Breathing, Gas Exchange and Diffusion in the Lungs

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Every cell in your body needs oxygen for respiration, and every cell makes carbon dioxide as a waste product. Your respiratory system brings oxygen into the body and removes carbon dioxide. On this page you will learn how the parts of the system are built for this job, how muscles make you breathe, and how gases cross from the air into the blood by diffusion.

The parts of the respiratory system

When you breathe in, air travels along this route: nose or mouth → trachea → bronchi → bronchioles → alveoli. The lungs sit inside the chest (the thorax), protected by the ribs. Underneath the lungs is the diaphragm, which separates the chest from the abdomen.

Exam vocabulary: say bronchus for one tube and bronchi for two; alveolus for one air sac and alveoli for many. Checkpoint mark schemes usually accept phonetic spellings, but learn the correct ones.

Breathing: how muscles move air

Your lungs have no muscles of their own. They are moved by the diaphragm and the intercostal muscles. The idea that explains breathing is about volume and pressure: when the volume inside the chest gets bigger, the air pressure inside the lungs becomes lower than the air pressure outside, so air is pushed in. When the volume gets smaller, the pressure inside becomes higher than outside, so air is pushed out. (Air always moves from higher pressure to lower pressure.)

1. To describe breathing IN, write the steps in this order: muscles contract → ribs up and out / diaphragm flattens → volume of chest increases → pressure decreases → air moves in.
2. To describe breathing OUT, reverse every step: muscles relax → ribs down and in / diaphragm domes up → volume decreases → pressure increases → air moves out.

Try it: put your hands on your ribs and breathe in deeply. You can feel your ribs move up and out. Put a hand on your stomach: it moves out a little because the flattened diaphragm pushes down on the organs below.

A model of breathing: the bell jar

Scientists use models to show ideas we cannot easily see. The bell jar model is an analogy: each part of the model stands for a part of the body.

Gas exchange in the alveoli

Each bronchiole ends in a bunch of tiny air sacs called alveoli. Each alveolus is wrapped in a net of capillaries (the smallest blood vessels). This is where gas exchange happens. It happens by diffusion: particles of a gas move from a place where they are in high concentration to a place where they are in low concentration. No muscles or energy are needed; the particles move by themselves because they are always moving randomly.

1. Blood arriving at the lungs from the body has little oxygen and a lot of carbon dioxide (the cells used the oxygen and made carbon dioxide in respiration).
2. The air in the alveolus has a lot of oxygen (fresh air was just breathed in) and little carbon dioxide.
3. Oxygen diffuses from the air in the alveolus, through the alveolus wall and the capillary wall, into the blood. It is carried by the red blood cells.
4. Carbon dioxide diffuses the other way: from the blood (mostly the plasma) into the air in the alveolus. It is then breathed out.
5. The blood, now rich in oxygen, flows back to the heart, which pumps it to the rest of the body.

Why the alveoli are so good at gas exchange

Exam technique: for "explain how the alveoli are adapted", always link the feature to why it helps: "the walls are one cell thick, so the diffusion distance is short". A feature on its own usually earns only half the credit.

Inhaled air and exhaled air

Because of gas exchange, the air you breathe out is different from the air you breathe in. These are typical values:

Notice that exhaled air still contains a lot of oxygen (about 16%). That is why rescue breaths in first aid can help someone who has stopped breathing. You can show that exhaled air has more carbon dioxide with limewater: it turns milky (cloudy) much faster when you breathe out through it than when inhaled air is bubbled through it.

Breathing is NOT respiration

Breathing and gas exchange supply the oxygen that respiration needs and remove the carbon dioxide it makes. Plants respire too, but they do not breathe. You will study respiration fully on the Aerobic Respiration page.

Worked examples

Worked example 1 (the route of air) Describe the route taken by air from the nose to the place where gas exchange happens. [2]

• nose → trachea → bronchus / bronchi → bronchioles → alveoli
• 1 mark for the correct structures, 1 mark for the correct order. Ending at the alveoli is essential — "lungs" alone is too vague.
• Common mistake: putting bronchioles before bronchi. Big tubes come first: trachea, then bronchi, then the small bronchioles.

