Lesson 1 of 6 / Respiration in humans
Follow air into the lungs
Which structures move air, and where do gases actually cross into blood?
I can identify the airway structures, exchange surfaces and the role of cilia.
I can identify airways, alveoli and capillaries and explain their roles.Syllabus
Syllabus K325, Topic 6(a). Identify airways, alveoli and capillaries and explain their roles.
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 4 min
Make a guess. You are not marked.
Air reaches the alveoli deep in your lungs. Which tubes does it travel through to get there?
Show the answer
Trachea, bronchi, then bronchioles.
Air stays in airways all the way. Capillaries carry blood beside the alveoli, not air.
The key idea
Airways conduct air to alveoli; the closely associated capillaries carry blood past the exchange surface.
Explanation
Air passes through the larynx, or voice box, into the trachea. The trachea is an airway supported by cartilage that helps prevent collapse. It divides into two bronchi, one leading into each lung.
The bronchi branch into smaller bronchioles. These distribute air through the lungs and lead to alveoli, the tiny air spaces where gas exchange takes place. A branching airway brings air close to a large total exchange surface.
Capillaries closely surround the alveoli. They bring blood with relatively little oxygen and carry blood away after it has gained oxygen. Air is inside the alveolus; blood stays inside the capillary. These are neighbouring spaces, not one shared tube.
Mucus in larger airways traps dust and pathogens. Cilia are microscopic projections on the lining cells that beat to move mucus towards the throat, where it can be swallowed or removed. Cilia do not create the pressure changes that draw air into the lungs.
Keep three processes separate: ventilation moves air into and out of the lungs, gas exchange transfers oxygen and carbon dioxide by diffusion, and cellular respiration releases energy inside cells.
Step by step
- 1
Start at the top
Trace the larynx and trachea before the branching bronchi and bronchioles.
- 2
Find the exchange site
Name the alveoli and the capillaries beside them.
- 3
Assign the role
Airway conduction, mucus clearance and gas exchange are different functions.
Worked example
A trapped dust particle
A dust particle sticks to mucus in the trachea. Explain how the airway lining helps remove it.
One way to explain it
Mucus traps the particle. Cilia on the lining cells beat to move the mucus towards the throat, allowing it to be swallowed or removed instead of remaining in the airway.
Why this answer works
- Mucus traps; cilia move.
- State the direction towards the throat.
- Do not confuse ciliary action with inhalation.
Watch out for this
A student says: "Air travels through capillaries to reach the alveoli." What is wrong with this?
Show the answer
Air travels through bronchioles to alveoli. Capillaries carry blood beside the alveoli.
Check your understanding
Which route correctly takes inhaled air towards its main gas-exchange surface?
Show the answer
Larynx -> trachea -> bronchi -> bronchioles -> alveoli
Gas exchange occurs across the thin alveolar and capillary walls at the end of this route.
Quick recall
Which structures move air, and where do gases actually cross into blood?