CAPF AC 2020 Q15: Science & Technology – Biology
During respiration in human beings, the actual place of gaseous exchange is
- (a) Bronchi
- (b) Nasal passage
- (c) Alveoli
- (d) Trachea
Answer: (c) Alveoli
Explanation
Where gases are exchanged in the lungs
How to think: separate the tubes from the sacs.
Tubes only carry air.
Exchange needs a thin wall next to blood, which only the air sacs have.
Air travels through a series of tubes before it reaches the alveoli.
The tubes only carry the air.
The alveoli are where oxygen enters the blood and carbon dioxide leaves it.
| Part | Job | Gas exchange? |
|---|---|---|
| Nasal passage | Filters, warms and moistens the air | No |
| Trachea (windpipe) | Carries air; kept open by cartilage rings | No |
| Bronchi | Two branches of the trachea, one to each lung | No |
| Alveoli | Tiny air sacs at the ends of the bronchioles | Yes |
Why alveoli suit gas exchange:
| Feature | Detail |
|---|---|
| Thin wall | Only one cell thick |
| Blood supply | Covered by a dense net of capillaries |
| Large area | Millions of alveoli give a very large surface |
| Process | Gases move by simple diffusion |
Answer: (c) Alveoli.
Why each option is right or wrong
- (a) Incorrect. The bronchi only carry air into each lung.
- (b) Incorrect. The nasal passage filters, warms and moistens the air.
- (c) CORRECT. The alveoli are the actual place of gaseous exchange with the blood.
- (d) Incorrect. The trachea is the windpipe; it only conducts air.
Key point
Path of air: nasal passage, pharynx, larynx, trachea, bronchi, bronchioles, alveoli. Gaseous exchange happens only at the alveoli, by simple diffusion.
Remember
Gaseous exchange happens in the alveoli, by diffusion.
Common mistakes
- Choosing bronchi because they are inside the lungs.
- Thinking gases move by active transport; it is simple diffusion.
- Confusing breathing (moving air in and out) with gaseous exchange at the alveoli.
Exam tip
Tubes conduct, sacs exchange. Nasal passage, trachea and bronchi are tubes; alveoli are sacs.
Sources
- NCERT Class XI Biology — Breathing and Exchange of Gases
- NCERT Class X Science — Life Processes (respiration)