Lung Volumes and Capacities: Definition, Formula, Measurement

Lung Volumes and Lung capacities are two related terms in the respiratory system. The quantities of air the lungs can receive, hold or expel under different conditions are called Lung volumes. These are also called Pulmonary volumes. Combinations of two more Pulmonary Volumes are called Pulmonary Capacities or Lung Capacities.These volumes tend to vary, depending on the depth of respiration, ethnicity, gender, age, body composition, and certain respiratory diseases.

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Key Terms: Lungs, Respiratory System, Human Body, Breathing, Pulmonary Volumes, Spirometry

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Lung OR Pulmonary Volumes

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1. Tidal Volume (TV): It is the volume of air that is taken in or given out of the lung during quiet normal breathing. It is about 500 ml. This depicts the functions of the respiratory muscles and the movement of the lungs and chest center. It is also referred to as the total air that is exhaled or inhaled during a single respiratory cycle.
2. Inspiratory Reserve Volume (IRV): It is the volume of air that can be taken into the lungs by forced inspiration over and above the tidal volume. It is about 2000-3300 ml.
3. Expiratory Reserve Volume (ERV): It is the volume of air that can be breathed out of the lungs by forced expiration after normal expiration. It is about 1000 ml. It tends to decrease with obesity or after upper abdominal surgery.
4. Residual Volume (RV): It is the amount of air that remains inside the lungs after forced expiration. It is about 1200 ml. It is measured indirectly by subtracting ERV from FRC. There is no method to measure this value directly. Residual Volume = FRC – ERV.

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Lung or Pulmonary Capacities

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1. Inspiratory Capacity (IC): It is the maximum volume of air that can be inspired from the end-expiration position and thus it is the sum total of tidal volume and inspiratory reserve volume. It is nearly 3500 ml.
2. Vital Capacity (VC): It is the volume of air that can be breathed out of the lungs by forced expiration after a forced inspiration.

Vital capacity = Inspiratory capacity + Expiratory reserve volume.

It is about- (i) 4,800 ml in male and (ii) 3,100 ml in females.

It varies with age and body size. VC reflects on the ability to breathe deeply and cough, showing the strength of the Inspiratory and Expiratory muscles.

Vital Capacity is measured by means of an instrument called a Spirometer. It is used to detect respiratory diseases.

3. Functional Residual Capacity (FRC): It is the volume of the air that remains in the lungs after a normal expiration. It varies from 2500 ml to 3000 ml. It is the sum of Residual volume and Expiratory reserve volume.

FRC = RV + ERV (1200+1000) ml = 2200 ml.

4. Total Lung Volume (TLV) or Total Lung Capacity (TLC): It is the volume of air that the lungs can hold after maximal inspiration. It is the sum total of vital capacity and residual volume. TLV = VC+ RV

It varies from 5000-6000 ml in an adult male.

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How are Lung volumes measured?

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It is essential to calculate the Lung or pulmonary volumes to aid in carrying out the correct physiological diagnosis. A lot of underlying disorders can be understood by knowing an individual’s Lung volumes. Measurement of Lung volumes is done by using various methods, the most common being spirometry. Other methods include- Body plethysmography, Nitrogen washout, and Helium dilution.

Spirometry measures the airflow in and out of the lungs and also estimates the lung’s size. It employs the use of an instrument called a Spirometer. With this technique, most of the lung volumes are measured- Tidal volume, Inspiratory Reserve Volume, and Expiratory Reserve Volume. However, the measurement of Residual Volume, Functional Residual Capacity, and Total Lung Capacity is done by body plethysmography, Nitrogen washout, and Helium dilution technique.

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Things to remember

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• Lung volumes are the parameters that tell us about the air that our lungs can receive, hold or expel under different conditions.
• Lung capacities are the parameters derived from the Lung volumes.
• Lung capacities help in assessing the normal functioning of the lungs and give an insight into the presence of any ailment or disorder.
• By using spirometry we can measure Tidal volume, Inspiratory Reserve Volume and Expiratory Reserve Volume.
• Residual volume cannot be measured directly, but instead it is measured indirectly by subtracting ERV from FRC.
• Residual Volume, Functional Residual Capacity, and Total Lung Capacity can be measured by using body plethysmography, Nitrogen washout, and Helium dilution technique.
• Total Lung Capacity is the volume of air that the lungs can hold after maximal inspiration.

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