Lung Volumes - MCAT Biology | Premier MCAT Prep

Lung Volumes

8 min read Updated Mar 26, 2026

Imagine your lungs as a water bottle. During quiet breathing, you only sip from the top (tidal volume). But you could chug much more if you tried (inspiratory reserve). After a normal sip, you could force out more water by squeezing (expiratory reserve). And no matter how hard you squeeze, some water always stays at the bottom, trapped in the crevices (residual volume). Understanding these volumes - and how they combine into capacities - is essential for MCAT questions about pulmonary function.

Spirometry tracing showing tidal volume, inspiratory reserve volume, expiratory reserve volume, residual volume, vital capacity, total lung capacity, and functional residual capacity — A spirometry tracing showing the standard lung volumes and capacities. Tidal volume (TV) is the normal breathing range; inspiratory and expiratory reserves extend above and below it. Residual volume remains after maximal exhalation. *Credit: OpenStax Anatomy & Physiology 2e, CC BY 4.0*

The Four Lung Volumes

There are four non-overlapping lung volumes that cannot be subdivided further:

Volume	Abbreviation	Definition	Typical Value
Tidal Volume	TV	Air moved in or out during a normal, quiet breath	~500 mL
Inspiratory Reserve Volume	IRV	Extra air you can inhale beyond a normal tidal breath (with maximal effort)	~3,000 mL
Expiratory Reserve Volume	ERV	Extra air you can exhale beyond a normal tidal exhalation (with effort)	~1,200 mL
Residual Volume	RV	Air that remains in the lungs even after maximal forced exhalation	~1,200 mL

The residual volume exists because the airways and alveoli do not completely collapse during exhalation. RV prevents alveolar collapse and maintains gas exchange even between breaths.

Lung Capacities

Lung capacities are combinations of two or more volumes. There are four:

Capacity	Formula	Definition	Typical Value
Inspiratory Capacity (IC)	TV + IRV	Maximum air you can inhale from the end of a normal exhalation	~3,500 mL
Functional Residual Capacity (FRC)	ERV + RV	Air remaining in the lungs after a normal exhalation	~2,400 mL
Vital Capacity (VC)	TV + IRV + ERV	Maximum air you can move in one breath (deepest inhale to hardest exhale)	~4,700 mL
Total Lung Capacity (TLC)	TV + IRV + ERV + RV	Total air the lungs can hold at maximum inflation	~5,900 mL

Spirometry

Spirometry is the clinical test used to measure lung volumes and capacities. The patient breathes into a device that records volume changes over time, producing a spirogram.

Important limitation: spirometry cannot measure residual volume directly. Since RV is the air you cannot blow out, it stays in the lungs during the test. This means spirometry also cannot directly measure FRC or TLC (both include RV).

To measure RV, clinicians use indirect methods such as gas dilution techniques. The key point for the MCAT: spirometry alone cannot give you RV.

Minute Ventilation and Alveolar Ventilation

Minute ventilation is the total volume of air moved per minute:

But minute ventilation overestimates how much air is actually useful, because it includes dead space air. Alveolar ventilation corrects for this:

Why Breathing Pattern Matters

Consider two patients who both have a minute ventilation of 6 L/min:

Patient	TV	RR	Minute Ventilation	Alveolar Ventilation
A	500 mL	12/min	6,000 mL/min	(500-150) x 12 = 4,200 mL/min
B	200 mL	30/min	6,000 mL/min	(200-150) x 30 = 1,500 mL/min

Patient B has the same minute ventilation but far less alveolar ventilation. Shallow, rapid breathing is inefficient because a larger proportion of each breath is wasted filling dead space. This is why deep, slow breaths are more effective than shallow, fast ones.

Obstructive vs. Restrictive Lung Disease

Pulmonary function tests are used to distinguish two broad categories of lung disease:

Feature	Obstructive Disease	Restrictive Disease
Problem	Airflow out is blocked	Lungs cannot fully expand
Examples	Asthma, COPD (emphysema, chronic bronchitis)	Pulmonary fibrosis, scoliosis, obesity
FEV1 (air exhaled in 1 sec)	Decreased	Decreased
FVC (total air exhaled forcefully)	Normal or slightly decreased	Decreased
FEV1/FVC ratio	Decreased (< 0.7)	Normal or increased (> 0.7)
TLC	Normal or increased (air trapping)	Decreased
RV	Increased (can’t blow it all out)	Decreased
Compliance	Increased (emphysema)	Decreased (stiff lungs)

Simple labeled diagram of lung anatomy showing trachea, bronchi, bronchioles, and lung lobes — Lung anatomy overview. **Focus on:** the branching airway from trachea to bronchi to bronchioles, and how obstructive diseases (emphysema, asthma) vs. restrictive diseases affect airflow and volumes differently. *Credit: Patrick J. Lynch, CC BY 2.5*

Which lung volumes and capacities cannot be measured by spirometry alone?

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Residual volume (RV), functional residual capacity (FRC), and total lung capacity (TLC). Spirometry measures air flow in and out, but RV is the air that remains after maximal exhalation - it never leaves the lungs. Since FRC = ERV + RV and TLC = VC + RV, both include RV and cannot be determined by spirometry alone.

A patient has a FEV1/FVC ratio of 0.55. Is this obstructive or restrictive disease, and why?

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Obstructive disease. A normal FEV1/FVC ratio is about 0.80 (80% of air is exhaled in the first second). A ratio of 0.55 means the patient can only blow out 55% in the first second - exhalation is prolonged due to airway obstruction. In restrictive disease, both FEV1 and FVC decrease proportionally, so the ratio stays normal or even increases.