Respiratory Volumes and Capacities

Spirometry measures various lung volumes and capacities to assess respiratory function.

Lung Volumes (Four Primary Volumes)

1. Tidal Volume (TV)

• Volume of air inhaled or exhaled during normal, quiet breathing
• Approximately 500 mL

2. Inspiratory Reserve Volume (IRV)

• Additional volume of air that can be inhaled after a normal tidal inspiration
• Approximately 3,100 mL in males, 1,900 mL in females

3. Expiratory Reserve Volume (ERV)

• Additional volume of air that can be exhaled after a normal tidal expiration
• Approximately 1,200 mL in males, 700 mL in females

4. Residual Volume (RV)

• Volume of air remaining in the lungs after maximal expiration
• Approximately 1,200 mL
• Cannot be measured by spirometry (requires specialized techniques)
• Prevents alveolar collapse

Lung Capacities (Combinations of Two or More Volumes)

1. Inspiratory Capacity (IC)

• Maximum volume of air that can be inhaled after normal expiration
• IC = TV + IRV
• Approximately 3,600 mL

2. Functional Residual Capacity (FRC)

• Volume of air remaining in lungs after normal expiration
• FRC = ERV + RV
• Approximately 2,400 mL
• Represents the resting volume of the lungs

3. Vital Capacity (VC)

• Maximum volume of air that can be exhaled after maximal inspiration
• VC = TV + IRV + ERV
• Approximately 4,800 mL in males, 3,100 mL in females
• Decreases with age, disease

4. Total Lung Capacity (TLC)

• Maximum volume of air the lungs can hold
• TLC = TV + IRV + ERV + RV or TLC = VC + RV
• Approximately 6,000 mL in males, 4,200 mL in females

Volume/Capacity	Definition	Approximate Value	Formula
Tidal Volume (TV)	Normal breathing	500 mL	-
Inspiratory Reserve Volume (IRV)	Extra inhalation	3100 mL	-
Expiratory Reserve Volume (ERV)	Extra exhalation	1200 mL	-
Residual Volume (RV)	Air remaining	1200 mL	-
Inspiratory Capacity (IC)	Maximum inhalation from rest	3600 mL	TV + IRV
Functional Residual Capacity (FRC)	Air after normal expiration	2400 mL	ERV + RV
Vital Capacity (VC)	Maximum breath cycle	4800 mL	TV + IRV + ERV
Total Lung Capacity (TLC)	Maximum lung volume	6000 mL	VC + RV

Other Important Measurements

Minute Ventilation (Respiratory Minute Volume)

• Total volume of air moved into and out of the lungs per minute
• Minute Ventilation = Tidal Volume × Respiratory Rate
• At rest: 500 mL × 12 breaths/min = 6,000 mL/min (6 L/min)
• Can increase to 100-150 L/min during maximal exercise

Forced Expiratory Volume in 1 Second (FEV₁)

• Volume of air that can be forcefully exhaled in the first second after maximal inspiration
• Normally approximately 80% of FVC
• Reduced in obstructive diseases (asthma, COPD)

Forced Vital Capacity (FVC)

• Maximum volume of air that can be forcefully exhaled after maximal inspiration
• Similar to vital capacity but measured during forced expiration

FEV₁/FVC Ratio

• Percentage of vital capacity exhaled in the first second
• Normal: >70-80%
• Obstructive diseases (asthma, COPD): Ratio decreased (<70%)
• Restrictive diseases (pulmonary fibrosis): Ratio normal or increased, but both FEV₁ and FVC are reduced

Key Takeaways for Pharmacy Students

1. Four volumes (TV, IRV, ERV, RV) are the building blocks
2. Four capacities (IC, FRC, VC, TLC) are combinations of volumes
3. Residual volume cannot be measured by simple spirometry
4. Vital capacity is the most clinically useful measurement
5. Restrictive diseases decrease all volumes; obstructive diseases increase RV and FRC
6. Values vary with age, sex, height, fitness, and body position