Massachusetts Comprehensive Assessment System (MCAS) Biology Practice Test

What is the primary site for gas exchange in the lungs?

1. Bronchioles

2. Alveoli

3. Bronchi

4. Trachea

The correct answer is: Alveoli

The alveoli are the primary site for gas exchange in the lungs due to their unique structure and function. These tiny, balloon-like sacs are located at the end of the bronchioles and provide a large surface area for the diffusion of gases. This surface area is essential because it allows oxygen to enter the bloodstream and carbon dioxide to exit efficiently. Alveoli are surrounded by a network of capillaries, which are small blood vessels that facilitate this gas exchange process. The walls of the alveoli and the capillaries are extremely thin, creating a short diffusion path that further enhances the efficiency of gas exchange. As oxygen moves from the alveoli into the blood, carbon dioxide moves from the blood into the alveoli to be exhaled. In contrast, the bronchi, bronchioles, and trachea primarily serve roles in conducting air to and from the lungs, rather than participating directly in gas exchange.