Lesson 2 of 6 / Respiration in humans
How breathing moves air
How can moving the ribs and diaphragm draw air into the lungs?
I can explain inhalation and expiration through muscle action, chest volume and air pressure.
I can explain breathing muscles, rib movement and ciliary clearance.Syllabus
Syllabus K325, Topic 6(b). Explain breathing muscles, rib movement and ciliary clearance.
About 5 min
Make a guess. You are not marked.
When you breathe in, which comes first: air rushing in, or your chest getting bigger?
Show the answer
The chest gets bigger first, then air flows in.
Muscles enlarge the chest, the lung pressure falls below the air outside, and air flows in to fill the space.
The key idea
Muscles change chest volume; this changes pressure in the lungs and causes air to flow.
Air moves from higher pressure to lower pressure. Compare the pressure inside the lungs with atmospheric pressure outside.
Positions exaggerate the changes to make the mechanism visible. Air pressure is compared with atmospheric pressure; no gas-exchange arrow is shown here.
Explanation
During inhalation, the diaphragm contracts and becomes flatter, moving downwards. The external intercostal muscles between the ribs contract, raising the ribs upwards and outwards. The internal intercostals involved in forced expiration relax.
These movements increase the volume of the chest cavity and the lungs expand with it. Pressure in the lungs falls below atmospheric pressure, so air flows in. The pressure difference follows the volume change: air entering is not what first makes the diaphragm contract.
During quiet expiration, the diaphragm and external intercostal muscles relax. The diaphragm returns to a dome shape, ribs move down and in, and elastic recoil helps reduce lung volume. Lung pressure rises above atmospheric pressure, so air flows out.
Internal intercostal muscles help pull the ribs down and in during forced expiration, such as forceful breathing out. Quiet expiration does not require them to contract on every breath. Abdominal muscles can also assist forced expiration, but their detailed action is not required here.
The diaphragm is below the lungs. The lungs themselves do not contain skeletal muscles that actively pull air in. Ventilation is a mechanical process; diffusion across alveoli is a separate process.
Step by step
- 1
Name the muscle action
Say contracts or relaxes for the diaphragm and relevant intercostal muscles.
- 2
Follow the volume change
Explain what happens to rib position, diaphragm shape and chest volume.
- 3
Compare air pressures
Lower lung pressure brings air in; higher lung pressure drives air out.
Worked example
Explain inhalation in one chain
Explain how contraction of the diaphragm results in air entering the lungs.
One way to explain it
The diaphragm contracts, flattens and moves down. Chest volume increases and the lungs expand. Lung pressure falls below atmospheric pressure, so air flows into the lungs down the pressure difference.
Why this answer works
- Begin with muscle action.
- Connect shape to volume.
- Compare internal pressure with atmospheric pressure before stating air direction.
Watch out for this
A student says: "The internal intercostal muscles must contract during every quiet exhalation." What is wrong with this?
Show the answer
Quiet exhalation mainly results from relaxation and elastic recoil. Internal intercostals help drive forced expiration.
Check your understanding
The diaphragm flattens and the ribs move up and out. Which sequence follows?
Show the answer
Chest volume increases -> lung pressure falls -> air enters
Air enters because pressure in the expanding lungs falls below atmospheric pressure.
Exam-style questions
Check your understanding
Which set of changes happens when you breathe out at rest?
Show the answer
Diaphragm relaxes and domes up; chest volume falls; pressure rises
A smaller chest volume raises the pressure in the lungs above air pressure, so air moves out.
Quick recall
How can moving the ribs and diaphragm draw air into the lungs?