Functional residual capacity (FRC)

Functional residual capacity and Lung Volume Capacity

Question One

Residual volume is the amount of air that remains in the lungs after a person engages in maximum forceful respiration. According to Lofrese & Lappin (2020), the quantity of air remains the same despite the time when the expiration process started and the size of the lungs. Functional residual capacity (FRC) is the amount of air that remains in the lungs after a normal mechanism of expiration. The expiration process is facilitated by opposing forces that emerge from the expansion of the chest wall and the elastic recoil of the lungs (Hopkins & Sharma, 2020). Total lung capacity (TLC) is the size of the lungs at maximum inspiration (Lofrese & Lappin, 2020). Total lung capacity (TLC) gives the total amount of gas inflated in the airways and pulmonary parenchyma in the thorax.

Functional residual capacity (FRC) is measured using Body plethysmography, inert gas dilution, and nitrogen washout.  Body plethysmography applies functional residual capacity and precise airway resistance as the key components. According to Criée et al. (2011), the measurement of body plethysmography gives a reflective of the numerous functions and structures of the lungs. The analysis adopts the volume in the box, the pressure in the box, and the pressure in the chest as the essential variables that assess the amount of air that remains in the lungs after a normal mechanism of expiration.

Inert gas dilution underestimates the exact value of Functional residual capacity because the technique evaluates specific sections that link the mouth and the lungs. The technique does not examine the areas with trapped gases. The nitrogen washout method examines the concentration of nitrogen in the exhaled gas with the key variable of 100% FiO2. Nitrogen is measured as exhaled volumes.

Question Two

The most appropriate technique of measuring Functional residual capacity (FRC) is using body plethysmography. The method assesses the amount of air in the lungs after taking a deep breath and the quantity of air left in the lungs after the exhalation process. The technique uses airway resistance and intrathoracic gas volume to give an accurate outcome of the assessment. Besides, the method gives all sections of the respiratory system equal treatment and consequently, facilitates the achievement of quality and effective results. Through an examination of the lungs, body plethysmography begins with breathing at a resting position and follows with a shutter maneuver, which is characterized by spirometric measurements. In such a case, there is an adequate measurement of the mechanism of the lungs during forced and normal breathing processes.

Question Three

The first image depicts an extrathoracic obstruction, which occurs because of the paralysis of the unilateral vocal cord resulting in a decreased inspiratory flow. In the case of forced expiration, the expiration flow becomes unpaired and consequently, blocks the airways. The second image shows an obstructive disorder, which results from the dominance of expiratory flow. The high level of the expiratory flow tends to block the airways. The third image is a restrictive ventilator defect.  The restrictive disorder results from diminishing lung volumes whereby the airflow is greater compared to normal lung capacity. The fourth image shows the intrathoracic obstruction. The disorder occurs when negative pressure that holds the trachea open. In the event of forced expiration, the trachea narrows down due to a lack of support and the diminishing flow of the plateau.


Criée, C. P., Sorichter, S., Smith, H. J., Kardos, P., Magnussen, H., Worth, H., et al. (2011). Body plethysmography – Its principles and clinical use. Respiratory Medicine , 105 (7), 959-971.

Hopkins, E., & Sharma, S. (2020). Physiology, functional residual capacity. StatPearls Publishing.

Lofrese, J. J., & Lappin, S. L. (2020). Physiology, residual volume. StatPearls Publishing.