Rate of breathing is controlle mainly by

Options

A. `CO_(2)` is level in blood
B. pH in blood
C. `O_(2)` level in blood
D. `O_(2)` level and pH in blood.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the primary factor that controls the rate of breathing.

2. **Identifying Key Factors**: The factors that can influence the rate of breathing include levels of carbon dioxide (CO2), oxygen (O2), and pH in the blood.

3. **Role of Carbon Dioxide**: Among these factors, the level of carbon dioxide in the blood is the most significant. When CO2 levels rise, it indicates that the body is producing more metabolic waste and needs to expel it.

4. **Signal to the Respiratory Center**: The increase in CO2 levels sends signals to the respiratory center located in the medulla oblongata of the brain.

5. **Response of the Respiratory Center**: The respiratory center responds by increasing the rate and depth of breathing. This is necessary to expel the excess CO2 and take in more oxygen.

6. **Conclusion**: Therefore, the rate of breathing is primarily controlled by the level of carbon dioxide in the blood.

**Final Answer**: The rate of breathing is controlled mainly by the level of carbon dioxide (CO2) in the blood.

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