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You are watching: At functional residual capacity (frc), the recoil of the lungs is:

StatPearls . Sweetheart Island (FL): StatPearls Publishing; 2021 Jan-.



Functional residual volume (FRC), is the volume remaining in the lung after a normal, passive exhalation. In a normal individual, this is around 3L. The FRC additionally represents the suggest of the breathing cycle where the lung tissue elastic recoil and also chest wall surface outward development are balanced and equal. Thus, the FRC is unique in the it is both a volume and related directly to two respiratory structures.

FRC is the total amount of waiting in a person’s lungs at the lowest suggest of their tidal volume (TV), where the tidal volume is the volume of waiting a person usually inspires and expires. The FRC is a lung capacity, consists of the amount of 2 or more volumes. It likewise cannot be measured straight using spirometry and also has to it is in calculated. This since FRC is a combination of the expiratory make reservation volume (ERV) and the residual volume (RV). The residual volume is the amount of air staying the lung after expelling as much air indigenous the lungs as possible. <1> The residual volume can never be exhaled; thus, it can not be measured using spirometry and also is the air causing the alveoli to continue to be open. The expiratory reserve volume (ERV) is the reserve amount of air that have the right to be exhaled forcefully, ~ passive exhalation. Therefore, the FRC deserve to be represented as the equation: FRC= RV+ERV

FRC is likewise the point at which two forces are at equilibrium; the within recoil forces of the lung as result of the elastic organization of the alveoli,  and the chest wall surface which wants to increase outwards.<2><3><4>


The FRC is important due to the fact that it is related to several components such together airway and vascular resistance, work-related of breathing, compliance, oxygen reserve, close up door capacity, and also V/Q mismatch.  

Reduced lung volumes an outcome in decreased FRC. Low lung volumes result in much less alveolar anxiety pulling the lung airways open, and the airway narrowing results in enhanced airway resistance.
Pulmonary vascular resistance is a mix of alveolar and also extra-alveolar ship resistances and is U-shaped. Https:// Thus, over there are larger resistances at TLC and also RV, and the shortest resistance is at the FRC volume.
At FRC, the work to inflate the lungs is the lowest, as the inward and outward lung compliances room balanced.
The compliance of the lung counts on the elastic recoil the the lung tissue. To reduce in this result in an boosted FRC.
The FRC results in an oxygen reserve, the residual wait volume in the lungs allows for oxygen exchange. This oxygen reserve, and FRC, are important throughout the induction that anesthesia.
A decreased FRC can result in shunts and also atelectasis. This occurs as soon as the FRC decreases listed below the closing volume of the lung; the volume in ~ which the respiratory bronchioles collapse.

 The FRC is influenced by conditions that impact lung compliance; a mix of the inward elastic recoil the the lung, and also outward growth of the chest wall. This include illness or problems with alters in lung organization compliance (emphysema, and also interstitial lung diseases), reduced chest motions (kyphoscoliosis), or decreased thoracic volume (obesity, pregnancy). Other determinants affecting FRC encompass acute transforms in location such as lying supine, age, height, and gender.


FRC is changed by the patients’ positioning, v it being biggest when upright and also decreasing as soon as supine or prone, <5> the last resulting in airway closure of some lung regions. Also larger transforms can be observed with patients in the Trendelenburg and head down positions.


As human beings age, our pulmonary function also declines due to decreases in respiratory tract muscle mass, and tissue elasticity.  ns of elasticity in connective tissue boosts the job-related of breathing; similar to chronic obstructive pulmonary disease (COPD) (but come a lesser extent), the air becomes harder to expel and the lungs perform not as easily return to normal size after inspiration. Thus the FRC rises slightly v age.

Height and Gender

A high person had actually a bigger lung volume and thus a greater FRC. Gender likewise affects FRC. Males tend to have a considerably larger lung volume even when compared to women of the very same height and age.<6> This is due to structural differences in between men and women. Women have s smaller sized ribcage, ribs that room angled or inclined differently than men, and a shorter diaphragm length. <6> However, because of the difference in the rib angle, women have actually a better capacity to broaden their lungs which is most likely to help physiological changes that occur during pregnancy. <6> 


In pregnant women, spirometry remains within typical limits, but structural and also volumes/capacities readjust significantly. The diaphragm relaxes due to hormonal changes), and also the growing fetus begins to exert push on the thoracic cavity. This causes both the RV and also ERV to decrease, which leads to a diminished FRC. Because of the lower FRC and pressure ~ above the thorax, a pregnant woman is more susceptible come atelectasis. <7>

 Ascites and obesity

FRC additionally changes v ascites or obesity. These FRC decreases are because of increased push on the diaphragm, and a palliation of thoracic volume. This is one of the causes of shortness that breath.


Anesthetics alter FRC by affect the tone or be safe of the respiratory muscles. Over there is a debate as to the contribution of the rib cage and also diaphragm to the decreased FRC.

Related Testing

Lung volumes are adhered to to track a patient’s respiratory tract disease. While no routinely provided in clinical practice, one way to measure up residual volume and total lung volume (TLC) is to measure a person’s FRC. 

FRC can be measured/calculated by using methods such together the totality body plethysmograph method (based ~ above Boyle’s Law), and also the helium dilution technique (based ~ above the regulation of preservation of Mass). <8>

Clinical Significance

 In restrictive diseases, the TLC decreases, resulting in decreased FRC, and also the lung tissues or chest wall expansion are minimal or restricted.  One example of restriction due to chest wall surface issues is serious kyphosis or weakness of spinal bones. Kyphosis is defined elsewhere. <9> Restrictive pathology can additionally be due to lung tissues, and also one instance is idiopathic pulmonary fibrosis. This condition is described elsewhere. <10>

With obstructive illness such as emphysema, the FRC is increased. V emphysema, the lungs come to be increasingly compliant, because of alveolar destruction. As the alveoli room destroyed, waiting is trapped in the lungs, and TLC is increased.  The boosted volume and also lung organization compliance causes the chest wall surface to expand, hence, the usual barrel chest seen in those through emphysema.

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While various other lung worths are an ext widely offered clinically, useful residual capacity (FRC) has utility both in understanding the respiratory cycle and in clinical practice. Because FRC is the equilibrium suggest for the pressures of the chest wall surface and lung, that is an efficient starting point once learning about the chest wall/lung system. Both clinicians and researchers use techniques to calculation FRC to attain values the cannot it is in measured by standard spirometry.<11><12><13>

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