Chemosensitive areas are situated close to respiratory rhythm centers and are found in CNS. These are mainly sensitive to

Options

A. `CO_2` but not `H^+` changes
B. `H^+` but not `CO_2` changes
C. `CO_2 and H^+` changes both
D. `CO_2 and O_2` but not `H^+` changes

Answer

To solve the question regarding the sensitivity of chemosensitive areas located close to respiratory rhythm centers in the Central Nervous System (CNS), we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Question**:
- The question asks about the sensitivity of chemosensitive areas that are near respiratory rhythm centers in the CNS. We need to determine what these areas are mainly sensitive to.

2. **Identifying the Role of Chemosensitive Areas**:
- Chemosensitive areas play a crucial role in regulating respiration. They monitor the levels of certain chemicals in the blood to maintain homeostasis.

3. **Identifying Key Substances**:
- The primary substances that influence respiration are carbon dioxide (CO2) and hydrogen ions (H+).
- Increased levels of CO2 lead to an increase in hydrogen ions, which indicates a drop in pH (acidosis).

4. **Evaluating the Options**:
- **Option 1**: CO2 but not hydrogen ion changes - This is incorrect because both CO2 and H+ levels are important.
- **Option 2**: Hydrogen ions but not CO2 changes - This is also incorrect for the same reason.
- **Option 3**: CO2 and hydrogen ion changes both - This is likely correct as both are critical for respiratory regulation.
- **Option 4**: CO2 and O2 but not hydrogen ion changes - This is incorrect because hydrogen ion levels are crucial for the regulation of respiration.

5. **Conclusion**:
- The correct answer is **Option 3**: CO2 and hydrogen ion changes both. These changes activate the respiratory rhythm centers, which then adjust the respiratory process accordingly.

### Final Answer:
- **Option 3: CO2 and hydrogen ion changes both.**