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Physiology, Functional Residual Capacity.


Hopkins E1, Sharma S2.


StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019-.
2019 Mar 14.

Author information

Lincoln Memorial University- Debusk COM
Mery Fitzgerald Hospital


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.[1][2][3]

Copyright © 2019, StatPearls Publishing LLC.

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