Correct Option: C. Alveoli Detailed Explanation: The primary structure in the respiratory system responsible for the exchange of gases (oxygen and carbon dioxide) between the air and the blood is the alveoli. Here’s a step-by-step breakdown of why this is the correct answer:

1. Structure of the Alveoli:
2. Alveoli are tiny, balloon-like structures located at the end of the bronchioles in the lungs. They are surrounded by a network of capillaries (small blood vessels).
3. The walls of the alveoli are extremely thin (one cell thick), which facilitates the diffusion of gases.
4. Function of the Alveoli:
5. The primary function of the alveoli is gas exchange. When air enters the lungs, it travels down the trachea, through the bronchi, and into the bronchioles, finally reaching the alveoli.
6. Oxygen from the inhaled air diffuses through the thin walls of the alveoli into the blood in the capillaries. Simultaneously, carbon dioxide from the blood diffuses into the alveoli to be exhaled.
7. Surface Area:
8. The alveoli provide a large surface area for gas exchange. The human lungs contain approximately 300 million alveoli, which significantly increases the area available for oxygen and carbon dioxide exchange.
9. Role in Respiration:
10. The process of gas exchange in the alveoli is crucial for respiration. Oxygen is essential for cellular metabolism, while carbon dioxide is a waste product that needs to be expelled from the body.

Why the Other Options Are Incorrect:

• A. Trachea:
• The trachea, or windpipe, is the main airway that conducts air from the larynx to the bronchi. While it is essential for directing air into the lungs, it does not participate in gas exchange. Its walls are made of cartilage and are lined with cilia and mucus to trap particles, but it does not have the thin walls necessary for gas diffusion.
• B. Bronchi:
• The bronchi are the large air passages that branch from the trachea into each lung. Like the trachea, they serve as conduits for air but do not facilitate gas exchange. The bronchi lead to smaller bronchioles, which eventually end in the alveoli where gas exchange occurs.
• D. Larynx:
• The larynx, or voice box, is located above the trachea and is primarily involved in sound production and protecting the airway during swallowing. It does not play a role in gas exchange.

Summary of Key Points:

• Alveoli are the primary sites for gas exchange in the lungs due to their large surface area and thin walls.
• Gas Exchange Process: Oxygen diffuses into the blood, and carbon dioxide diffuses out into the alveoli.
• Other Structures: The trachea, bronchi, and larynx are important for air conduction but do not participate in gas exchange.
• Understanding the function of alveoli is crucial for grasping how the respiratory system supports cellular respiration and overall metabolism.

This thorough understanding of the respiratory system's anatomy and function is essential for any biology exam focused on human physiology.