In human lungs, gas exchange occurs at the :

Options

A. two bronchi
B. alveolar sacs
C. pleural sacs
D. both (b) and (c)

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks where gas exchange occurs in the human lungs. This requires knowledge of the respiratory system and the specific structures involved in gas exchange.

2. **Identifying Key Structures**: The main structures of the respiratory system include the bronchi, alveolar sacs, and pleural sacs. It is important to understand the function of each of these structures.

3. **Function of the Respiratory System**: The primary function of the respiratory system is to facilitate gas exchange, which involves taking in oxygen and expelling carbon dioxide. This is crucial for cellular respiration and maintaining homeostasis in the body.

4. **Role of the Lungs**: The lungs are the main organs of the respiratory system, located in the thoracic cavity. They are responsible for the exchange of gases between the air and the blood.

5. **Focus on Alveoli**: The alveoli are tiny air sacs within the lungs where gas exchange occurs. They are clustered in groups called alveolar sacs. The structure of the alveoli is designed to maximize surface area for efficient gas exchange.

6. **Characteristics of Alveoli**: Alveoli have thin membranes, which facilitate the diffusion of gases. They are also highly vascularized, meaning they have a rich blood supply that allows for the transport of oxygen and carbon dioxide.

7. **Conclusion**: Based on the above points, the correct answer to the question is that gas exchange occurs at the alveolar sacs (option B).

**Final Answer**: In human lungs, gas exchange occurs at the alveolar sacs.

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