Breathing and respiration
Explain how the diaphragm and ribcage draw air into the lungs, and how breathing differs from respiration.
On this page
Breathing · about 25–35 min
What you need to understand
Breathing happens in the chest; respiration happens in cells.
Definitions
- breathing
the mechanical movement of air into and out of the lungs
For example: The diaphragm and ribcage change chest volume during breathing.- respiration
a cellular process that releases usable energy from food
For example: Respiration occurs in cells and is not another word for inhalation.- ventilation
movement of air into and out of the lungs
For example: Breathing ventilates the lungs for gas exchange.- diaphragm
a sheet of muscle below the lungs that changes chest volume
For example: It contracts and flattens during inhalation.- bronchus
one of the two main air passages leading from the windpipe into a lung
For example: The windpipe divides into two bronchi.
Key idea
- LookDescribe what the diaphragm and ribcage do during inhalation.
- ThinkBreathing happens in the chest; respiration happens in cells.
- DoDistinguish the mechanical movement of air from the cellular energy-releasing process.
Explanation
Air enters through the nose or mouth, travels down the windpipe and passes through the two bronchi into the lungs. In the lungs, oxygen passes from air into the blood and carbon dioxide passes from blood into the air. Blood carries oxygen to cells and brings carbon dioxide back to the lungs.
During inhalation (breathing in), the diaphragm contracts and flattens. Muscles between the ribs move the ribcage up and out. The space inside the chest increases and the lungs expand. Air pressure inside the lungs falls below the pressure outside, so air flows in. The diaphragm does not pull air directly: its movement helps change the volume of the lungs.
During normal exhalation (breathing out) at rest, the diaphragm and rib muscles relax. The diaphragm becomes dome-shaped and the ribcage moves down and in. The lungs recoil towards their resting size. Air pressure inside rises above the pressure outside, so air flows out.
The ribcage also protects the lungs. The windpipe and bronchi carry air; the lungs provide the surfaces for gas exchange. These parts work together, but they have different jobs.
Breathing, also called ventilation, moves air. Cellular respiration releases usable energy from food. Cells need this energy for activities such as movement and growth. Breathing supports respiration by supplying oxygen and removing carbon dioxide; chest movement is not the energy-releasing process itself.
Pause and say it: Breathing happens in the chest; respiration happens in cells.
Common mistake
Tempting wrong idea: Cellular respiration stops while a person inhales.
Why it fails: Cellular respiration releases energy inside living cells all the time. Breathing in and out only moves air into and out of the lungs; it does not switch respiration on or off.
Use this instead: Cells continue releasing usable energy during both inhalation and exhalation. Chest movement ventilates the lungs; it does not switch cellular respiration off.
Practical work
What to show: Distinguish the mechanical movement of air from the cellular energy-releasing process.
Before you finish: The heart pumps blood; the lungs do not pump themselves like the heart.
Practical link: The topic investigation is taught in “Compare inhaled and exhaled air”
6. Worked examples
Modelled example 1
Inhalation
Problem
Describe what the diaphragm and ribcage do during inhalation.
Study the worked solution
Reason from the evidence
Method
Link diaphragm and ribcage movement to the change in chest volume.
Reason
These mechanical changes ventilate the lungs; respiration is the separate energy-releasing process in cells.
Working
- The diaphragm contracts and flattens.
- The ribcage moves up and out.
- The space inside the chest increases and the lungs expand.
Air pressure inside the lungs falls below outside air pressure, so air enters.
State the conclusion
Working
These movements ventilate the lungs; they are not the cellular process of respiration.
Common misconception 2
Breathing and respiration are different processes
Learner response
“Respiration happens when the lungs breathe oxygen in and carbon dioxide out.” Correct the statement.
Define both processes
View solution step by step
Separate place and purpose
Method
Describe breathing as ventilation and respiration as a cellular process.
Reason
The linked processes occur in different places and do different jobs.
Working
Breathing moves air into and out of the lungs. Respiration happens in cells and releases usable energy from food. Breathing supports respiration by helping supply oxygen and remove carbon dioxide.
Challenge 3
Work backwards from chest movement
Problem
A scan shows that a person’s ribcage is moving down and in while the diaphragm becomes dome-shaped. Identify the phase of breathing and explain the resulting movement of air.
Build a movement → volume → air chain
Hints
Hint 1: volume
Do these movements increase or decrease chest volume?
Hint 2: direction
During which phase does air leave the lungs?
View solution step by step
Follow the mechanical chain
Method
Use the two observed movements to infer the volume change.
Reason
During normal breathing out at rest, the lungs recoil as chest volume decreases. Air moves from higher to lower pressure.
Working
The person is exhaling. The diaphragm has relaxed and become dome-shaped, and the ribcage moves down and in. The lungs recoil as chest volume decreases. Pressure inside the lungs rises above outside air pressure, so air moves out.
Guided practice
Try it with support
Describe inhalation using the ribcage, diaphragm and chest volume, then distinguish it from respiration.
Give the movement before the volume change.
State where respiration occurs.
Check the guided answer
Answer: The ribcage moves up and out, the diaphragm contracts and flattens. The lungs expand as chest volume increases. Pressure inside the lungs falls below outside air pressure, so air enters. Respiration is the cellular process that releases usable energy from food.
Check: The lungs provide gas-exchange surfaces; the ribcage also protects them.
Practise and continue
Practise this
Trace air from the windpipe into the lungs and state how the diaphragm helps during breathing.
Need a hint?
Use windpipe, bronchi and lungs in order.
Check your answer
Answer: Air travels through the windpipe, which divides into bronchi leading to the lungs. Diaphragm movement changes chest volume and helps move air in or out.
Check: The heart pumps blood; the lungs do not pump themselves like the heart.
Think like a scientist
A model has two balloons inside a sealed bottle. A tube connects them to the outside air, and a rubber sheet seals the open base. Pulling the sheet down inflates the balloons. What does this model help explain, and what does it leave out?
Check the reasoning
Pulling the sheet down increases the space around the balloons, allowing them to expand as air flows through the tube. It models how a volume change can move air. The balloons represent lungs, the tube represents air passages and the sheet represents the diaphragm. The bottle is rigid, so it cannot show ribcage movement. The sheet also does not contract like a real muscle, and the model does not show gas exchange with blood.
Remember: A model can explain one relationship without copying every feature.
One-minute check
Hide the page. Explain how diaphragm and ribcage movement causes air to enter the lungs.
Compare the diaphragm during breathing in and normal breathing out at rest.
Correct this common mistake: “Cellular respiration stops while a person inhales.”
Syllabus and review details
- SEC G1 Science 2027 · 2027
Content structure and syllabus content, PDF pages 8–17
Last reviewed: