Gaseous exchange in the lungs takes place in the

1. Analyze the question The question asks which part of the respiratory system is the actual site of gaseous exchange, where oxygen enters the blood and carbon dioxide leaves it.

2. Explain the concept Air travels along this path: Nose → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli

• The trachea, bronchi and bronchioles are air passages. They carry air to and from the lungs but do not exchange gases.
• The alveoli are tiny air sacs at the ends of the bronchioles. They are the site of gaseous exchange.

3. Solution, step by step Alveoli are built for efficient diffusion:

• Their walls are one cell thick (thin), so the diffusion distance is short.
• They are surrounded by a dense network of blood capillaries, which also have thin walls.
• They are moist, so gases dissolve and then diffuse.
• They have a very large surface area, since there are millions of them.

Oxygen diffuses from the alveolar air (high concentration) into the blood (low concentration). Carbon dioxide diffuses from the blood (high concentration) into the alveolar air (low concentration).

Checking the other options:

• B. Bronchi: air passages only
• C. Bronchioles: smaller air passages only
• D. Trachea: the windpipe, which only conducts air

4. Answer The correct option is A. alveoli.

5. Exam tip Remember: "Alveoli = Air exchange." The other options are all tubes, and tubes only transport air. Whenever a question asks where gaseous exchange happens in the lungs, look for the alveoli. In fish, the equivalent site is the gill filaments/lamellae, and in insects it is the tracheoles.