Proficient foundational knowledge of basic physiological principles leads to better utilization of their clinical application. This article intends to lend a better understanding of functional residual capacity (FRC), a term that denotes the volume in the lungs just after passive expiration. It is important to understand the basic definition, relevant relationships to other lung volumes/capacities, the forces of the chest wall and lung at FRC, and related clinical applications.
The breathing cycle consists of many named volumes. Two or more volumes are labeled capacities. The functional residual capacity (FRC) can be defined in multiple ways. FRC is the volume in the lungs at the end of a natural exhalation. However, there is still air left in the lungs. The residual volume (RV) is the amount of air an individual never physiologically expires. It is the volume remaining in the lungs after expelling as much air from the lungs as possible. The amount of air between RV and FRC is the expiratory reserve volume (ERV). Therefore, FRC= RV+ERV. FRC is the total amount of air in a person’s lungs at the lowest point of their tidal volume (TV). Tidal volume is the volume of air a person normally inspires and expires. So, after normal expiration, FRC is equal to the amount of air left in the lungs.
An important aspect of understanding FRC (and the respiratory cycle in general) is knowledge of the forces involved. There are 2 main mechanical forces throughout the breathing cycle: the force of the chest wall and the force of the lungs. Without the necessary physiological pressures, the chest wall would expand outward, and the lungs would collapse. This is what occurs when the vacuum is disrupted such as with a stab wound leading to a pneumothorax. FRC is the point at which these forces are at equilibrium; that is, the difference between the inner recoil forces of the lungs and the outer recoil forces of the chest wall are balanced.
Why is air needed in our lungs other than the amount we normally breathe? The 2 components of FRC can explain why. As previously mentioned, FRC contains both the RV and the ERV.
First, consider ERV. While sitting quietly or performing normal activities, the only air a person with no respiratory pathology breathes is their TV. However, when a person exerts energy, more oxygen is needed and the volume of air ventilated increases in both directions; more air is inspired, and more air is expired. Think of someone running a race. Their energy expenditure increases and more oxygen is needed to continue at an acceptable pace. This is one reason both ERV and IRV are needed.
RV is the amount of air a person may never physiologically expire. Why is RV essential? Imagine blowing up a balloon. The most difficult part of inflating a balloon is the first expelling of air. If all of the air left our lungs every time extra lung volume was needed, the work of breathing would increase tremendously. Our lungs would collapse, and any strenuous activity would be not only difficult, but also dangerous.
FRC is dependent upon many factors. Some of these are uncontrollable including age, height, and biological gender. Other elements that are sometimes controllable or reversible include pathological respiratory diseases (often due to smoking), some occupations that lead to lung disease, pregnancy, diseases that cause ascites, and acute changes in position such as laying supine.
As humans age, our pulmonary function also declines; muscle mass, the elasticity of tissue, alveolar surface area, and diffusion capability all decrease. A person’s ability to respond to insults also lessens. The decrease in muscle mass includes the muscles we use to breathe. 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. The lungs do not as readily return to normal size after inspiration.
Height and Gender
If a person is taller, their diaphragm is likely larger, and they need more oxygen to perform the same tasks as do smaller individuals. This means that they need more lung volume to accomplish this and therefore have an increased FRC. While it is tempting to assume this is the reason for differences of lung volume between genders, research has shown that men and women who are the same height and age still differ in lung volumes. Men tend to have a significantly larger lung volume, and because of this an increased FRC. This is likely due to structural differences between men and women. Women have ribs that are angled differently than men and a shorter diaphragm length. Due to the difference in rib angle, women have a greater capacity to expand their lungs. While women have a smaller rib cage and a shorter diaphragm length, the angle of a woman’s ribs has shown to lend an increased advantage in inspiratory capacity. This is likely to aid physiological changes that occur during pregnancy.
Pregnant women need more oxygen to have the necessary energy to fertilize an egg and create a fetus. Physiological hormone changes throughout pregnancy allow a pregnant woman’s respiratory status to change significantly. While spirometry remains within normal limits, structural and volumes/capacities change significantly. The diaphragm relaxes (due to aforementioned 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. However, a pregnant female’s oxygen requirements are increased. TV and IRV increase, and a pregnant woman’s ribcage grows in size while the subcostal angle increases. The previously mentioned inspiratory advantage due to rib angle likely helps a pregnant female compensate. Because of the lowered FRC and pressure on the thorax, a pregnant woman is more susceptible to atelectasis. This likelihood is increased when laying supine. The result is the possibility of a slightly reduced PaO2 if a blood gas is obtained, especially when in this position. It is also important to note that while minute ventilation (MV) increases throughout pregnancy, this is mostly due to the increase in TV. Respiratory rate is largely unchanged.
