Gas exchange in humans — Edexcel International GCSE Biology
The structure of the thorax, how ventilation works, how alveoli are adapted, the effects of smoking and the breathing practical.
Revision notes
5 short notes, in the order of the specification. Each one in short:
The thorax is the chest, which contains the lungs and heart and is protected by the ribs. Air passes through the trachea, bronchi and bronchioles to the alveoli, where gas exchange happens. The intercostal muscles between the ribs and the diaphragm below the lungs move the chest, and fluid between the pleural membranes reduces friction.
Ventilation is the movement of air into and out of the lungs. When you breathe in, the intercostal muscles and diaphragm contract, the ribs move up and out and the diaphragm flattens. The volume of the thorax increases, so the pressure falls below atmospheric pressure and air moves in. Breathing out reverses these changes.
Alveoli are tiny air sacs in the lungs where oxygen diffuses into the blood and carbon dioxide diffuses out. They are adapted for gas exchange: millions of alveoli give a very large surface area, walls one cell thick give a short diffusion distance, and a rich capillary network and ventilation keep a steep concentration gradient.
Smoking damages the lungs and circulatory system. Tar contains carcinogens that cause lung cancer and destroys cilia, so mucus is not removed. Carbon monoxide combines with haemoglobin, so red blood cells carry less oxygen. Nicotine is addictive and raises heart rate and blood pressure. Smoking also causes chronic bronchitis and emphysema and increases the risk of coronary heart disease.
Limewater shows that exhaled air contains more carbon dioxide than inhaled air, because the exhaled-air tube turns cloudy much faster. The carbon dioxide comes from respiration in body cells. After exercise the breathing rate is higher and breathing is deeper, because exercising muscles respire faster, so extra carbon dioxide must be removed and more oxygen taken in.
Exam questions
7 exam-style questions (24 marks), each with its mark scheme.
Answer the questionsFlashcards
13 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of structure and functions in living organisms on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Quick check questions
Name the muscles that contract when you breathe in.
Show answer
The intercostal muscles and the diaphragm.
What happens to the pressure in the thorax when you breathe in?
Show answer
It decreases, because the volume of the thorax increases.
Why are alveolus walls only one cell thick?
Show answer
To give a short diffusion distance so gases diffuse quickly.
Which substance in tobacco smoke combines with haemoglobin?
Show answer
Carbon monoxide.
What happens to limewater when carbon dioxide is bubbled through it?
Show answer
It turns cloudy (milky).
Frequently asked questions
How are alveoli adapted for gas exchange?
Alveoli are adapted for gas exchange by diffusion. Millions of alveoli give a very large surface area, and their walls and the capillary walls are each one cell thick, giving a short diffusion distance. A rich capillary network and ventilation maintain a steep concentration gradient, and the moist lining lets gases dissolve.
What happens when you breathe in?
When you breathe in, the intercostal muscles contract and move the ribs up and out, and the diaphragm contracts and flattens. This increases the volume of the thorax, so the pressure inside falls below atmospheric pressure. Air moves from the higher pressure outside into the lungs. The lungs themselves have no muscle.
How does smoking affect the lungs?
Smoking damages the lungs because tar destroys the cilia, so mucus and trapped microorganisms are not removed, causing infections and a smoker's cough. Smoke irritates the airways, causing chronic bronchitis. In emphysema the alveolus walls break down, reducing the surface area for gas exchange. Carcinogens in tar can cause lung cancer.
Why does carbon monoxide reduce oxygen in the blood?
Carbon monoxide reduces the oxygen carried in the blood because it combines with haemoglobin in red blood cells, so less haemoglobin is available to carry oxygen. The heart has to work harder to supply the body with enough oxygen. This is one way that smoking increases the risk of coronary heart disease.
Why does breathing rate increase during exercise?
Breathing rate increases during exercise because the exercising muscles respire faster, making more carbon dioxide and needing more oxygen. Faster, deeper breathing removes the extra carbon dioxide and takes in more oxygen for respiration. After exercise, the breathing rate takes a few minutes to return to the resting rate.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026