Introduction
Lung volume refers to the volume of air in the lungs at a given duration during the respiratory cycle. The summation of different lung volumes provides the lung capacity. The lung volumes vary depending on age, gender, body composition, depth of respiration, and certain respiratory diseases. Functional residual capacity is the remaining air in the lungs after passive exhalation.
What Is Functional Residual Capacity?
Functional residual capacity (FRC) is the remaining amount of air present in the lungs at the end of passive exhalation. It is the total amount of air at the lowest point of tidal volume in the lungs. The tidal volume is the volume of air that is inspired and expired. The functional residual capacity is about three liters in a normal individual. It also represents the stage of the respiratory cycle where the lung tissue elastic recoil and chest wall expansion are balanced and equal.
The functional residual capacity is the sum of two or more lung volumes. It has to be calculated and cannot be measured using spirometry. It is the combination of the expiratory reserve volume (ERV) and the residual volume (RV). The residual volume is the volume of air present in the lungs after expelling as much air from the lungs as possible. It cannot be measured using spirometry because it cannot be exhaled. The expiratory reserve volume is the amount of air that can be forcefully exhaled after passive exhalation. Hence, the functional residual capacity of the lungs is represented as;
FRC = RV+ERV
What Is the Significance of Functional Residual Capacity?
The functional residual capacity is significant because it is related to factors like airway and vascular resistance, oxygen reserve, work of breathing, compliance, closing capacity, and V/Q mismatch (ventilation/perfusion ratio).
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A decrease in lung volume decreases the functional residual capacity. Low lung volumes reduce the alveolar tension and open the airways, and airway narrowing causes increased airway resistance.
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Pulmonary vascular resistance is the result of combined alveolar and extra-alveolar vessel resistance. The resistance is more at total lung capacity and residual volume and lowest at functional residual capacity volume.
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The inward and outward lung compliances are balanced at functional residual capacity. Hence, the load to inflate the lungs is less.
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Lung compliance is dependent on the elastic recoil of the lung tissues. A decrease in lung compliance increases the functional residual capacity.
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The residual air volume allows oxygen exchange, and the functional residual capacity results in an oxygen reserve. The functional residual capacity and oxygen reserve are essential during the induction of anesthesia.
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A reduced functional residual capacity can cause shunts and atelectasis. It is due to the decrease in the functional residual capacity below the closing capacity of the lung. The total closing capacity of the lung is the volume at which the respiratory bronchioles collapse.
The conditions that affect lung compliance, like inward elastic recoil of the lung and outward expansion of the chest wall, also affect the functional residual capacity. It also includes diseases or conditions like changes in lung compliance (emphysema and interstitial lung diseases), decreased chest movements (kyphoscoliosis), or decreased thoracic volume due to obesity or pregnancy. Other factors that affect the functional residual capacity are age, height, gender, and lying in the supine position.
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Position - Functional residual capacity changes with the patient’s position. It is increased in an upright position and decreases in the supine or prone position.
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Age - Lung function declines with age due to a decrease in respiratory muscle mass and tissue elasticity. Loss of elasticity makes it harder to expel the air, and the lungs do not return to normal size after inspiration, thereby increasing the work of breathing. This increases the functional residual capacity.
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Height and Gender - A taller person has a larger lung volume and a greater functional residual capacity. Men have a larger lung volume compared to women. This is due to the difference in rib cage structure between men and women.
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Pregnancy - The growing fetus exerts pressure on the thoracic cavity, and this reduces the residual volume and expiratory reserve volume, resulting in a decrease in functional residual capacity.
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Ascites and Obesity - Ascites and obesity increase the pressure on the diaphragm and reduce the thoracic volume, thereby decreasing the functional residual capacity. This results in shortness of breath.
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Anesthesia - Anesthesia affects the relaxation of the respiratory muscles and alters the functional residual capacity.
What Are the Diseases That Affect the Functional Residual Capacity?
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Restrictive lung diseases like kyphosis and idiopathic pulmonary fibrosis decrease the total lung capacity, resulting in a decrease in functional residual capacity.
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Obstructive diseases like emphysema increase lung compliance due to alveolar destruction. The alveolar destruction causes air trapping and increases total lung capacity, thereby increasing the functional residual capacity.
Why Is Functional Residual Capacity Significant?
Functional residual capacity is significant because, for every respiratory cycle, it keeps the small airways open and prevents the complete emptying of the lungs. If the functional residual capacity is not present, the alveolar PO₂ (partial pressure of oxygen) and PCO₂ (partial pressure of carbon dioxide) will widely vary during breathing and will interfere with the diffusion of respiratory gases. If the functional capacity reduces, it results in shunt and atelectasis. If the functional residual capacity is increased, the equilibrium between elastic recoil and the outward expansion of the chest wall is disturbed because functional residual capacity is the point at which the inner recoil forces of the lung and the chest wall are in equilibrium.
What Are the Tests Performed to Calculate Functional Residual Capacity?
Functional residual capacity can be calculated using a whole-body plethysmograph technique and the helium dilution method. It is calculated as the summation of residual volume and expiratory reserve volume.
Conclusion
Functional residual capacity is the volume of air present in the lungs after a passive exhalation. It is measured to evaluate lung function. Functional residual capacity is the indicator of the elasticity of the lungs. Functional residual capacity is essential to prevent the diffusion of respiratory gases. A lowered or elevated functional residual capacity is an indicator of respiratory disease.
