Correct Option: C. Alveoli Detailed Explanation: The primary structure in the respiratory system responsible for the exchange of oxygen and carbon dioxide between the air and the bloodstream 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 (tiny 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 we inhale, oxygen from the air enters the alveoli. At the same time, carbon dioxide (a waste product of metabolism) from the blood enters the alveoli to be exhaled.
6. This exchange occurs through a process called diffusion, where gases move from an area of higher concentration to an area of lower concentration. Oxygen moves from the alveoli (where it is in higher concentration) into the blood (where it is in lower concentration), and carbon dioxide moves in the opposite direction.
7. Role in the Respiratory System:
8. The alveoli increase the surface area available for gas exchange, making the process more efficient. The human lungs contain approximately 300 million alveoli, providing a vast surface area (about the size of a tennis court) for gas exchange.

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 primary role 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 off from the trachea and lead into each lung. Like the trachea, they serve as conduits for air but do not facilitate gas exchange. The bronchi further divide into smaller bronchioles, which eventually lead to 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.
• Diffusion allows oxygen to enter the bloodstream and carbon dioxide to exit through the alveolar walls.
• Other structures like the trachea, bronchi, and larynx are important for air passage and protection but do not participate in gas exchange.

Revision Summary:

• The alveoli are the key structures for gas exchange in the respiratory system.
• They facilitate the movement of oxygen into the blood and carbon dioxide out of the blood through diffusion.
• Other respiratory structures (trachea, bronchi, larynx) serve different functions and do not engage in gas exchange.