Which part does not only have involuntary muscles :

Options

A. Urethra
B. Iris
C. Heart muscles
D. Blood vessels

Answer

To solve the question "Which part does not only have involuntary muscles?", we need to analyze the options provided and understand the characteristics of involuntary and voluntary muscles.

### Step-by-Step Solution:

1. **Understanding Muscle Types**:
- Involuntary muscles are those that cannot be consciously controlled. They function automatically, such as the muscles in the heart and digestive system.
- Voluntary muscles are those that can be controlled consciously, such as the muscles in the arms and legs.

2. **Analyzing the Options**:
- **Urethra**: The urethra has two types of sphincters. The internal sphincter (between the bladder and urethra) is involuntary, while the external sphincter (which helps in urination) is voluntary. Therefore, the urethra contains both involuntary and voluntary muscles.
- **Iris**: The iris of the eye is controlled involuntarily. It adjusts the size of the pupil to regulate light entering the eye, and we do not have conscious control over it.
- **Heart Muscles**: The heart is made up of cardiac muscle, which is involuntary. It beats automatically and cannot be controlled consciously.
- **Blood Vessels**: The muscles in blood vessels (smooth muscles) are also involuntary. They regulate blood flow without conscious control.

3. **Identifying the Correct Answer**:
- Among the options, the urethra is the only structure that contains both involuntary (internal sphincter) and voluntary (external sphincter) muscles. The other options consist solely of involuntary muscles.

### Conclusion:
The part that does not only have involuntary muscles is the **urethra**.

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