IGCSE · Practice questions · Breathing and exercise

Faster, deeper, controlled by CO₂.

Six original Cambridge-style questions on the composition of inspired and expired air, investigating breathing during exercise, calculating volumes of air breathed, and the mechanism and control of ventilation.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Muscles, volume, pressure, air.

Expired vs inspired: less oxygen (16% vs 21%), more carbon dioxide (4% vs 0.04%), more water vapour.
Limewater: turns milky with carbon dioxide; expired air turns it milky faster.
Extended ventilation: muscles contract, volume rises, pressure falls, air flows in; reverse to breathe out.
Extended control: more carbon dioxide in the blood, detected by the brain, gives faster and deeper breathing.

Analysis

[4 marks]

Describe how you would use limewater to show that expired air contains more carbon dioxide than inspired air. State the results you would expect.

Set up two boiling tubes with the same volume of limewater, connected to a mouthpiece so that breathing in draws air through one tube (A) and breathing out bubbles air through the other (B). ✓
Breathe in and out gently through the mouthpiece for the same number of breaths (for example 10). ✓
Expected: the limewater in B (expired air) turns milky quickly. ✓
The limewater in A (inspired air) stays clear or turns only slightly cloudy after much longer, so expired air contains more carbon dioxide. ✓

Calculation

[3 marks]

The table shows breathing measurements for a student.

Activity	Rate / breaths per min	Depth / dm³
rest	12	0.5
jogging	28	1.8

Calculate the volume of air breathed per minute at rest and while jogging, and how many times greater it is while jogging.

Rest: 12 × 0.5 = 6.0 dm³ per min. ✓
Jogging: 28 × 1.8 = 50.4 dm³ per min. ✓
50.4 ÷ 6.0 = 8.4 times greater. ✓

Data

[3 marks]

Air samples were analysed. Inspired air contained 21% oxygen, 0.04% carbon dioxide and 1% water vapour. Expired air contained 16% oxygen, 4% carbon dioxide and 6% water vapour.

(a) Calculate the decrease in oxygen. (b) Calculate how many times greater the carbon dioxide percentage is in expired air. (c) State one other way expired air differs from inspired air.

(a) 21 − 16 = 5 percentage points. ✓
(b) 4 ÷ 0.04 = 100 times greater. ✓
(c) Expired air contains more water vapour (6% compared with 1%), or it is warmer. ✓

Analysis

[4 marks]

Plan an investigation into the effect of physical activity on the rate of breathing.

Count the number of breaths in one minute while the person sits at rest. ✓
Have the person do a set exercise for a fixed time, for example step-ups for two minutes, then immediately count breaths for one minute. ✓
Control: the same person (or same age and fitness), the same type, intensity and duration of exercise, counting at the same time after exercise. ✓
Repeat several times, with rest in between so breathing returns to normal, and calculate a mean for each condition. ✓

Explain

[5 marks]

Extended. Explain how the ribs, intercostal muscles and diaphragm cause air to enter the lungs.

The external intercostal muscles contract (the internal intercostals relax). ✓
This pulls the ribs upwards and outwards. ✓
The diaphragm muscle contracts and flattens. ✓
Both changes increase the volume of the thorax, so the pressure inside the lungs falls below atmospheric pressure. ✓
Air is pushed into the lungs from the higher pressure outside, through the trachea, bronchi and bronchioles. ✓

Explain

[4 marks]

Extended. During a race a runner breathes faster and more deeply. Explain how the body brings about this change and why it is useful.

Muscle cells respire faster to release more energy, producing more carbon dioxide. ✓
The carbon dioxide concentration in the blood increases. ✓
This is detected by the brain, which sends impulses to the intercostal muscles and diaphragm to increase the rate and depth of breathing. ✓
More carbon dioxide is removed from the blood, and more oxygen is taken in for aerobic respiration in the muscles. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.