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LESSON B7 · EXPLAINER
Breathing, Gaseous Exchange and Smoking
An amoeba needs no lungs. You need 300 million alveoli. What’s the difference?
Narration uses your device’s built-in voice. No sound? Switch to “Captions only” — the captions advance on their own.
Explore in 3D
Breathing in 3D
See how the ribs, diaphragm and lungs move during breathing.
Step 2 · Read
Key notes
Copy these into your notebook in your own words — that is how they stick.
Inhalation
External intercostals contract (ribs up/out), diaphragm contracts (flattens) → volume ↑, pressure ↓ → air in.
Exhalation
Muscles relax, ribs down/in, diaphragm domes → volume ↓, pressure ↑ → air out.
Large surface area, thin, moist, well supplied with blood/ventilated.
SA:V ratio
Small organisms rely on diffusion; large ones need specialised surfaces and transport.
Plants
Stomata and lenticels; net CO₂ uptake in light, net O₂ uptake in dark.
Smoking
Nicotine (addiction, heart rate), CO (less O₂ transport), tar (cancer, cilia damage), emphysema, bronchitis.
Government action
Tobacco taxes, bans on advertising and smoking in public places, health warnings, education.
EXAM TIPS & COMMON MISTAKES
Volume and pressure change in OPPOSITE directions — say both.
Carbon monoxide → REDUCED OXYGEN TRANSPORT; nicotine → increased heart rate and addiction. P1 tests which is which.
Gas exchange explanation must mention DIFFUSION and CONCENTRATION GRADIENT.
Step 2 · Learn from examples
Worked examples & study resources
The kind of support a teacher gives after the lesson: worked examples to attempt first, a board illustration, key terms, common misconceptions and memory aids.
Worked example 1
Inhaled and exhaled air
Gas
Inhaled air
Exhaled air
Oxygen
21%
16%
Carbon dioxide
0.04%
4%
Nitrogen
78%
78%
Water vapour
variable
saturated
(a) How many times more carbon dioxide is there in exhaled air? (b) Explain the change in oxygen and carbon dioxide. (c) Why is nitrogen unchanged?
Try it in your notebook first, then reveal the solution one step at a time.
(a) 4 ÷ 0.04 = 100 times more carbon dioxide.
(b) Oxygen diffuses from the alveoli into the blood and is used by cells in respiration, so less is breathed out. Carbon dioxide made in respiration diffuses from the blood into the alveoli and is breathed out.
(c) Nitrogen is not used or made by the body, so the amount stays the same.
Exhaled air is also warmer and saturated with water vapour, picked up from the moist lining of the airways.
Answer. (a) 100 times. (b) O₂ is absorbed into the blood and used in respiration; CO₂ produced in respiration diffuses out into the alveoli. (c) The body does not use or produce nitrogen.
Examiner’s tip. Note that exhaled air still contains 16% oxygen — enough for mouth-to-mouth resuscitation to work.
Worked example 2
Surface area to volume ratio
Calculate the surface area : volume ratio of cubes with sides of 1 cm, 2 cm and 4 cm, and use the results to explain why large animals need lungs.
Try it in your notebook first, then reveal the solution one step at a time.
Surface area = 6 × side²; volume = side³.
1 cm cube: SA = 6 cm², V = 1 cm³ → ratio 6 : 1.
2 cm cube: SA = 24 cm², V = 8 cm³ → ratio 3 : 1.
4 cm cube: SA = 96 cm², V = 64 cm³ → ratio 1.5 : 1.
As size increases, the SA : V ratio falls. A large animal’s body surface is too small for its volume to supply enough oxygen by diffusion, and the distance to the centre is too great. It needs a specialised exchange surface (lungs with millions of alveoli) and a transport system.
Answer. Ratios 6 : 1, 3 : 1 and 1.5 : 1. Larger organisms have a smaller SA : V ratio, so diffusion through the body surface is too slow — they need lungs with a large surface area, plus blood to transport the gases.
Examiner’s tip. Single-celled organisms such as Amoeba have a large SA : V ratio and exchange gases through their whole surface.
On the board · Breathing in and breathing out
Inhalation (breathing in)
Exhalation (breathing out, at rest)
External intercostal muscles
Contract
Relax
Ribs
Move up and out
Move down and in
Diaphragm
Contracts and flattens
Relaxes and domes upwards
Volume of thorax
Increases
Decreases
Pressure in lungs
Decreases (below atmospheric)
Increases (above atmospheric)
Air flow
Into the lungs
Out of the lungs
Air always moves from higher to lower pressure. Bigger chest volume → lower pressure → air rushes in.
Key terms
Inhalation
Breathing in.
Exhalation
Breathing out.
Alveolus (pl. alveoli)
Tiny air sac in the lungs where gaseous exchange happens.
Gaseous exchange
Oxygen diffusing into the blood and carbon dioxide diffusing out, across a respiratory surface.
Diaphragm
Sheet of muscle below the lungs that flattens to increase chest volume.
Intercostal muscles
Muscles between the ribs that move the rib cage.
Surface area : volume ratio
How much surface an organism has compared with its size.
Common misconceptions
✗ We breathe in oxygen and breathe out carbon dioxide only.✓ Exhaled air is still about 16% oxygen and only about 4% carbon dioxide.
✗ The lungs pull air in with their own muscles.✓ Lungs have no muscle. The diaphragm and intercostal muscles change the chest volume.
✗ Smoking only damages the lungs.✓ Smoking also causes heart disease, strokes and many cancers; nicotine is addictive and carbon monoxide reduces oxygen carried by the blood.
Memory aid
Breathing in: air In, ribs Up and Out, diaphragm Down (flat). Breathing out is the exact opposite.
Real-life link
Several Caribbean countries, including Jamaica and Trinidad and Tobago, ban smoking in enclosed public places, and health warnings are printed on cigarette packs — measures that cut second-hand smoke and help smokers quit.
Try this
Count your breaths per minute at rest, then after one minute of star jumps. Time how long it takes to return to the resting rate.
Play & practise
Games
4 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness.
Step 3 · Practise
Paper 1 practice — multiple choice
8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better.
Step 4 · Apply
Paper 2 practice — structured questions
Write full answers, then reveal the mark scheme and tick the points you earned (13 marks in total). Your answers are saved on this device.
Based on CSEC P2 May/June 2023, Question 5(b)–(c)
Question 1 [8 marks]
Breathing and gaseous exchange in humans.
(a) Name the THREE muscles responsible for breathing in humans. [3]
MARK SCHEME — (a) (max 3)
(b) Explain how gaseous exchange occurs in the lungs. [5]
MARK SCHEME — (b) (max 5)
Based on CSEC P2 May/June 2024, Question 4(d)
Question 2 [5 marks]
Mr Smith has smoked cigarettes for ten years.
(a) Explain TWO negative effects smoking would have had on his health. [4]
MARK SCHEME — (a) (max 4)
(b) Suggest ONE action a government can take to address smoking nationally. [1]
MARK SCHEME — (b) (max 1)
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Sources
Where this topic appears in CSEC papers
Find these in your own past-paper booklets for extra timed practice. The practice above is adapted from them.