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

1. Understanding the Role of 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. This thinness is crucial because it allows oxygen to pass from the air in the alveoli into the blood in the capillaries and carbon dioxide to move from the blood into the alveoli to be exhaled.
4. Gas Exchange Process:
5. When you inhale, air enters the alveoli, where oxygen concentration is higher than in the blood. Due to the concentration gradient, oxygen diffuses from the alveoli into the blood.
6. Conversely, carbon dioxide, which is at a higher concentration in the blood than in the alveoli, diffuses from the blood into the alveoli to be expelled from the body during exhalation.
7. Importance of 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, making the process efficient.

Why 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 primary function is to provide a passage for air and to filter, warm, and moisten it.
• B. Bronchi:
• The bronchi are the large air passages that branch from the trachea into each lung. They further divide into smaller bronchioles. Like the trachea, the bronchi are involved in conducting air but do not facilitate gas exchange. They serve as conduits for air to reach the alveoli.
• 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 occurs through diffusion, where oxygen moves into the blood and carbon dioxide moves out.
• The trachea, bronchi, and larynx are important for air conduction but do not participate in gas exchange.

Revision Summary:

• The alveoli are the structures responsible for gas exchange in the respiratory system.
• They allow oxygen to enter the blood and carbon dioxide to exit through diffusion.
• Other structures like the trachea, bronchi, and larynx are involved in air conduction but not in gas exchange.
• Understanding the function of each part of the respiratory system is crucial for grasping how breathing and gas exchange work.