Which of the following is the primary organ involved in gas exchange during respiration in humans?

1. Analyze the question The question asks which organ is the primary site of gas exchange (where oxygen enters the blood and carbon dioxide leaves it). Note that this is different from organs that merely help air move in and out.

2. Explain the concept Respiration in humans involves two ideas:

• Breathing (ventilation): moving air in and out of the body. This is done by the diaphragm, intercostal muscles, trachea, and bronchi.
• Gas exchange: the actual swapping of O₂ and CO₂ between air and blood. This happens in the alveoli, the tiny air sacs inside the lungs.

The alveoli are ideal for exchange because they:

• have very thin walls (one cell thick)
• are surrounded by a dense network of capillaries
• have a large total surface area
• are kept moist, so gases can dissolve and diffuse

3. Step-by-step solution

• A. Diaphragm: a muscle that contracts and relaxes to change chest volume. It aids breathing but does no gas exchange. ❌
• B. Bronchi: air passages that carry air into the lungs. They are tubes only, with no exchange. ❌
• C. Lungs: contain millions of alveoli, where O₂ diffuses into the blood and CO₂ diffuses out. ✅
• D. Trachea: the windpipe, which carries air to the bronchi. It is a passage, not an exchange site. ❌

4. Answer The correct option is C. Lungs.

5. Exam tip Remember: "Diaphragm moves it, tubes carry it, alveoli swap it." If a question asks about gas exchange, think alveoli → lungs. If it asks about air passage, think trachea and bronchi. If it asks about breathing movement, think diaphragm and intercostal muscles.