Q11 - Anatomy and physiology of humans

The correct answer is D) Alveoli.

Gas exchange needs a very thin surface with a huge area, in close contact with blood. The alveoli are tiny air sacs with walls one cell thick, each wrapped in capillaries. O₂ diffuses from the air into the blood, and CO₂ diffuses the opposite way.

A) Incorrect: The left atrium receives blood that has already been oxygenated in the lungs. No gas exchange happens there.

B) Incorrect: The diaphragm is the main breathing muscle. It moves air in and out, but no gas crosses it.

C) Incorrect: The trachea only conducts air to the bronchi. Its walls are too thick for gas exchange.

D) Correct: The alveoli are the site of gas exchange between air and blood.

E) Incorrect: The aorta carries oxygenated blood from the left ventricle to the body. It does not exchange gas with air.