Lesson 1 of 5 / Respiration in humans
Follow air into the lungs
Which structures move air, and where do gases actually cross into blood?
In this lesson: Identify airway structures, alveoli and associated capillaries and state their functions.
About 4 min
The key ideaAirways 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 conducts air and divides into two bronchi, one entering each lung. These structures provide an open route for air, rather than being the main gas-exchange surface.
Each bronchus branches into smaller bronchioles. These distribute air throughout the lungs and lead to alveoli, the small air spaces where oxygen and carbon dioxide are exchanged with blood.
Capillaries lie close to alveoli. They bring blood with relatively little oxygen and carry blood away after it has gained oxygen. Air is inside an alveolus while blood stays inside a capillary: these are adjacent spaces, not one continuous tube.
Keep three processes separate. Ventilation moves air into and out of the lungs. Gas exchange transfers gases by diffusion across the alveolar and capillary walls. Cellular respiration releases energy from glucose inside cells.
Step by step
- 1
Trace the air
Follow larynx, trachea, bronchi, bronchioles and alveoli.
- 2
Locate the blood
Capillaries carry blood beside alveoli, not inhaled air.
- 3
Name the crossing
Oxygen and carbon dioxide diffuse between the two spaces.
Worked example
Two routes meet at an exchange surface
Trace air from the larynx to an alveolus, then explain what the nearby capillary carries.
One way to explain it
Air passes through the larynx, trachea, a bronchus and branching bronchioles to alveoli. The nearby capillary carries blood, which gains oxygen by diffusion from alveolar air and loses carbon dioxide to it.
Why this answer works
- Give the airway order.
- Keep blood separate from air.
- Link the adjacent routes through diffusion, not a shared tube.
Is this true? "Air travels through capillaries to reach the alveoli."
Air travels through bronchioles to alveoli. Capillaries carry blood beside the alveoli.