Functional Residual Capacity Calculator

Functional Residual Capacity equals Expiratory Reserve Volume plus Residual Volume

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What Is Functional Residual Capacity?

Functional Residual Capacity (FRC) is the volume of air remaining in the lungs after a normal, passive exhalation. It serves as a reservoir that stabilizes oxygen and carbon dioxide levels in the blood between breaths. FRC consists of two components: the expiratory reserve volume (ERV, the additional air that can be forcefully exhaled) and the residual volume (RV, the air that remains trapped in the lungs and cannot be voluntarily expelled).

Worked Example

A pulmonologist measures a patient's expiratory reserve volume at 1.1 L and residual volume at 1.2 L during body plethysmography. Using the formula FRC = ERV + RV, the functional residual capacity is 1.1 + 1.2 = 2.3 L. This value falls within the normal adult range of approximately 2.2 to 2.4 L, suggesting normal resting lung volume.

Normal Values

A normal FRC in adults is approximately 2.2 to 2.4 liters. ERV typically ranges from 1.0 to 1.2 L and RV from 1.1 to 1.2 L. FRC increases with age and height and is generally higher in males. Conditions like COPD and emphysema increase FRC due to air trapping, while restrictive diseases such as pulmonary fibrosis decrease it.

Frequently Asked Questions

How is FRC measured clinically?

FRC cannot be measured with standard spirometry because it includes the residual volume. Instead, body plethysmography, helium dilution, or nitrogen washout techniques are used. Body plethysmography tends to give higher values in patients with air trapping.

Why is FRC clinically important?

FRC helps maintain gas exchange between breaths and prevents alveolar collapse. A reduced FRC (as seen in obesity, pregnancy, or general anesthesia) increases the risk of atelectasis and hypoxemia. An elevated FRC (as in emphysema) indicates hyperinflation and reduced elastic recoil.

How does body position affect FRC?

FRC decreases when moving from an upright to a supine position because abdominal contents push the diaphragm cephalad, reducing thoracic volume. This is clinically important during general anesthesia and mechanical ventilation, where supine positioning further lowers FRC and increases the risk of atelectasis.

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