Lesson focus: ventilationBreathing: moving air in and out of the lungs. moves air; gas exchange diffuses gases; respiration releases energy in cells.
Explain inhalationBreathing in, when air moves into the lungs. and quiet exhalationBreathing out, when air moves out of the lungs. using muscles, volume and pressure.
Build on earlier learning
Gas exchangeDistinguish movement of air from diffusion at alveoli.
Practical
Bell-jar lung model
- Pull the rubber sheet (diaphragm) down: the volume inside the jar increases and the pressure falls.
- Air enters the balloons (lungs), which inflate.
- Push the sheet up: the volume decreases, the pressure rises and air leaves the balloons.
Record: A table of each movement and its effect on volume, pressure and air flow, then the limits of the model (no ribs, rigid jar).
Bell-jar model of breathing
Pulling the rubber sheet down is like the diaphragm contracting: the volume inside goes up, the pressure goes down and air is pushed into the balloons by the air outside.
Controls
The model has no ribs. In the body the intercostal muscles also move the ribs up and out.
You will be successful when you can…
- Describe diaphragm and rib movement.
- Link thoracic volume to pressure.
- Explain airflow down a pressure gradient.
- Distinguish ventilation, gas exchange and respiration.
Words that carry the science
- inhalation
- Movement of air into lungs.
- exhalation
- Movement of air out of lungs.
- diaphragm
- Sheet of muscle beneath lungs involved in ventilation.
- intercostal muscles
- Muscles between ribs that move the rib cage.
- thoracic volume
- Volume inside the chest cavity.
Connect muscle action to pressure and airflow
01Inhalation is active
The diaphragmA sheet of muscle under the lungs that helps you breathe in and out. contracts and flattens; external intercostal musclesMuscles between the ribs that move the ribcage when you breathe. contract, lifting ribs up and out. thoracic volumeThe amount of space inside your chest, which gets bigger when you breathe in. increases.
Lung pressure falls below atmospheric pressure, so air flows in down the pressure gradient. In simple steps: the diaphragmA sheet of muscle under the lungs that helps you breathe in and out. (a sheet of muscle under the lungs) contracts and flattens. The rib musclesMuscles between the ribs that pull the ribs up and out when you breathe in. contract and lift the ribs up and out. The chest gets bigger, so the pressure inside drops. Air rushes in from outside, where the pressure is higher.
02Quiet exhalation uses recoil
The diaphragm and external intercostals relax. Elastic recoil reduces lung and chest volume, pressure rises above atmospheric pressure and air flows out.
Forced exhalationBreathing out, when air moves out of the lungs. recruits muscles, so not all exhalation is passive.
03Three processes, three locations
ventilationBreathing: moving air in and out of the lungs. moves air; gas exchange diffuses gases across alveoli; cellular respiration releases energy in cells. Exercise raises cellular demand, so circulation and ventilation adjust to support gradients and transport.
Explain inhalation as a chain
Why does contracting the diaphragm move air into the lungs?
- Contraction flattens the diaphragm.
- thoracic volumeThe amount of space inside your chest, which gets bigger when you breathe in. increases.
- Pressure inside lungs falls below atmospheric pressure.
- Air moves into lungs down the pressure gradient.
How to make the reasoning visible: Direction of airflow follows pressure difference, not the diaphragm “sucking” air.
Ventilation model interpretation
A bottle model can represent pressure–volume changes but has limits.
| Model part | Represents | Useful idea | Limitation |
|---|---|---|---|
| Bottle | Chest cavity | Enclosed volume | Rigid unlike rib cage |
| Balloons | Lungs | Inflate as pressure falls | Do not show alveoli |
| Rubber sheet | Diaphragm | Volume change alters pressure | Pulling by hand differs from contraction |
| Tube | Trachea/bronchi | Air pathway | Simplified branching |
What happens when the rubber sheet is pulled down?
Why does the model not fully represent ribs?
In the bell-jar model, what happens to the balloons when the rubber sheet is pushed back up? Why?
Check your understanding
Answer from memory, then check against the suggested answer.
What does the diaphragm do during inhalation?
Why does air enter?
What is gas exchange?
What happens to the ribs when you breathe in?
What happens to the diaphragm when you breathe out?
During quiet exhalation, what happens to volume and pressure?
Why do breathing rate and depth increase during exercise?
Breathing, gas exchange and respiration are three different things. Say where each one happens.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Answer the parts to explain how air gets into the lungs when you breathe in.
What does the diaphragm do?
(1)What do the rib muscles do to the ribs?
(1)What happens to the size of the chest?
(1)What happens to the pressure inside the chest?
(1)So which way does the air move?
(1)Total for this question: 5 marks
Command word: Follow the order: muscles → chest size → pressure → air moves.