The respiratory centre in the brain which regulates the process of respiration is extremely sensitive to blood concentrations of:

Options

A. `O_2`
B. `CO_2`
C. hydrogencarbonates
D. oxyhaemoglobin

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the sensitivity of the respiratory center in the brain concerning blood concentrations. We need to identify which substance the respiratory center is particularly responsive to.

2. **Function of the Respiratory System**: The primary function of the respiratory system is to facilitate gas exchange, allowing oxygen to enter the body and carbon dioxide to be expelled. This process is vital for maintaining homeostasis in the body.

3. **Role of the Respiratory Center**: The respiratory center, located in the medulla oblongata of the brain, regulates the rhythm and rate of breathing. It does not operate in isolation; it responds to changes in the blood's chemical composition.

4. **Chemosensitive Area**: Adjacent to the respiratory rhythm center is a chemosensitive area that detects changes in blood chemistry, particularly the levels of carbon dioxide (CO2) and hydrogen ions (H+).

5. **Response to CO2 Levels**: When the concentration of CO2 or H+ ions in the blood increases, the chemosensitive area is activated. This activation sends signals to the respiratory center to adjust the breathing rate accordingly.

6. **Conclusion**: The respiratory center is extremely sensitive to the concentration of carbon dioxide in the blood. An increase in CO2 levels triggers a response to increase the breathing rate to eliminate excess CO2 and restore balance.

### Final Answer:
The respiratory center in the brain is extremely sensitive to blood concentrations of **carbon dioxide (CO2)**.

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