Lung Volumes and Spirometry: GCSE Biology

What are the key lung volumes at GCSE?

Pulmonologists describe lung function using defined volumes. You need four at GCSE level:

Inspiratory reserve volume (IRV) — the extra air that can be inhaled beyond a normal tidal breath — and expiratory reserve volume (ERV) — the extra air that can be exhaled beyond a normal tidal breath — may appear in extension questions:

VC = TV + IRV + ERV

Residual volume cannot be exhaled because the elastic recoil of the lungs and the structure of the airways prevent complete collapse. This air prevents the alveoli from collapsing and sticking together between breaths.

What is a spirometer and how does it work?

A spirometer is a device that measures the volume of air breathed in and out over time. A simple bell-type spirometer consists of:

1. A sealed, oxygen-filled chamber (a bell or drum) that floats on water and rises when the person exhales and falls when they inhale.
2. A mouthpiece connected to the chamber via a tube.
3. A recording device (traditionally a pen on a rotating drum; modern versions are electronic) that produces a spirogram — a trace of volume against time.

The person breathes into and out of the spirometer through the mouthpiece, which includes a CO₂ absorber (soda lime) to prevent re-breathing carbon dioxide.

Key features of a spirogram trace:

• The baseline rises as oxygen is absorbed (the chamber shrinks as the person takes in O₂ and the CO₂ is absorbed).
• The peaks of the trace represent maximum inhalation; the troughs represent maximum exhalation.
• Tidal breathing produces regular small fluctuations.
• Deep breathing shows much larger fluctuations, reaching vital capacity at the extremes.

What does a spirogram reveal about breathing rate and tidal volume?

Worked example: A spirometer trace shows 12 complete breaths over 60 seconds. The volume at peak inhalation is 3.2 L and at trough exhalation (normal) is 2.7 L.

• Breathing rate = 12 breaths per minute
• Tidal volume = 3.2 − 2.7 = 0.5 L per breath
• Ventilation rate = tidal volume × breathing rate = 0.5 L × 12 = 6.0 L/min

Ventilation rate represents the total volume of air moved into and out of the lungs per minute. It increases during exercise because both breathing rate and tidal volume increase.

How do lung volumes change during exercise?

At rest, a healthy adult breathes about 12–16 times per minute with a tidal volume of ~0.5 L — a ventilation rate of ~6–8 L/min.

During vigorous exercise:

• Breathing rate increases to 20–30+ breaths per minute (the hypothalamus and brainstem respond to rising CO₂ in the blood).
• Tidal volume increases to 2–3 L per breath (drawing on the inspiratory and expiratory reserve volumes).
• Ventilation rate can reach 100–150 L/min in a trained athlete.

The residual volume does not change with exercise — it is a fixed structural property of the lungs and thoracic cage.

What do spirograms reveal about lung disease?

Spirometry is a standard clinical test for diagnosing and monitoring obstructive and restrictive lung diseases:

• Asthma: narrowing of airways due to bronchospasm and inflammation. The vital capacity may be normal but the forced expiratory volume in one second (FEV₁) — the volume exhaled in the first second of a forced exhalation — is reduced because airflow is restricted. After a bronchodilator, FEV₁ typically improves, confirming reversible obstruction.
• COPD (Chronic Obstructive Pulmonary Disease, including emphysema): destruction of alveolar walls, which reduces elastic recoil of the lungs and traps air. Residual volume increases, vital capacity decreases, and FEV₁ is persistently reduced. Smoking is the primary cause.
• Fibrosis: scarring of lung tissue reduces compliance (the lungs are stiff). Total lung capacity and vital capacity are both reduced. All volumes are smaller than expected.

Frequently asked questions

Why does the spirometer trace gradually rise over time?

In a traditional bell spirometer filled with oxygen, the person breathes in O₂ and breathes out CO₂, which is absorbed by the soda lime. Over time, the volume of gas in the bell decreases because oxygen is being consumed (taken in and used in respiration) and CO₂ is removed by the absorber rather than returned to the bell. The baseline of the trace therefore drifts upward (the bell gets lighter and rides higher), even though the person is breathing normally. In electronic spirometers measuring airflow, this baseline drift does not occur.

What is the difference between vital capacity and total lung capacity?

Vital capacity is the maximum volume of air you can move — from a full exhalation to a full inhalation (or vice versa). It does not include the air that remains in the lungs after maximum exhalation, which is the residual volume. Total lung capacity = vital capacity + residual volume. You cannot measure residual volume directly with a simple spirometer (it cannot be exhaled), so special techniques such as helium dilution or body plethysmography are needed to measure it clinically.

How does smoking affect the spirogram?

Smoking damages the alveolar walls over years, leading to emphysema — a form of COPD. Alveolar destruction reduces the elastic recoil of the lungs, meaning they cannot expel air effectively. On a spirogram: residual volume increases (trapped air), vital capacity decreases, and FEV₁ is markedly reduced. The tidal breathing trace may look similar to a healthy person at rest, but the forced exhalation manoeuvre reveals the obstructed airflow. Spirometry is used to detect COPD in smokers before severe symptoms develop.

Why cannot the residual volume be measured by a spirometer?

A standard spirometer measures the volume of air exchanged with the atmosphere through the mouthpiece. The residual volume never leaves the lungs — after maximum exhalation, this air is still in the airways and alveoli but cannot be pushed out. Because it never passes through the spirometer, the device cannot measure it. It is measured indirectly by breathing in a known concentration of an inert gas (helium), waiting for it to mix with all the lung air, and then calculating the total volume from the diluted concentration. Total lung capacity minus vital capacity then gives residual volume.

---

For Socratic GCSE biology with Professor Darwin — linking lung structure, function and disease in one integrated picture — visit aitutors.me.