The shape of the lesson
By the end of the lesson, learners can
- Volume up, pressure down: Extended.
- Inspired and expired air: In the limewater apparatus, air breathed in is drawn through tube A and air breathed out bubbles through tube B.
- Exercise: faster and deeper: Choose resting, walking, jogging or sprinting and watch the spirometer trace.
- 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.
One picture every idea returns to
Expired air contains less oxygen (about 16% instead of 21%), more carbon dioxide (about 4% instead of 0.04%) and more water vapour than inspired air, and turns limewater milky much faster. During physical activity breathing becomes faster and deeper. Extended: muscles change the volume of the thorax, which changes the pressure, so air moves in and out; extra carbon dioxide in the blood during exercise is detected by the brain, which increases the rate and depth of breathing.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, In with oxygen, out with carbon dioxide.. Breathing changes the air: the air we breathe out has less oxygen, more carbon dioxide and more water vapour than the air we breathe in. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Volume up, pressure down | Extended. Project tab 2 of the lab (Ventilation of the lungs) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Inspired and expired air | In the limewater apparatus, air breathed in is drawn through tube A and air breathed out bubbles through tube B. Project tab 4 of the lab (Testing inspired and expired air with limewater) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Exercise: faster and deeper | Choose resting, walking, jogging or sprinting and watch the spirometer trace. Project tab 5 of the lab (Exercise and breathing) and let a student drive it while the class predicts. | Think-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 23 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Exercise: faster and deeper, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Air does not rush in and then make the chest expand. | The order is: muscles contract, volume increases, pressure falls, then air flows in. And the diaphragm flattens when it contracts; it domes upwards when it relaxes. |
| Expired air still contains plenty of oxygen (about 16%), which is why rescue breaths work. | Do not write that expired air has "no oxygen" or is "all carbon dioxide". |
| In the Extended explanation the trigger is the rise in carbon dioxide in the blood, detected by the brain. | Do not say the lungs detect a lack of oxygen, or that the muscles decide how fast to breathe. |
Breathing and exercise: worksheet
6 questions, 23 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how you would use limewater to show that expired air contains more carbon dioxide than inspired air. State the results you would expect.
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.
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.
Plan an investigation into the effect of physical activity on the rate of breathing.
Extended. Explain how the ribs, intercostal muscles and diaphragm cause air to enter the lungs.
Extended. During a race a runner breathes faster and more deeply. Explain how the body brings about this change and why it is useful.
Answer key and marking notes
- 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.
- 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.
- (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.
- 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.
- 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.
- 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.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which result shows that a gas contains carbon dioxide?
- ALimewater turns milky
- BA glowing splint relights
- CLimewater stays clear
- DA lighted splint gives a squeaky pop
About what percentage of expired air is carbon dioxide?
- A4%
- B0.04%
- C16%
- D21%
The spirometer trace was recorded from a person at rest. What is their breathing rate?
- A15 breaths per minute
- B5 breaths per minute
- C25 breaths per minute
- D0.25 breaths per minute
Using the same spirometer trace, what is the depth of each breath (the volume breathed in)?
- A0.8 dm3
- B3.2 dm3
- C2.4 dm3
- D0.4 dm3
A student counts her breaths for one minute before and immediately after two minutes of skipping. Which variable must she keep the same to make the comparison fair between repeats?
- AThe time for which she skips
- BHer breathing rate after skipping
- CHer breathing depth after skipping
- DThe number of breaths she counts
Extended: during quiet breathing out at rest, what happens to the diaphragm?
- AIt relaxes and domes upwards
- BIt contracts and flattens
- CIt contracts and domes upwards
- DIt relaxes and flattens
Extended: why does air move out of the lungs when the volume of the thorax decreases?
- AThe pressure in the lungs rises above atmospheric pressure
- BThe pressure in the lungs falls below atmospheric pressure
- CThe intercostal muscles squeeze air out of each alveolus
- DCarbon dioxide in the lungs pushes the oxygen out
Extended: a person at rest breathes air that contains 3% carbon dioxide. Their breathing becomes faster and deeper. What is the best explanation?
- ACarbon dioxide in the blood rises and is detected by the brain
- BOxygen in the air falls, so the lungs breathe faster
- CThe muscles respire faster, so they use more oxygen
- DCarbon dioxide irritates the cilia, which speeds up breathing
Quiz key
Carbon dioxide turns limewater milky (cloudy). A glowing splint relighting is the test for oxygen, limewater staying clear means little or no carbon dioxide, and a squeaky pop is the test for hydrogen.
Expired air contains about 4% carbon dioxide. 0.04% is the figure for inspired air, 16% is the oxygen in expired air, and 21% is the oxygen in inspired air.
There are 5 complete breaths (peaks) in 20 s, so 5 × 3 = 15 breaths per minute. 5 is the count for 20 s only, not per minute, 25 comes from adding the time and the count, and 0.25 is breaths per second.
The trace rises from 2.4 to 3.2 dm3 in each breath, so the depth is 3.2 − 2.4 = 0.8 dm3. 3.2 and 2.4 are the single readings at the peak and trough, and 0.4 is only half the rise (from the middle line to a peak).
The length (and type) of the exercise is a control variable that she sets and must keep the same each time. Breathing rate after exercise and the number of breaths counted are what she measures (dependent variable), and depth is also an outcome, not something she can fix.
When breathing out at rest the diaphragm relaxes and returns to its dome shape, reducing the volume of the thorax. It contracts and flattens when we breathe in. Contracting does not make it dome, and relaxing does not flatten it.
A smaller volume raises the pressure inside the lungs above atmospheric pressure, so air flows out to the lower pressure outside. A pressure below atmospheric would draw air in, the intercostal muscles act on the ribs not on each alveolus, and gases do not push each other out.
Less carbon dioxide can leave the blood, so its concentration in the blood rises; the brain detects this and increases the rate and depth of breathing. The oxygen in the air is barely changed and the lungs do not control breathing, the person is at rest so the muscles are not respiring faster, and cilia do not control breathing rate.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.