Second Wind

Second Wind: the breathing practical, lung volumes and the effect of exercise

This free game is part of our Pearson Edexcel iGCSE Biology (4BI1) course for exams in EXAM YEARS 2026-2029.

Measuring breathing with a spirometer

A spirometer measures the volume of air breathed in and out of the lungs. Its trace shows the key lung volumes:

• Tidal volume: the air breathed in and out in one normal, quiet breath (about 0.5 litres, or 500 cm³).
• Vital capacity: the most air you can breathe out after taking the deepest possible breath in (about 4.6 litres for a teenager).
• Residual volume: the air that always stays in the lungs, even after breathing out as hard as you can (about 1.2 litres). It stops the alveoli collapsing.

You can count the peaks on a spirometer trace to work out the breathing rate: breaths counted ÷ seconds × 60 = breaths per minute.

Practical part 1: do we breathe out carbon dioxide?

This is part of the Pearson Edexcel International GCSE Biology practical for spec point 2.50: investigate breathing in humans, including the release of carbon dioxide and the effect of exercise.

Limewater turns cloudy (milky) when carbon dioxide bubbles through it. With a two-tube apparatus, breathed-in air is pulled through tube A and breathed-out air bubbles through tube B. Tube B turns cloudy much faster, because breathed-out air contains far more carbon dioxide:

• Oxygen: about 21% in breathed-in air, about 16% in breathed-out air.
• Carbon dioxide: about 0.04% in breathed-in air, about 4% in breathed-out air (around 100 times more).
• Nitrogen: about 78% in both.
• Breathed-out air is also warmer and has more water vapour.

The extra carbon dioxide is made by respiration in the body’s cells. It diffuses from the blood into the alveoli and is breathed out. Hydrogen-carbonate indicator can be used instead of limewater: it turns from red/orange to yellow when carbon dioxide rises. Wear eye protection and never get limewater in your mouth.

Practical part 2: the effect of exercise on breathing

1. Sit quietly, then count the breaths for one minute to get the resting breathing rate (a baseline).
2. Do a set exercise for a fixed time, such as 2 minutes of step-ups.
3. Count the breaths again straight away, then every minute until breathing returns to the resting rate.
4. Repeat and calculate a mean to make the results more reliable.

The independent variable is the exercise, the dependent variable is the breathing rate, and the control variables include the same person, the same exercise for the same time and the same rest before measuring.

During exercise the muscles respire faster, so they need more oxygen and make more carbon dioxide. Breathing becomes faster and deeper to take in oxygen and remove carbon dioxide. After exercise, breathing stays fast for a while to repay the oxygen debt: extra oxygen is needed to break down the lactic acid made by anaerobic respiration. Fitter people have a smaller rise in breathing rate and recover faster.

How to play Second Wind

• Hold SPACE (or the HOLD button) to breathe in and let go to breathe out. Sam’s X-ray suit shows the ribs lifting, the diaphragm flattening and the lungs filling.
• Lab 1, Meet your lungs: take calm breaths and one giant breath to see tidal volume, vital capacity and residual volume on the spirometer.
• Lab 2, The limewater test: predict which tube goes cloudy, then breathe through the apparatus to find out.
• Lab 3, The exercise test: hold on the glowing bars to breathe for Sam from rest to a sprint and recovery, keeping the oxygen and carbon dioxide in the blood balanced.
• Lab 4, Data Desk: count the breaths on your own trace, calculate breathing rate and percentage increase, and read a fitness graph.

Exam tips

• Link exercise to respiration: “the muscles respire faster, so more carbon dioxide is produced and more oxygen is needed, so breathing rate and depth increase”.
• Show your working for rates: count ÷ time in seconds × 60. For percentage increase use (new − old) ÷ old × 100.
• Counting for a full minute is more accurate than counting for 15 seconds and multiplying by 4, because breathing is not perfectly regular.
• Say that limewater turns cloudy or milky with carbon dioxide, not that it “changes colour”.