Trace the gas exchange system
Follow air through the human gas exchange system from trachea to alveoli and connect alveoli to their capillary network.
Section 9.1 · The gas exchange system
AS Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; microscope recognition, biological drawing and supervised practical competence require practical work, while human subject-teacher review remains pending.
Before you begin
- Know that gas exchange is diffusion across a surface and that ventilation renews air while blood flow renews the blood side.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which structures directly provide the main gas-exchange surface?
What you will learn
- Describe the system using lungs, trachea, bronchi, bronchioles, alveoli and capillary network
- Trace the route between external air and blood
Identify larynx trachea bronchi and bronchioles in a diagram
Build the explanation
Air travels through the trachea, which branches into bronchi, then into progressively smaller bronchioles and finally to alveoli. The trachea and bronchi conduct air; bronchioles distribute air within the lungs; alveoli provide the exchange surface. Each alveolus is closely associated with a capillary network, which carries blood alongside the air space. The lungs contain branching airways and many alveoli rather than one large chamber.
Apply it carefully
Keep the route and the exchange boundary distinct: inhaled air reaches an alveolar air space; oxygen diffuses across the thin wall into blood in nearby capillaries, while carbon dioxide diffuses in the opposite direction. Bronchi and bronchioles conduct air but are not the principal gas-exchange surface. The syllabus structure list does not require adding extra organs when the question asks only for the stated route.
Pause and explain
What travels alongside the alveoli for exchange with blood?
Worked example
Complete the route: trachea → bronchi → ___ → alveoli.
Show the worked solution
- Follow the branches from larger airways to smaller ones.
- Bronchioles are the smaller airways that lead to alveoli.
Answer Trachea → bronchi → bronchioles → alveoli.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Trace air from the trachea to an alveolus and name the blood vessel network at the exchange surface.
Give me a hint
Move from larger to smaller branching airways.
Compare my reasoning
- Trachea branches into bronchi.
- Bronchi branch into bronchioles, which lead to alveoli with an associated capillary network.
Air passes through the trachea, bronchi and bronchioles to the alveoli. A capillary network lies alongside alveoli for exchange with blood.
Look for these in your work
- Airway order correct
- Alveoli identified
- Capillary network included
Explain why a bronchus should not be called the main gas-exchange surface.
Give me a hint
Compare air conduction with exchange at a thin surface.
Compare my reasoning
- A bronchus conducts air to smaller airways.
- Gas exchange takes place mainly between alveolar air and capillary blood.
A bronchus is an airway that conducts air and branches into bronchioles. The main exchange occurs at the alveoli, where air and capillary blood are separated by a thin barrier.
Look for these in your work
- Bronchus role stated
- Alveolar exchange location stated
A diagram labels a capillary within the alveolar air space. Correct the relationship.
Give me a hint
The air and blood compartments do not merge.
Compare my reasoning
- The alveolus contains air.
- Capillaries run outside the air space, closely against the alveolar wall.
The alveolus is an air space. Capillaries lie in the surrounding tissue close to the alveolar wall, so gases diffuse across the separating barrier; blood does not enter the air space.
Look for these in your work
- Air/blood compartments separated
- Capillary location corrected
- Diffusion boundary stated
Common mistakes
- Placing bronchioles after alveoli
- Describing a bronchus as the main exchange surface
- Drawing blood inside the alveolar air space
What can you explain now?
Which structures directly provide the main gas-exchange surface?
Compare with the explanation
Alveoli
Alveoli provide a large total surface closely supplied by capillaries.
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Make it stick
What is the airway sequence to an alveolus, and what lies on the blood side of its wall?
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