Correct Option: B. 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 blood in our circulatory system. 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 (windpipe) is the main airway that conducts air from the larynx to the bronchi.
4. The bronchi are the two main branches that lead from the trachea into each lung, further dividing into smaller bronchioles.
5. The alveoli are tiny, balloon-like structures located at the end of the bronchioles.
6. Structure of Alveoli:
7. Alveoli are small, thin-walled sacs surrounded by a network of capillaries (tiny blood vessels).
8. The walls of the alveoli are only one cell thick, which allows for efficient gas exchange.
9. Gas Exchange Process:
10. When air enters the alveoli, oxygen diffuses across the alveolar wall into the capillaries, where it binds to hemoglobin in red blood cells.
11. Simultaneously, carbon dioxide, which is a waste product of cellular metabolism, diffuses from the blood into the alveoli to be exhaled.
12. This process is driven by the concentration gradients of the gases: oxygen moves from an area of higher concentration (in the alveoli) to lower concentration (in the blood), and carbon dioxide moves in the opposite direction.
13. Importance of Alveoli:
14. The large surface area provided by millions of alveoli in the lungs maximizes the efficiency of gas exchange.
15. The thin walls and proximity to capillaries ensure that gases can move quickly and effectively.

Why Other Options Are Incorrect:

• A. Trachea:
• The trachea is primarily a passageway for air; it does not participate in gas exchange. Its main function is to conduct air to the bronchi and lungs.
• C. 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 small, thin-walled sacs surrounded by capillaries, allowing for efficient diffusion of oxygen and carbon dioxide.
• Other structures like the trachea, bronchi, and larynx serve different functions and do not participate in gas exchange.
• Understanding the role of alveoli is crucial for grasping how the respiratory system functions in oxygenating blood and removing carbon dioxide.

This comprehensive understanding of the alveoli's role in gas exchange is essential for anyone studying biology, particularly in the context of human physiology and respiratory health.