How does air get into and out of the lungs when we breathe?
Describe the parts of the human breathing system and explain how breathing in and breathing out happen
A scaffolded answer to the N(A)-Level Biology outcome on the breathing system. The path of air to the lungs, the role of the ribs and diaphragm, and how the chest changes shape to breathe in and out.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
This outcome wants you to name the main parts of the human breathing system and trace the path of air to the lungs. You should be able to explain how breathing in and breathing out happen, using the diaphragm, the ribs, and changes in the volume and pressure of the chest. The marks reward the correct order of structures and a clear, step-by-step account of ventilation. Remember that breathing is the movement of air, not the same as respiration.
The answer
The parts of the breathing system
Air travels to the lungs along this path:
- Nose and mouth. Air enters here. The nose warms and moistens the air and traps dust.
- Windpipe (trachea). A tube carrying air down towards the lungs. It is held open by rings of cartilage.
- Bronchi. The trachea splits into two bronchi, one going to each lung.
- Bronchioles. Each bronchus branches into many smaller tubes called bronchioles.
- Alveoli. The bronchioles end in millions of tiny air sacs called alveoli, where gas exchange takes place.
The lungs sit inside the chest, protected by the ribs. Below them is a sheet of muscle called the diaphragm.
How we breathe in
Breathing in is called inhaling. It happens like this:
- The diaphragm contracts and moves down, becoming flatter.
- The rib muscles contract and pull the ribs up and out.
- This makes the space inside the chest larger (its volume increases).
- A bigger space means lower pressure inside the lungs than outside, so air is pushed in.
How we breathe out
Breathing out is called exhaling. It is the opposite:
- The diaphragm relaxes and moves up, becoming dome-shaped again.
- The rib muscles relax and the ribs move down and in.
- This makes the space inside the chest smaller (its volume decreases).
- A smaller space means higher pressure inside the lungs, so air is pushed out.
Examples in context
Example 1. Why you breathe faster when you exercise. During exercise the muscles respire faster and need more oxygen, and they make more carbon dioxide. You breathe faster and deeper to take in more oxygen and remove the extra carbon dioxide. This links breathing (moving air) to respiration (using the oxygen in the cells).
Example 2. Why smoking damages the breathing system. Tobacco smoke damages the tiny hairs that sweep dust and germs out of the airways, and it harms the delicate alveoli. This makes breathing harder and gas exchange less efficient, which is why smokers often become breathless. It shows how the structure of the airways suits keeping them clean.
Try this
Q1. Name the tiny air sacs in the lungs where gas exchange happens. [1 mark]
- Cue. The alveoli.
Q2. State what happens to the diaphragm when you breathe in. [1 mark]
- Cue. It contracts and moves down (flattens).
Q3. Explain why air flows into the lungs when the chest gets bigger. [2 marks]
- Cue. A bigger chest means lower pressure inside the lungs than outside, so air is pushed in from the higher pressure outside.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksDescribe the path that air takes from the nose to the place where gas exchange happens in the lungs. Name the main parts it passes through.Show worked answer →
Air enters through the nose (or mouth), where it is warmed and moistened. It passes down the windpipe (trachea), which branches into two tubes called bronchi, one going to each lung. Each bronchus branches again and again into smaller tubes called bronchioles. The bronchioles end in tiny air sacs called alveoli, where gas exchange takes place.
What markers reward: the correct order (nose, trachea, bronchus, bronchioles, alveoli) and naming the alveoli as the place where gas exchange happens. Spelling does not need to be perfect, but the order must be right.
Original5 marksExplain what happens to the diaphragm, the ribs and the volume of the chest when a person breathes in.Show worked answer →
When a person breathes in (inhales): the diaphragm contracts and moves down, becoming flatter. The rib muscles contract and pull the ribs up and out. Both of these make the space inside the chest larger, so its volume increases. This lowers the pressure inside the lungs below the air pressure outside, so air is pushed into the lungs from outside.
What markers reward: diaphragm contracts and flattens (moves down), ribs move up and out, chest volume increases, pressure inside drops, so air flows in. Saying the diaphragm moves up when breathing in is the most common error.
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