OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

In which part of the lungs does gas exchange occur?

1. Bronchi

2. Trachea

3. Alveoli

4. Diaphragm

The correct answer is: Alveoli

Gas exchange occurs in the alveoli, which are tiny, balloon-like structures located at the end of the bronchioles in the lungs. They are specialized for this process due to their large surface area and thin walls, which allow for efficient diffusion of oxygen and carbon dioxide between the air in the alveoli and the blood in the surrounding capillaries. The walls of the alveoli are extremely thin, facilitating the rapid exchange of gases; oxygen diffuses into the blood and carbon dioxide is expelled from the blood into the alveoli to be exhaled. The other structures mentioned do not perform gas exchange. The bronchi are the large air passages that lead from the trachea to the lungs, playing a role in transporting air but not in the exchange of gases. The trachea serves as the windpipe, conducting air to the bronchi and preventing the entry of foreign materials, but it also does not facilitate gas exchange. The diaphragm is a muscle that assists with breathing by contracting and relaxing to change the pressure in the thoracic cavity, allowing air to be drawn into and pushed out of the lungs, but it too does not directly engage in the gas exchange process.