Correct Option: C. Alveoli Detailed Explanation: The primary structure in the respiratory system responsible for gas exchange between the air and the 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 (tiny 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 inside 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 vast surface area (about the size of a tennis court when combined) for gas exchange, making the process efficient. This large surface area is essential for meeting the oxygen demands of the body and removing carbon dioxide effectively.

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 clear airway for air to enter and exit the lungs.
• B. Bronchi:
• The bronchi are the large air passages that branch from the trachea into each lung. They serve as conduits for air to reach the bronchioles and alveoli. Like the trachea, they do not facilitate gas exchange; their role is primarily to transport air.
• D. Larynx:
• The larynx, or voice box, is located above the trachea and is involved in sound production and protecting the airway during swallowing. It does not play a role in gas exchange, making it irrelevant to the question.

Summary of Key Points:

• Alveoli are the primary sites for gas exchange in the lungs due to their thin walls and large surface area.
• 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 have thin walls and a large surface area, facilitating efficient diffusion of gases.
• The trachea, bronchi, and larynx are involved in air transport but not in gas exchange.