRC is altered by the patients’ positioning, with it being greatest when upright and decreasing when supine or prone,  the latter resulting in airway closure of some lung regions. Even larger changes can be observed with patients in the Trendelenburg and head down positions.
As humans age, our pulmonary function also declines due to decreases in respiratory muscle mass, and tissue elasticity. Loss of elasticity in connective tissue increases the work of breathing; similar to chronic obstructive pulmonary disease (COPD) (but to a lesser extent), the air becomes harder to expel and the lungs do not as readily return to normal size after inspiration. Thus the FRC increases slightly with age.
Height and Gender
A tall person had a larger lung volume and thus a greater FRC. Gender also affects FRC. Men tend to have a significantly larger lung volume even when compared to women of the same height and age. This is due to structural differences between men and women. Women have s smaller ribcage, ribs that are angled or inclined differently than men, and a shorter diaphragm length.  However, due to the difference in the rib angle, women have a greater capacity to expand their lungs which is likely to aid physiological changes that occur during pregnancy. 
In pregnant women, spirometry remains within normal limits, however structural and volumes/capacities change significantly. The diaphragm relaxes due to hormonal changes), and the growing fetus begins to exert pressure on the thoracic cavity. This causes both the RV and ERV to decrease, which leads to a decreased FRC. Because of the lowered FRC and pressure on the thorax, a pregnant woman is more susceptible to atelectasis. 
Ascites and obesity
FRC also changes with ascites or obesity. These FRC decreases are due to increased pressure on the diaphragm, and a reduction of thoracic volume. This is one of the causes of shortness of breath.
Anesthetics alter FRC by affecting the tone or relaxation of the respiratory muscles. There is a debate as to the contribution of the rib cage and diaphragm to the decreased FRC.
Lung volumes are followed to track a patient’s respiratory disease. While not routinely used in clinical practice, one way to measure residual volume and total lung capacity (TLC) is to measure a person’s FRC.
FRC can be measured/calculated by using techniques such as the whole body plethysmograph method (based on Boyle’s Law), and the helium dilution method (based on the Law of Conservation of Mass). 
In restrictive diseases, the TLC decreases, resulting in decreased FRC, and the lung tissues or chest wall expansion are limited or restricted. One example of restriction due to chest wall issues is severe kyphosis or weakness of spinal bones. Kyphosis is described elsewhere.  Restrictive pathology can also be due to lung tissues, and one example is idiopathic pulmonary fibrosis. This disease is described elsewhere. 
With obstructive diseases such as emphysema, the FRC is increased. With emphysema, the lungs become increasingly compliant, due to alveolar destruction. As the alveoli are destroyed, air is trapped in the lungs, and TLC is increased. The increased volume and lung tissue compliance causes the chest wall to expand, hence, the typical barrel chest seen in those with emphysema.
While other lung values are more widely used clinically, functional residual capacity (FRC) contains utility both in understanding the respiratory cycle and in clinical practice. Since FRC is the equilibrium point for the forces of the chest wall and lung, it is an efficient starting point when learning about the chest wall/lung system. Both clinicians and researchers use methods to calculate FRC to obtain values that cannot be measured by standard spirometry.