Lung Volumes and Capacities

Writer: Mustafa A. Shahrori, MD.

Keywords: Pulmonary function, spirometry, lung volumes, lung capacities, respiratory physiology, tidal volume, inspiratory reserve volume, expiratory reserve volume, residual volume, functional residual capacity, vital capacity, total lung capacity, pulmonary function testing, respiratory assessment

Overview

Lung volumes and capacities are fundamental respiratory measurements essential for diagnosing and monitoring pulmonary function. The four primary lung volumes (tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume) represent discrete air quantities during different breathing phases. The four capacities (inspiratory capacity, functional residual capacity, vital capacity, and total lung capacity) combine these volumes. Measurement techniques, normal values, clinical patterns in obstructive and restrictive diseases, and physiological factors affecting these parameters provide crucial insights for respiratory assessment and diagnosis.

Definition and Classification

Pulmonary function tests (Figure 1) (Figure 2) (Figure 3) measure two primary categories of respiratory parameters:

1.     Lung volumes - discrete, non-overlapping quantities of air in the lungs during different phases of the respiratory cycle

2.     Lung capacities - combinations of two or more lung volumes

Figure 1. Pulmonary function test (PFT) procedure using spirometry 

Figure 2. Lung volumes and capacities

Kapwatt. (2014, May 31). Added the “Inspiration Capacity” (IC) metric, which is the sum of Inspiratory Reserve Volume and Tidal Volume [Diagram]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Lungvolumes_updated.svg

Figure 3. Lung volumes and capacities

The Four Lung Volumes

Tidal volume (VT)

• The volume of air moved into or out of the lungs during normal quiet breathing

• Typical value: 6-8 mL/kg of ideal body weight (approximately 500 mL in an average adult)

• Measured during regular breathing without additional effort

NOTE: Tidal volume increases during exercise and can decrease during shallow breathing patterns such as those seen in restrictive lung diseases.

Inspiratory reserve volume (IRV)

• The additional volume of air that can be inhaled with maximum effort beyond normal tidal inspiration

• Typical value: 2.5-3.0 L in adults

• Represents the reserve capacity for increased ventilation during exercise or respiratory distress

Expiratory reserve volume (ERV)

• The additional volume of air that can be forcefully exhaled after a normal tidal expiration

• Typical value: 1.0-1.5 L in adults

• Decreased in obesity, pregnancy, and conditions causing abdominal distension

Residual volume (RV)

• The volume of air remaining in the lungs after maximal expiration

• Typical value: 1.0-1.5 L in adults

• Cannot be voluntarily exhaled and requires indirect measurement techniques

• Prevents complete lung collapse and maintains gas exchange between breaths

ALERT: Residual volume and any lung capacity that includes RV cannot be measured directly by simple spirometry. They require indirect measurement techniques such as nitrogen washout, helium dilution, or body plethysmography.

The Four Lung Capacities

Inspiratory capacity (IC)

• Definition: IRV + VT

• The maximum volume of air that can be inhaled after a normal quiet expiration

• Typical value: 3.0-3.5 L in adults

• Clinical significance: Decreased in restrictive lung diseases

Functional residual capacity (FRC)

• Definition: ERV + RV

• The volume of air present in the lungs at the end of normal quiet expiration

• Typical value: 2.0-2.5 L in adults

• Represents the point where the outward elastic recoil of the chest wall is balanced by the inward elastic recoil of the lungs

Vital capacity (VC)

• Definition: IRV + VT + ERV

• The maximum volume of air that can be exhaled after a maximum inspiration

• Typical value: 4.0-5.0 L in adults, but varies with age, sex, height, and ethnicity

• Often measured as forced vital capacity (FVC) during spirometry

Total lung capacity (TLC)

• Definition: IRV + VT + ERV + RV, or simply VC + RV

• The total volume of air in the lungs after maximal inspiration

• Typical value: 5.5-6.0 L in adults

• Clinical significance: Decreased in restrictive diseases, increased in obstructive diseases

Measurement Techniques

Spirometry

• Measures directly: VT, IRV, ERV, VC, and IC

• Cannot measure: RV, FRC, and TLC (since these include RV)

• Most commonly used pulmonary function test

• Results are typically expressed as absolute values and as percentages of predicted values based on age, sex, height, and ethnicity

NOTE: Spirometry (Figure 4) is the primary tool for diagnosing obstructive and restrictive lung diseases, monitoring disease progression, and evaluating response to therapy.

Figure 4. Spirometry

Jmarchn. (2013, June 10). Doing a spirometry [Photograph]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:DoingSpirometry.JPG

For a more detailed discussion, refer to the Spirometry article.

Gas dilution techniques

• Used to measure FRC, which then allows calculation of RV and TLC

• Methods include:

  • Nitrogen washout technique

  • Helium dilution method

• Based on the principle of dilution of a known concentration of inert gas in a closed system

Body plethysmography

• Uses Boyle's law (pressure × volume = constant) to determine all lung volumes and capacities

• Most accurate method, especially in patients with air trapping or bullous disease

• Measures the total volume of gas in the thoracic cavity, including trapped gas

Clinical Significance and Patterns of Abnormality

Obstructive lung diseases

• Examples: Asthma, COPD, emphysema, chronic bronchitis

• Pattern of abnormalities:

  • Increased RV and FRC due to air trapping

  • Normal or increased TLC

  • Decreased VC due to increased RV

  • Decreased forced expiratory flow rates

ALERT: In obstructive lung diseases, RV can increase by more than 300% of predicted values, leading to hyperinflation and increased work of breathing.

Restrictive lung diseases

• Examples: Pulmonary fibrosis, pneumonia, acute respiratory distress syndrome, neuromuscular disorders

• Pattern of abnormalities:

  • Decreased TLC

  • Proportionally decreased all lung volumes

  • Normal or increased FEV1/FVC ratio

Mixed pattern

• Features of both obstructive and restrictive patterns

• Examples: Bronchiectasis with fibrosis, cystic fibrosis

Factors Affecting Lung Volumes and Capacities

Physiological factors

1.     Age

o    TLC remains relatively stable until older age

o    RV increases with age as elastic recoil decreases

o    VC decreases with age

2.     Sex

o    Males typically have larger lung volumes than females of the same age and height

o    Differences become apparent after puberty

3.     Body size and position

o    Taller individuals have larger lung volumes

o    FRC decreases when changing from standing to supine position

o    Obesity decreases ERV and FRC due to diaphragmatic displacement

NOTE: Lung volumes should always be interpreted with reference to predicted values that account for the individual's age, sex, height, and ethnicity.

Pathological conditions

1.     Respiratory muscle weakness

o    Decreased TLC, VC, and IC

o    Normal or increased RV

2.     Pregnancy

o    Progressive decrease in FRC as pregnancy advances

o    Relatively preserved VC and TLC

o    Changes due to elevation of diaphragm by enlarging uterus

3.     Altitude

o    Increased ventilation at high altitude

o    Increased TLC and VC with long-term adaptation

References

Guyton, A. C., & Hall, J. E. (2021). Guyton and Hall textbook of medical physiology (14th ed.). Elsevier.