Worked example 2 (explaining inhaling) Explain how the intercostal muscles and the diaphragm make air move into the lungs. [4]

1. The intercostal muscles contract and pull the ribs up and out. [1]
2. The diaphragm contracts and flattens (moves down). [1]
3. This increases the volume inside the chest. [1]
4. So the pressure inside the lungs decreases (becomes lower than outside) and air is pushed into the lungs. [1]
5. Common mistake: "the lungs suck air in" or "the air goes in, so the chest gets bigger". The muscles act first; the air moves because of the pressure change.

Worked example 3 (calculation) At rest, Karim takes 15 breaths per minute. Each breath moves 0.5 litres of air into his lungs. (a) Calculate the volume of air he breathes in each minute. [1] (b) During a race his breathing rate rises to 40 breaths per minute and each breath is 2.0 litres. Calculate how many times more air per minute he breathes during the race. [2]

• (a) 15 × 0.5 = 7.5 litres per minute
• (b) During the race: 40 × 2.0 = 80 litres per minute. 80 ÷ 7.5 = 10.7 times (to 1 decimal place). [1 for 80, 1 for the answer; ecf from (a)]
• Exam technique: write the units at each step and show the calculation, so you can still score if one number is wrong.

Worked example 4 (gas exchange) Explain how oxygen gets from the air in an alveolus into the blood. Use the idea of concentration in your answer. [3]

• The concentration of oxygen is higher in the alveolus than in the blood. [1]
• Oxygen diffuses from high to low concentration, through the thin walls of the alveolus and capillary (one cell thick). [1]
• It enters the blood and is carried by red blood cells; the blood flows away, so the concentration in the blood stays low. [1]
• Common mistake: writing "oxygen is pumped into the blood". Diffusion needs no pump.

Worked example 5 (breathing and respiration) Salma says, "Breathing and respiration are the same thing." Explain why she is wrong. [2]

• Breathing is the movement of air into and out of the lungs (caused by muscles). [1]
• Respiration is a chemical reaction in cells that releases energy from glucose (using oxygen). [1]
• Common mistake: saying "respiration happens in the lungs". It happens in every cell, including the cells of the lungs.

Worked example 6 (evaluating a model) The bell jar model is used to show breathing. Give two ways the model is different from the real chest. [2]

• Any two: the jar wall is rigid but ribs move / there are no intercostal muscles / the rubber sheet is pulled by hand but the diaphragm contracts / balloons are hollow but lungs are spongy with alveoli / no gas exchange happens in the model.
• Exam technique: compare both sides — "the jar … but the ribs …". A difference must say what the real body does.

Practice

All pages in this course

Start here

• Stage 8 Science: The Tests and How to Prepare
• Thinking and Working Scientifically: Models, Investigations and Analysing Results

Biology

• Joints and Antagonistic Muscles
• Blood and Its Components
• Breathing, Gas Exchange and Diffusion in the Lungs
• A Balanced Diet, Energy Stores and Lifestyle
• Aerobic Respiration
• Ecosystems, Bioaccumulation and Invasive Species

Chemistry

• The Structure of the Atom
• Purity, Concentration and Chromatography
• Word Equations, Exothermic and Endothermic Reactions
• The Reactivity of Metals
• Impure Products and Solubility of Salts

Physics

• Speed and Distance–Time Graphs
• Balanced and Unbalanced Forces, and Moments
• Pressure, Particles and Diffusion
• Reflection and Refraction
• Dispersion and Colour
• Magnetic Fields and Electromagnets

Earth and space

• The Earth's Magnetic Field and Earth's Resources
• Climate, Weather and Climate Change
• Galaxies and Asteroids

Practice tests

• Stage 8 Science — Practice Test 1
• Stage 8 Science — Practice Test 2
• Stage 8 Science — Practice Test 3