Respiratory Volumes, Capacities and Spirometry

Why breathing volumes differ

A normal breath uses only part of the air a person can inhale or exhale. After an ordinary inspiration, most people can inhale additional air if they breathe in as deeply as possible. After an ordinary expiration, they can exhale additional air with effort. Even after forced expiration, some air remains inside the lungs. Respiratory physiology therefore distinguishes simple volumes that refer to individual portions of lung air from capacities that combine two or more portions. These measurements help describe how the lungs and chest-wall mechanics work together.

Four fundamental respiratory volumes

Tidal volume (TV) is the air moved in a normal quiet inspiration or expiration, conventionally about 500 mL in the textbook adult example. Inspiratory reserve volume (IRV) is the extra air that can be inhaled after a normal inspiration through maximum inspiration, about 2500–3000 mL in the source. Expiratory reserve volume (ERV) is the extra air expelled by forced expiration following a normal expiration, about 1000–1100 mL. Residual volume (RV) is the air remaining after the strongest expiration, approximately 1100–1200 mL in the source.

These numbers are illustrative adult ranges, not fixed values for every age, body size or health condition. The named volumes are defined by manoeuvres, not by whether a spirometer display happens to use a certain colour. Residual volume is especially important because lungs do not empty completely after forceful exhalation. If they did, small air spaces would be much harder to keep open and gas exchange between breaths would not work in the same way.

Capacities are sums of individual volumes

Inspiratory capacity (IC) is the maximum amount a person can inhale after a normal expiration, so IC = TV + IRV. Expiratory capacity (EC) is the amount one can exhale after a normal inspiration, EC = TV + ERV. Functional residual capacity (FRC) is the air left inside the lungs after a normal quiet expiration, FRC = ERV + RV. Vital capacity (VC) is the greatest air volume moved between maximal inspiration and maximal expiration, VC = IRV + TV + ERV. Total lung capacity (TLC) is the air contained after a maximal inspiration, TLC = VC + RV.

Consider representative values: TV = 500 mL, IRV = 2800 mL, ERV = 1000 mL and RV = 1200 mL. Then IC = 3300 mL, EC = 1500 mL, FRC = 2200 mL, VC = 4300 mL and TLC = 5500 mL. Check each total by confirming what remains in the lungs: FRC includes residual air plus additional air that could still be forcibly expired, while VC excludes residual air because that air cannot be voluntarily expelled.

How a spirometer measures airflow and volume

A spirometer records breathed air volumes over time and can be used to estimate tidal volume and certain reserves and capacities. Ordinary spirometry cannot directly measure residual volume because this air cannot be expelled into the instrument. Consequently FRC and TLC also cannot be obtained by simple direct spirometric measurement alone; methods such as gas dilution or body plethysmography are needed to determine lung volumes that include RV. This limitation provides a practical interpretation of what can and cannot be inferred from a breathing trace.

A typical spirometric tracing shows small regular oscillations during resting breaths, a larger upward displacement during forced inspiration and a larger downward displacement during forced expiration. Volumes vary with patient effort, body position, age and physiological condition. The chapter's aim is to understand volume definitions and the relationships between components, not to diagnose a particular pulmonary disorder from one simplified graph.

Minute ventilation is not equal to every respiratory capacity

Tidal volume can be multiplied by breathing frequency to calculate total minute ventilation. At 12 breaths per minute and 500 mL per breath, the result is 6000 mL/min, or 6 L/min. Over one hour at the same constant rate, 6 × 60 = 360 L of total air is moved. This does not mean a person has a 360 L lung capacity: lung capacity describes the volume of air present or mobilisable at a particular stage, whereas minute ventilation sums repeated breaths across time.

Fresh air reaching the alveoli per minute is smaller than total minute ventilation because some inhaled air fills conducting airways, often termed anatomical dead space. The exact amount varies; a commonly used adult illustration is around 150 mL per breath, but that value should not replace the source's central 500 mL tidal-volume example. The distinction matters when someone breathes very rapidly and shallowly: moving the same total volume of air does not necessarily supply identical alveolar ventilation.

A method for capacity calculations

First write the four primary volumes and identify whether the question asks about maximum movable air, maximum contained air or air remaining after a normal expiration. Next substitute the appropriate sum. Avoid adding RV into vital capacity; it belongs in TLC and FRC. Check units before arithmetic: some questions give volumes in millilitres while others request litres. Convert 1000 mL = 1 L only after calculating a consistent total.

Quick Quiz

Model Answers

1. TV ~500 mL in quiet breathing; IRV ~2500–3000 mL additional forced inspiration; ERV ~1000–1100 mL additional forced expiration; RV ~1100–1200 mL remaining after maximal expiration.

2. IC 3300 mL; EC 1500 mL; FRC 2200 mL; VC 4300 mL; TLC 5500 mL. All follow from the standard sums of primary volumes.

3. VC excludes RV because it is the maximum air voluntarily moved; TLC adds the remaining RV: TLC = VC + RV.

4. Air that remains after maximal expiration cannot be blown into the device, so RV and capacities containing it require additional measurement techniques.

5. 12 × 500 = 6000 mL/min = 6 L/min. In 60 minutes this totals 360 L of ventilated air, assuming a constant rate and tidal volume.

6. Minute ventilation sums repeated breaths per minute (TV × rate); lung capacity refers to a combined volume in the lungs, such as VC or TLC, at a particular breathing state.