Lung Volumes and Spirometer

Spec Mapping — OCR H420 Module 3.1.1 — Exchange surfaces, content statements covering the use of a spirometer to measure tidal volume, vital capacity, ventilation rate and oxygen uptake; the interpretation of spirometer traces; and the precautions required for safe operation (refer to the official OCR H420 specification document for exact wording). This is one of OCR's classic practical+calculation topics — the spirometer is both an instrument students may use under PAG conditions and the source of routine 6-mark data-interpretation questions.

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.

The spirometer was invented by John Hutchinson in 1846, a British surgeon working on life-insurance actuarial science: he found that vital capacity was the single best predictor of longevity in his cohort of 2,130 male patients. Modern spirometry remains a routine test, used to diagnose asthma, chronic obstructive pulmonary disease (COPD), pulmonary fibrosis and occupational lung diseases such as silicosis and asbestosis. The instrument is the cornerstone of respiratory medicine.

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.

RV (~1.2 dm³) cannot be measured by spirometry

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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

A typical spirometer trace shows a regular waveform over time. Between calm breaths the peaks and troughs are separated by the tidal volume. The frequency gives the breathing (ventilation) rate in breaths per minute. A deep forced breath gives a sudden excursion equal to the vital capacity, from which IRV and ERV can be read off.

Pulmonary ventilation rate (also called minute ventilation) is defined as:

$\text{Ventilation rate} = \text{Tidal volume} \times \text{Breathing rate}$Ventilation rate=Tidal volume×Breathing rate

For example, a person at rest breathing at 12 breaths per minute with a tidal volume of 0.5 dm³ has a ventilation rate of 0.5 × 12 = 6 dm³ min⁻¹. During strenuous exercise, both TV and breathing rate can rise considerably, so ventilation might reach 100 dm³ min⁻¹ or more.

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FEV₁ and FVC

• Forced vital capacity (FVC) — the maximum volume of air that can be expired in one forced breath after taking a maximum inspiration.
• Forced expiratory volume in 1 second (FEV₁) — the volume of air exhaled in the first second of a forced expiration from maximum inspiration.

The FEV₁ / FVC ratio is a clinically important measure of lung function:

• In a healthy person, FEV₁ is about 75–80% of FVC.
• In obstructive disease (e.g., asthma, emphysema, chronic bronchitis), airway narrowing reduces FEV₁ disproportionately, so the ratio falls below ~70%.
• In restrictive disease (e.g., pulmonary fibrosis), both FEV₁ and FVC fall in proportion, so the ratio remains normal or even high, but FVC itself is much reduced.

Exam Tip: OCR questions often provide spirometer traces for healthy and diseased subjects and ask you to compare them. Look for: (1) smaller tidal volume, (2) smaller vital capacity, (3) slower FEV₁ (shallower initial slope), and (4) increased breathing rate as a compensation.

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Calculating Oxygen Uptake

Because CO₂ is removed by soda lime, the only net volume change in the spirometer chamber arises from oxygen being absorbed by the subject. The gradient of the baseline on the trace therefore gives the rate of oxygen consumption:

$\text{Rate of O}_2 \text{ uptake} = \frac{\text{Volume decrease}}{\text{Time}}$Rate of O2​ uptake=TimeVolume decrease​

For example, if the baseline falls by 1.2 dm³ over 2 minutes, then oxygen consumption is 0.6 dm³ min⁻¹. This figure can be converted into an estimate of metabolic rate.

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Worked Example

A student breathes quietly into a spirometer and produces the following values:

• Tidal volume = 0.45 dm³
• Breathing rate = 16 breaths min⁻¹
• Baseline fall over 3 minutes = 0.9 dm³

Ventilation rate = 0.45 × 16 = 7.2 dm³ min⁻¹ Rate of O₂ uptake = 0.9 / 3 = 0.3 dm³ min⁻¹

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How body size, age and fitness affect lung volumes

Lung volumes scale predictably with several factors:

• Body size: Vital capacity scales approximately with height squared. A 1.5 m adult has a VC of ~3.2 dm³; a 1.9 m adult has ~5.5 dm³. Predicted values use a regression on height, age and sex.
• Age: VC peaks at age 20–25 and falls by ~30 cm³ per year thereafter, due to gradual loss of elastic fibres and a stiffening of the chest wall. An 80-year-old typically has 70–80% of their early-adult VC.
• Sex: Male VC is on average 20–25% higher than female VC of the same height, due to larger thoracic dimensions.
• Fitness: Endurance-trained athletes have VCs at the upper end of the predicted range; resistance-trained athletes (weightlifters) show smaller increases. Training does not greatly increase TLC, but it lowers resting respiratory rate and increases tidal volume at submaximal exercise.
• Smoking: Loss of FEV₁ accelerates from the normal ~30 mL year⁻¹ to ~60–80 mL year⁻¹ in smokers; the Fletcher–Peto curve is a famous representation of this divergence.

A worked spirometry case

A 17-year-old swimmer enters the school PAG laboratory. Her trace shows:

• Tidal volume = 0.6 dm³
• Breathing rate = 12 min⁻¹
• IRV = 3.2 dm³
• ERV = 1.4 dm³
• FEV₁ = 4.4 dm³
• FVC = 5.2 dm³

Calculations:

• Vital capacity = TV + IRV + ERV = 0.6 + 3.2 + 1.4 = 5.2 dm³
• Ventilation rate = TV × rate = 0.6 × 12 = 7.2 dm³ min⁻¹
• FEV₁ / FVC = 4.4 / 5.2 = 0.85 (above normal — indicating excellent lung function)

The unusually high VC and ratio are consistent with the cumulative effect of regular endurance training on the elastic and mechanical properties of the lung and chest wall.

Common Exam Mistakes

• Confusing tidal volume with vital capacity. Tidal volume is a normal breath; vital capacity is a maximum breath.
• Including residual volume inside vital capacity. Residual volume is additional to VC and cannot be exhaled.
• Saying the chamber contains "pure" oxygen without justifying it. Oxygen is used so that the subject can breathe for longer; a soda lime canister absorbs the CO₂.
• Forgetting the nose clip. Loss of exhaled air through the nose introduces a major error in tidal volume measurement.
• Muddling FEV₁ and FVC. FEV₁ is measured in the first second; FVC is the total volume exhaled.
• Reading the baseline as "air leaking out". The baseline fall is due to the subject absorbing oxygen and CO₂ being removed from the chamber.

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Comparison: obstructive vs restrictive lung disease