Introduction

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 because it is both a volume and related directly to 2 respiratory structures.

FRC is the total air in a person’s lungs at the lowest point of their tidal volume (TV), where the tidal volume is the air a person normally inspires and expires. The FRC is a lung capacity consisting of 2 or more volumes. It also cannot be measured directly using spirometry and has to be calculated. FRC combines the expiratory reserve volume (ERV) and the residual volume (RV). The residual volume is the amount of air remaining in the lungs after expelling as much air from the lungs as possible.[1] The residual volume can never be exhaled; thus, it cannot be measured using spirometry. 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 2 forces are at equilibrium: the lung's inner recoil forces due to the alveoli's elastic tissue and the chest wall, which wants to expand outwards.[2][3][4]

Function

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.

1. 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.
2. Pulmonary vascular resistance is a U-shaped combination of alveolar and extra-alveolar vessel resistances. 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.
3. At FRC, the work to inflate the lungs is the lowest, as the inward and outward lung compliances are balanced.
4. The lung's compliance depends on the elastic recoil of the lung tissue. Decreases in this result in an increased FRC.
5. The FRC results in an oxygen reserve, the lung's residual air volume allowing for oxygen exchange. This oxygen reserve and FRC are important during the induction of anesthesia.
6. A reduced FRC can result in shunts and atelectasis. This occurs when the FRC decreases below the lung's closing capacity, the volume at which the respiratory bronchioles collapse.

The FRC is affected by conditions that affect lung compliance, a combination of the lung's inward elastic recoil and the chest wall's outward expansion. 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 position changes such as lying supine, age, height, and gender.

Position

FRC is altered by the patients’ positioning, which is greatest when upright and decreases when supine or prone, resulting in airway closure of some lung regions. Even larger changes can be observed with patients in the Trendelenburg and head-down positions.[5]

Age

As humans age, our pulmonary function also declines due to decreased 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 return to normal size as readily 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 than women of the same height and age.[6] This is due to structural differences between men and women. Women have smaller ribcages, ribs that are angled or inclined differently than men, and a shorter diaphragm length.[6] However, due to the difference in the rib angle, women have a greater capacity to expand their lungs, likely aiding physiological changes during pregnancy.[6] 

Pregnancy

In pregnant women, spirometry remains within normal limits; however, structural and volume/capacity changes significantly. The diaphragm relaxes due to hormonal changes), and the growing fetus exerts pressure on the thoracic cavity. This causes the RV and ERV to decrease, leading to a decreased FRC. Because of the lowered FRC and pressure on the thorax, a pregnant woman is more susceptible to atelectasis.[7]

Ascites and obesity

FRC also changes with ascites or obesity. These decreases are due to increased pressure on the diaphragm and a reduction of thoracic volume, which is 1 cause of shortness of breath.

Anesthesia

Anesthetics alter FRC by affecting the tone or relaxation of the respiratory muscles. The contribution of the rib cage and diaphragm to decreased FRC is debated.

Related Testing

Lung volumes are followed to track a patient’s respiratory disease. While not routinely used in clinical practice, 1 way to measure residual volume and total lung capacity (TLC) is to measure a person’s FRC. FRC can be measured/calculated using techniques such as the whole body plethysmograph method (based on Boyle’s law) and the helium dilution (based on the law of conservation of mass).[8]

Clinical Significance

 In restrictive diseases, the TLC decreases, resulting in decreased FRC, and the lung tissues or chest wall expansion is limited or restricted. One example of restriction due to chest wall issues is severe kyphosis or weakness of spinal bones. Kyphosis is described elsewhere.[9] Restrictive pathology can also be due to lung tissues; 1 example is idiopathic pulmonary fibrosis. This disease is described elsewhere.[10]

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) is useful in understanding the respiratory cycle and 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.[11][12][13]