Correct Option: C. Alveoli Detailed Explanation: The primary function of the respiratory system is to facilitate the exchange of gases—specifically oxygen (O₂) and carbon dioxide (CO₂)—between the air we breathe and the bloodstream. The structure that is primarily responsible for this gas exchange is the alveoli. Step-by-Step Explanation:

1. Anatomy of the Respiratory System:
2. The respiratory system includes several key structures: the trachea, bronchi, bronchioles, and alveoli.
3. The trachea is the windpipe that conducts air from the throat to the bronchi.
4. The bronchi are the two main branches that lead from the trachea into each lung.
5. The bronchi further divide into smaller bronchioles, which eventually lead to the alveoli.
6. Structure of Alveoli:
7. Alveoli are tiny, balloon-like structures located at the end of the bronchioles.
8. They are surrounded by a network of capillaries (small blood vessels) that facilitate the exchange of gases.
9. The walls of the alveoli are extremely thin (one cell thick), which allows for efficient diffusion of gases.
10. Gas Exchange Process:
11. When we inhale, air enters the alveoli, where oxygen from the air diffuses across the alveolar wall into the blood in the capillaries.
12. Simultaneously, carbon dioxide, which is a waste product of cellular metabolism, diffuses from the blood into the alveoli to be exhaled.
13. This process is driven by the concentration gradient: oxygen moves from an area of higher concentration (in the alveoli) to an area of lower concentration (in the blood), and vice versa for carbon dioxide.
14. Importance of Alveoli:
15. The large surface area provided by millions of alveoli in the lungs maximizes the efficiency of gas exchange.
16. The moist lining of the alveoli also aids in the diffusion of gases.

Why Other Options Are Incorrect:

• A. Trachea:
• The trachea is primarily a passageway for air; it does not participate in gas exchange. Its role is to conduct air to the bronchi and lungs.
• B. Bronchi:
• Similar to the trachea, the bronchi serve as air passageways. They branch off from the trachea and lead to the lungs but do not facilitate gas exchange themselves.
• D. Larynx:
• The larynx, or voice box, is involved in sound production and protecting the airway during swallowing. It does not play a role in gas exchange.

Summary of Key Points:

• The alveoli are the primary sites for gas exchange in the respiratory system.
• They are tiny, thin-walled structures that allow oxygen to enter the blood and carbon dioxide to exit.
• Other structures like the trachea, bronchi, and larynx serve different functions and do not participate in gas exchange.
• The efficiency of gas exchange is enhanced by the large surface area and thin walls of the alveoli.

This understanding of the respiratory system is crucial for grasping how our bodies obtain oxygen and eliminate carbon dioxide, which is vital for cellular respiration and overall metabolic processes.