Rate of respiration is directly affected by

Options

A. `CO_(2)` concentration
B. `O_(2)` in trachea
C. Concentration of `O_(2)`
D. Diaphragm expansion

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks what directly affects the rate of respiration. To answer this, we need to consider the factors that influence how we breathe.

2. **Identifying Key Factors**: The options provided are:
- Carbon dioxide concentration
- Oxygen in trachea
- Concentration of oxygen
- Diaphragm expansion

3. **Role of Chemoreceptors**: The rate of respiration is primarily regulated by chemoreceptors in the body. These receptors respond to changes in the levels of carbon dioxide (CO2) and oxygen (O2) in the blood.

4. **Central Chemoreceptors**:
- Located in the medulla oblongata.
- They detect changes in the pH of cerebrospinal fluid, which is influenced by CO2 levels.
- An increase in CO2 concentration leads to a decrease in pH (more acidic), stimulating an increase in the rate of respiration.

5. **Peripheral Chemoreceptors**:
- Include the aortic body and carotid body.
- They detect changes in blood oxygen and carbon dioxide levels.
- The carotid body can also detect pH changes, but their overall influence on respiration is less than that of central chemoreceptors.

6. **Conclusion**: Since the central chemoreceptors are primarily responsible for detecting elevated levels of carbon dioxide and directly influencing the rate of respiration, we conclude that the rate of respiration is directly affected by carbon dioxide concentration.

7. **Final Answer**: The correct answer is **carbon dioxide concentration** (Option 1).