4.5 Breathing and Gas Exchange

A free GCSE Biology lesson for AQA (8461) from OBA School: the structure of the lungs, ventilation, gas exchange in the alveoli and breathing rate. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Name the parts of the breathing system and describe the path of air into the lungs
• Describe how ventilation moves air in and out of the lungs
• Explain how the alveoli are adapted for efficient gas exchange
• Calculate breathing rate and use data on the composition of inhaled and exhaled air

Key words

• Trachea: The windpipe: a tube held open by rings of cartilage that carries air to the lungs.
• Bronchi: The two tubes that branch from the trachea, one to each lung (singular: bronchus).
• Bronchioles: Smaller tubes that branch from the bronchi and end in alveoli.
• Alveoli: Tiny air sacs where gas exchange happens (singular: alveolus).
• Ventilation: Breathing: moving air into and out of the lungs.
• Gas exchange: Oxygen diffusing from the air into the blood, and carbon dioxide diffusing from the blood into the air.

The breathing system

The lungs are in the thorax (chest), protected by the ribcage and separated from the abdomen by the diaphragm. Air enters through the nose or mouth, then passes down the trachea, into the two bronchi, through branching bronchioles and finally into millions of tiny alveoli.

Exam tip: Path of air: nose / mouth → trachea → bronchi → bronchioles → alveoli. Learn it in order.

Ventilation

You cannot pull air into your lungs directly. Instead, muscles change the volume of the thorax, which changes the pressure inside it.

• Intercostal muscles; Breathing in (inhaling) contract: ribcage moves up and out; Breathing out (exhaling) relax: ribcage moves down and in
• Diaphragm; Breathing in (inhaling) contracts and flattens; Breathing out (exhaling) relaxes and domes upwards
• Volume of thorax; Breathing in (inhaling) increases; Breathing out (exhaling) decreases
• Pressure inside; Breathing in (inhaling) decreases below air pressure outside; Breathing out (exhaling) increases above air pressure outside
• Air movement; Breathing in (inhaling) air is drawn in; Breathing out (exhaling) air is forced out

Key fact: Always link the steps: volume increases → pressure decreases → air moves in. Air moves from higher to lower pressure.

Gas exchange in the alveoli

Oxygen diffuses from the air in the alveoli into the blood in the capillaries, where it joins haemoglobin. Carbon dioxide diffuses the other way, from the blood into the alveoli, and is breathed out. Both gases move down their concentration gradients.

• millions of alveoli: a very large surface area (about 70 m² in an adult); Why it speeds up diffusion more gas can diffuse at the same time
• alveolus wall and capillary wall each one cell thick; moist lining; Why it speeds up diffusion short diffusion distance; gases dissolve before diffusing
• rich network of capillaries; Why it speeds up diffusion blood constantly carries oxygen away and brings carbon dioxide, keeping a steep concentration gradient
• ventilation; Why it speeds up diffusion replaces the air, keeping oxygen high and carbon dioxide low in the alveoli: a steep gradient

Worked example: total surface area

One lung contains about 240 million alveoli. The average surface area of one alveolus is 0.15 mm². Calculate the total surface area of this lung in m². (1 m² = 1 000 000 mm²)
total area = 240 000 000 × 0.15 = 36 000 000 mm²
36 000 000 ÷ 1 000 000 = 36 m²

Answer: 36 m² – about the floor area of a classroom

Breathing rate and the air we breathe

breathing rate (breaths per minute) = number of breaths ÷ number of minutes

Worked example: breathing rate

Ali counts 6 breaths in 20 seconds at rest. Calculate his breathing rate.
20 seconds = 1/3 of a minute, so multiply by 3: 6 × 3 = 18

Answer: 18 breaths per minute

• nitrogen; Inhaled air (%) 78; Exhaled air (%) 78
• oxygen; Inhaled air (%) 21; Exhaled air (%) 16
• carbon dioxide; Inhaled air (%) 0.04; Exhaled air (%) 4

Exhaled air has less oxygen (it was used in respiration) and much more carbon dioxide (made in respiration). It is also warmer and contains more water vapour.

Common mistakes

• Saying "air pressure increases so air rushes in". When you breathe in, the pressure in the thorax decreases.
• Saying the lungs "suck in" air or that the lungs themselves contract. The intercostal muscles and diaphragm do the work.
• Saying alveoli have "thin cell walls". Animal cells have no cell wall: the alveolus wall is one cell thick.
• Writing that exhaled air has no oxygen. It still contains about 16% oxygen.
• Forgetting that diffusion needs a concentration gradient; mention how blood flow and ventilation keep it steep.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the correct path of air into the lungs? (Choose from: bronchi → trachea → alveoli → bronchioles, trachea → bronchi → bronchioles → alveoli, trachea → bronchioles → bronchi → alveoli, alveoli → bronchioles → bronchi → trachea.)

Answer: trachea → bronchi → bronchioles → alveoli

Working: Air passes down the trachea, into the two bronchi, through the bronchioles and into the alveoli.

Question 2. True or false? When you breathe in, the diaphragm contracts and flattens.

Answer: True

Working: True. The diaphragm contracts and flattens and the intercostal muscles contract, so the volume of the thorax increases.

Question 3. Ali takes 8 breaths in 30 seconds. What is his breathing rate in breaths per minute?

Answer: 16

Working: 8 × 2 = 16 breaths per minute.

Related lessons

• Previous lesson: 4.4 Helping the Heart
• Next lesson: 4.6 Tissues and Organs in Plants
• In this chapter: 4.1 The Blood
• In this chapter: 4.2 The Blood Vessels
• In this chapter: 4.3 The Heart

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This lesson is section 4.5 (AGCBH0405) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.