Fill in the following blanks with suitable words:
If a person is doing exercise, his breathing rate …………..

Answer

**Step-by-Step Text Solution:**

1. **Understand the Context**: The question asks about the effect of exercise on a person's breathing rate.

2. **Identify the Need During Exercise**: When a person exercises, their muscle cells require more energy. This energy is produced through a process that needs oxygen.

3. **Recognize the Role of Breathing**: Breathing is the process that allows oxygen to enter the body and carbon dioxide to be expelled.

4. **Determine the Effect on Breathing Rate**: Since muscle cells need more oxygen during exercise, the body compensates by increasing the breathing rate.

5. **Conclude the Answer**: Therefore, the blank can be filled with the word "increases".

**Final Answer**: If a person is doing exercise, his breathing rate **increases**.

---