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IB DemystifiedMYP Sciences

Breathing and gas exchange

You take about 20 000 breaths a day without thinking. Each breath brings oxygen deep into your lungs, where it crosses into your blood through walls thinner than a tissue, and carbon dioxide leaves the same way.

Recommended for MYP 2 · About 3 lessons · Criteria A, B, C and D

air from bronchiolealveolus(air sac)blood low in oxygen →→ blood rich in oxygencapillaryoxygencarbon dioxidewalls only one cell thick
Figure 1. Oxygen and carbon dioxide diffuse across the thin walls of an alveolus.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Breathing in and out
  6. Breathing in the real world
  7. Worked examples
  8. Assessment tips
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • identify the parts of the breathing system
  • explain how the diaphragm and ribs move air in and out
  • explain gas exchange by diffusion in the alveoli
  • distinguish breathing from respiration
  • explain the effects of exercise, smoking, asthma and air pollution
  • discuss smoking, vaping and smog

Before you start

You will use these skills. If any feel shaky, review them first.

  • cells and organs (see Cell organisation)
  • pressure in gases (see Pressure in solids, liquids and gases)
  • percentages and ratios

Key vocabulary

Trachea
The windpipe, carrying air to the lungs.
Bronchi and bronchioles
Branching tubes carrying air into the lungs.
Alveoli
Tiny air sacs where gas exchange happens.
Diaphragm
A sheet of muscle below the lungs that helps us breathe.
Diffusion
Movement of particles from a higher to a lower concentration.
Respiration
The reaction in cells that releases energy from glucose.

Understanding the ideas

  1. What is it?

    Air travels through the nose or mouth, trachea, bronchi and bronchioles to millions of alveoli. The airways are lined with mucus and cilia that trap and remove dust and microbes.

  2. Why does it happen?

    To breathe in, the diaphragm contracts and flattens and the ribs move up and out; the chest's volume increases, its pressure falls, and air is pushed in. Breathing out reverses this. In the alveoli, oxygen diffuses into the blood and carbon dioxide diffuses out, helped by a huge surface area, thin walls, moisture and a rich blood supply.

  3. How do we know?

    Comparing inhaled and exhaled air shows that the body takes in oxygen and gives out carbon dioxide, made by respiration in every cell. Breathing is not the same as respiration.

  4. Why does it matter?

    Exercise increases breathing rate. Smoking, asthma and air pollution damage or narrow the airways and reduce gas exchange, which is why smoking laws and clean air matter.

  5. What does it connect to?

    Breathing links to respiration, blood and circulation in biology, to gas pressure in physics, and to air pollution in environmental science.

Breathing in and out

  • Breathing in: diaphragm contracts and flattens; intercostal muscles pull the ribs up and out; volume increases, pressure falls; air moves in.
  • Breathing out: muscles relax; diaphragm domes up; ribs move down and in; volume decreases, pressure rises; air moves out.
  • Gas exchange: oxygen diffuses into the blood; carbon dioxide diffuses into the alveoli.
  • Good exchange surfaces: large area, thin walls, moist, rich blood supply.

Breathing in the real world

Asthma inhalers relax the muscles around narrowed airways. Doctors use peak-flow meters to monitor lung health. During Lahore's smog season, doctors advise people with asthma to stay indoors and wear masks outside.

Worked examples

Example 1: air per minute

A person takes 18 breaths a minute, each 0.6 litres. How much air is breathed in per minute?

  1. 18 × 0.6.
  2. = 10.8 litres per minute.

Example 2: comparing air

Exhaled air has 16% oxygen; inhaled air has 21%. How much of the oxygen percentage is absorbed?

  1. 21 − 16 = 5 percentage points.
  2. This oxygen diffused into the blood in the alveoli.

Assessment tips

Breathing questions often use diagrams of the lungs, air data and exercise experiments. Expect to:

  • Label the breathing system and describe the path of air.
  • Explain breathing using volume and pressure.
  • Explain diffusion and alveolus adaptations.
  • Analyse data on air, exercise and health.

Common mistakes: confusing breathing with respiration; saying the lungs suck in air; saying all oxygen is absorbed; and mixing up bronchi, bronchioles and alveoli.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: is there more carbon dioxide in exhaled air?

Partially guided investigation · about 30 minutes · pairs

Research question
Does exhaled air contain more carbon dioxide than inhaled air?
Scientific background
Limewater turns cloudy (milky) when carbon dioxide bubbles through it. The faster it turns cloudy, the more carbon dioxide the air contains.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Two boiling tubes with fresh limewater, bungs with bent glass tubes (a breathing set-up so you breathe in through one tube and out through the other), clean mouthpieces, stopwatch.
Method
  1. Set up the two tubes so that inhaled air is drawn through one and exhaled air is blown through the other.
  2. Breathe gently in and out through the mouthpiece for one minute.
  3. Watch which tube of limewater turns cloudy first and time it.
  4. Repeat with a new mouthpiece and fresh limewater.

Safety. Wear eye protection. Limewater is an irritant: breathe gently and never suck limewater into your mouth. Use a new, clean mouthpiece for each person.

Then evaluate: why is it important to breathe gently, and why use fresh limewater each time?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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