Correct Option: B. Alveoli Detailed Explanation: The primary structure in the respiratory system responsible for gas exchange between the air and the bloodstream 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. Surface Area:
8. The alveoli provide a vast surface area for gas exchange. In a healthy adult, there are approximately 300 million alveoli, which collectively provide a surface area roughly the size of a tennis court. This large area is essential for efficient gas exchange.
9. Surfactant Production:
10. Alveoli also produce a substance called surfactant, which reduces surface tension and prevents the alveoli from collapsing. This is vital for maintaining their structure and function during breathing.

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 air passage, it does not participate in gas exchange. Its primary function is to provide a clear airway for air to enter and exit the lungs.
• C. 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 alveoli but do not facilitate gas exchange themselves. Like the trachea, their role is primarily structural and functional in terms of air transport.
• 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 unrelated 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 enters the blood and carbon dioxide is expelled.
• Other structures like the trachea, bronchi, and larynx are involved in air conduction but do not participate in gas exchange.
• Understanding the specific functions of each part of the respiratory system is crucial for grasping how breathing and gas exchange work.

This thorough understanding of the alveoli and their role in gas exchange is essential for anyone studying biology, especially in the context of the respiratory system.