Similar to pregnancy, FRC also changes with ascites, whether caused by liver disease, malignancy, or other pathology. With significant disease of the liver, portal hypertension causes fluid to leak out of the capillaries into the abdomen. Increased abdominal girth yields cephalad pressure on the diaphragm, decreasing FRC. This is one of the causes of shortness of breath in patients with ascites; unlike in pregnancy, TV and IRV are not increased.
Post-surgical patients also experience a drop in FRC. Medications given during anesthesia relax the muscles of respiration. When a person breathes, the diaphragm moves inferiorly to help create the negative pressure gradient that brings air into the lungs. When the accessory muscles of respiration are relaxed, and analgesic/anesthetic/paralytic medications have been consumed, the diaphragm experiences superior movement (cephalad) rather than inferior. This reduces FRC significantly, and some studies have suggested that the length of time that FRC is reduced is directly proportional to the likelihood of post-surgical pulmonary complications.
Lung volumes are followed to track a patient’s respiratory disease. While not routinely used in clinical practice, a way to measure residual volume and total lung capacity (TLC) is to measure a person’s FRC. FRC is approximately 2400 mL in an average sized person with no lung pathology.
While FRC can be estimated as about 35% of TLC, it is impossible to measure TLC, FRC, or RV using spirometry alone. 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). The whole body plethysmograph method is much more accurate under pathological conditions and involves placing a patient in a glass-walled box about the size of a telephone booth and measuring pressures using transducers.
Once FRC is measured, the clinician may calculate RV, and with that value may calculate the patient’s TLC. This is possible due to the following equations:
*TLC= total lung capacity, RV= residual volume, ERV= expiratory reserve volume, IRV= inspiratory reserve volume, FRC= functional residual capacity
The ERV can be measured. So, if a clinician measures the FRC, the ERV can also be obtained and subtracted from FRC to calculate the RV. The TLC can be obtained by adding all of the lung volumes together after RV has been calculated or by adding FRC, TV, and IRV (third equation).
If whole body plethysmograph is utilized, other informative values may be distinguished including the specific airway resistance (sRaw), airway resistance (Raw), and shift volume. All of these values together, when obtained, paint a detailed clinical picture of a patient’s lung function.
Years of studying patients’ various lung volumes led to the realization that by analyzing gathered data, equations could be used to predict the person’s likely lung volumes/capacities. Equations such as these are how clinicians can compare a patient’s lung function to what is considered normal for that patient.
Before discussing specific lung pathologies and their relationship to FRC, it is important to have a basic understanding of compliance and elasticity. Compliance and elasticity are defined mathematically as:
Compliance is the change in volume per change in unit pressure. Elasticity is the inverse of compliance: the change in unit pressure per change in volume. Simply stated, compliance is the ease of distension; elasticity is the ease at which a distended structure returns to the original volume. The more compliant an object, the less elastic, and vice versa.
In restrictive respiratory states, the lungs or chest wall are affected in a way that limits lung expansion. While some restrictive states can be due to chest wall issues (such as severe kyphosis), here restrictive pathology of the lung itself will be considered. An example of a restrictive disease of the lung is idiopathic pulmonary fibrosis. An increase in fibrous tissue in the lung causes a decrease in lung compliance; the lungs become more elastic and therefore harder to inflate. This decreases TLC, and therefore FRC, due to the lungs being harder to fill.
With obstructive respiratory pathology of the lungs such as emphysema, there is a reduction in airflow, and the lungs become increasingly compliant. Alveoli are destroyed, air is trapped, and TLC is increased. Consequently, FRC is also increased. The increased volume and compliance causes the chest wall to experience less resistance from the lungs, hence, the typical barrel chest seen in those with emphysema.
The previous paragraphs illustrate the importance of understanding basic principles and definitions in physiology to perceive the pathological conditions that plague patients correctly. Mastering this knowledge can also simplify the process of explaining a disease the patient. If a patient understands their pathology and why pharmacological interventions are important, one can infer that there will be an increase in patient compliance to treatments meant to lengthen life or improve symptoms.
While other lung values are more widely used clinically, functional residual capacity 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.
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