Lesson overview
Gas exchange and breathing: What changes in the air you breathe out?
Breathe in through one tube of limewater and out through another, breathe on a cold mirror with cobalt chloride paper, and find out what is different about the air you breathe out. Then look inside one alveolus, a tiny air sac in the lungs, pull the rubber sheet of a bell-jar model of the chest, and record breathing at rest and after step-ups.
Suggested sequence
- Lesson 1 · Air in, air out. Investigating inspired and expired air with limewater and cobalt chloride paper; how the two airs differ in oxygen, carbon dioxide and water vapour, and (Supplement) why. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Lungs and gas exchange. The parts of the breathing system, from the larynx to the alveoli and their capillaries; the features of a gas exchange surface; (Supplement) the intercostal muscles, the cartilage of the trachea, and goblet cells, mucus and cilia. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · Breathing in, breathing out. The effect of physical activity on the rate and depth of breathing; (Supplement) how the ribs, intercostal muscles and diaphragm change the volume and pressure in the chest, and why breathing speeds up. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 10 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 10 questions, 15 min, lesson closed.
Learning goals
Part 1: Air in, air out
- Investigate the differences between inspired and expired air using limewater as a test for carbon dioxide (11.1.3)
- Describe the differences in composition between inspired and expired air, limited to oxygen, carbon dioxide and water vapour (11.1.4)
- Supplement: explain the differences in composition between inspired and expired air (S11.1.9)
Part 2: Lungs and gas exchange
- Identify the lungs, diaphragm, ribs, intercostal muscles, larynx, trachea, bronchi, bronchioles, alveoli and associated capillaries (11.1.2)
- Describe the features of gas exchange surfaces in humans: large surface area, thin surface, good blood supply and good ventilation with air (11.1.1)
- Supplement: identify in diagrams and images the internal and external intercostal muscles (S11.1.6)
- Supplement: state the function of cartilage in the trachea (S11.1.7)
- Supplement: explain the role of goblet cells, mucus and ciliated cells in protecting the breathing system from pathogens and particles (S11.1.11)
Part 3: Breathing in, breathing out
- Supplement: explain the role of the ribs, the internal and external intercostal muscles and the diaphragm in producing volume and pressure changes in the thorax leading to the ventilation of the lungs (S11.1.8)
- Investigate and describe the effects of physical activity on the rate and depth of breathing (11.1.5)
- Supplement: explain the link between physical activity and the rate and depth of breathing in terms of an increased carbon dioxide concentration in the blood, detected by the brain (S11.1.10)
Misconceptions to listen for
- “W: expired air leaves the apparatus.” — W is on tube A, which only room air passes through. Expired air bubbles through B and leaves at Y.
- “Z: the person breathes in and out.” — The person breathes at the mouthpiece, X. Z is a small part inside a tube: a one-way valve.
- “Limewater turns from clear to blue.” — Limewater does not change colour. Carbon dioxide makes tiny white specks in it, so it turns cloudy.
- “Cobalt chloride paper turns from blue to pink.” — That is the test for water. Carbon dioxide turns limewater cloudy.
- “Limewater turns from cloudy to clear.” — Limewater starts clear. Carbon dioxide makes it cloudy, not the other way round.
- “Expired air contains no oxygen.” — Expired air is still about 16% oxygen. The body takes only part of it.
- “Inspired air contains no carbon dioxide.” — Inspired air has a little carbon dioxide, about 0.04%.
- “It contains no oxygen.” — It is still about 16% oxygen. The body takes only part of the oxygen in each breath.
- “It contains less nitrogen.” — The body does not use nitrogen, so there is about the same amount, 78%, in both.
- “Oxygen is used up in the lungs.” — Oxygen only crosses into the blood in the lungs. It is used by cells all over the body, in respiration.
Scope
Gas exchange in humans (11.1.1–11.1.5): the features of gas exchange surfaces, the parts of the breathing system and their capillaries, investigating inspired and expired air with limewater, the differences in their composition, and the effect of physical activity on the rate and depth of breathing; for the Supplement (11.1.6–11.1.11), the internal and external intercostal muscles, the cartilage of the trachea, how the ribs, intercostal muscles and diaphragm ventilate the lungs, why the two airs differ, how a rise in carbon dioxide in the blood speeds up breathing, and how goblet cells, mucus and cilia protect the breathing system. Respiration in cells and the oxygen debt are in “Respiration”; lung volumes other than the volume of one breath, and smoking, are not covered. The results in the experiment come from a teaching model.