All of the following statements regarding static lung volumes are correct EXCEPT:

Question

Accepted Answer

C. Total lung capacity (TLC) in an adult male is approximately 6 liters and can be directly measured by spirometry alone. ## Clarification of Static Lung Volumes

**Key Point:** Static lung volumes are subdivisions of the total air in the lungs. Spirometry can measure only those volumes that include the inspiratory reserve volume (IRV), tidal volume (TV), and expiratory reserve volume (ERV). Any volume that includes residual volume (RV) cannot be measured by spirometry alone.

### Measurable vs. Non-Measurable Volumes

| Volume | Definition | Measurable by Spirometry? |
| --- | --- | --- |
| Tidal Volume (TV) | Air breathed in and out at rest (~500 mL) | Yes |
| Inspiratory Reserve Volume (IRV) | Maximum air that can be inhaled after normal inspiration (~3100 mL) | Yes |
| Expiratory Reserve Volume (ERV) | Maximum air that can be exhaled after normal expiration (~1200 mL) | Yes |
| Residual Volume (RV) | Air remaining in lungs after maximal expiration (~1200 mL) | **No** — requires gas dilution or plethysmography |
| Vital Capacity (VC) | IRV + TV + ERV (can be measured directly) | Yes |
| Functional Residual Capacity (FRC) | ERV + RV (cannot be measured directly) | **No** — requires gas dilution or plethysmography |
| Total Lung Capacity (TLC) | VC + RV = IRV + TV + ERV + RV | **No** — requires gas dilution or plethysmography |

**High-Yield:** TLC cannot be measured by spirometry alone because it includes RV. TLC must be calculated using gas dilution (helium dilution, nitrogen washout) or body plethysmography.

### Why the Correct Answer Is Wrong

Option 3 states that TLC "can be directly measured by spirometry alone." This is **FALSE**. Spirometry can only measure VC (which is IRV + TV + ERV). To obtain TLC, you must measure RV separately using gas dilution or plethysmography, then add it to VC. Therefore, TLC cannot be directly measured by spirometry — it must be calculated.

**Clinical Pearl:** In clinical practice, when a patient's TLC is reported, it has been calculated from spirometry (VC) plus a separately measured RV using gas dilution or plethysmography.

[cite:Guyton and Hall Textbook of Medical Physiology Ch 37]

Answer

A. Vital capacity (VC) is the maximum volume of air that can be exhaled after maximum inspiration and is reduced in restrictive lung disease

Answer

B. Functional residual capacity (FRC) is the volume of air remaining in the lungs after normal expiration and equals RV plus expiratory reserve volume (ERV)

Answer

D. Residual volume (RV) cannot be measured by simple spirometry and requires gas dilution or body plethysmography

All of the following statements regarding static lung volumes are correct EXCEPT:

Explanation

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