Lung Volumes and Spirometer

Measuring how much air moves in and out of the lungs gives insight into the health of the respiratory system and allows scientists to quantify ventilation. The instrument traditionally used to make these measurements is the spirometer. This lesson introduces the standard lung volumes — tidal volume, vital capacity, residual volume, and inspiratory/expiratory reserve volumes — and explains how to interpret a spirometer trace. It also explores the derived quantities of ventilation rate (pulmonary ventilation) and FEV₁, and the precautions needed for safe use. This content covers OCR A-Level Biology A specification 3.1.1 (h).

Key Definitions:

• Tidal volume (TV) — the volume of air moved into or out of the lungs in a single normal breath at rest, typically about 500 cm³ (0.5 dm³).
• Vital capacity (VC) — the maximum volume of air that can be forcibly expelled after the deepest possible inhalation; typically 4.5–5 dm³ in an adult.
• Residual volume (RV) — the volume of air remaining in the lungs after the most forceful possible exhalation; approximately 1.0–1.5 dm³. It cannot be measured directly with a spirometer.
• Inspiratory reserve volume (IRV) — the extra air that can be drawn in above a normal tidal inhalation.
• Expiratory reserve volume (ERV) — the extra air that can be forced out after a normal tidal exhalation.
• Total lung capacity (TLC) — VC + RV; the total volume the lungs can hold.

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Standard Lung Volumes

Volume	Adult value (dm³)	Definition
Tidal volume (TV)	0.5	Normal resting breath
Inspiratory reserve (IRV)	3.0	Extra in beyond TV
Expiratory reserve (ERV)	1.1	Extra out beyond TV
Vital capacity (VC = TV + IRV + ERV)	4.6	Max out after max in
Residual volume (RV)	1.2	Left after max out
Total lung capacity (TLC = VC + RV)	5.8	Total lung volume

These values vary with age, sex, size and fitness. Endurance athletes typically have larger vital capacities, and healthy values fall gradually with age.

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The Spirometer

A classical spirometer consists of a chamber of air or oxygen floating on water. A tube leads from the chamber to a mouthpiece through which the subject breathes. As the subject inhales, the chamber falls; as they exhale, it rises. A pen attached to the chamber traces a graph on a rotating drum (kymograph), producing a characteristic trace of volume against time.

flowchart LR
    A[Subject] -->|Inhale| B[Chamber falls]
    B --> C[Pen moves up on trace]
    A -->|Exhale| D[Chamber rises]
    D --> E[Pen moves down on trace]

Components and Precautions

• Soda lime canister — absorbs carbon dioxide from exhaled air, so CO₂ does not re-enter the chamber or be re-inhaled.
• One-way valves — ensure that fresh air flows in one direction only.
• Medical-grade oxygen in the chamber — allows the subject to breathe safely for several minutes.
• Nose clip — prevents breathing through the nose and losing exhaled air to the atmosphere.
• Sterile mouthpiece — hygiene and to prevent cross-contamination.
• Healthy subject — subjects with asthma or other respiratory conditions should not participate without medical supervision.

Because CO₂ is absorbed by the soda lime, the total volume in the chamber gradually falls over time as the subject consumes oxygen. The gradient of this baseline fall gives the rate of oxygen uptake, which can be used to calculate metabolic rate.

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Interpreting a Spirometer Trace