Skin of man cannot act as respiratory organ
because

Options

A. It is dry
B. It is not thin
C. It is not permeable to `O_(2)` and `CO_(2)`
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks why the skin of humans cannot function as a respiratory organ.

2. **Identifying the Respiratory Organ**: In humans, the primary respiratory organ is the lungs, which are specialized for gas exchange.

3. **Characteristics of Lungs**:
- Lungs contain alveoli, which are tiny air sacs that facilitate gas exchange.
- Alveoli have thin and permeable membranes, allowing for efficient diffusion of oxygen and carbon dioxide.
- The extensive surface area provided by numerous alveoli enhances the capacity for gas exchange.
- Lungs are highly vascularized, meaning they have a rich supply of blood vessels that transport gases throughout the body.
- The moist environment of the alveoli aids in the solubility of gases, further facilitating diffusion.

4. **Characteristics of Skin**:
- The skin is primarily a protective barrier and is not designed for gas exchange.
- It is generally dry, which does not facilitate the diffusion of gases.
- The skin is composed of multiple layers (about 7), making it thick and not conducive to gas exchange.
- The skin is not permeable to oxygen and carbon dioxide, which are essential for respiration.

5. **Conclusion**: Given the characteristics of both the lungs and the skin, the skin cannot act as a respiratory organ because it is dry, thick, and not permeable to oxygen and carbon dioxide. Therefore, the correct answer is "all of these."

### Final Answer:
The skin of man cannot act as a respiratory organ because it is dry, it is not thin, it is not permeable to oxygen and carbon dioxide, and all of these factors contribute to its ineffectiveness for gas exchange.

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