Which blood vessels have walls only one cell thick?

Options

A. Capillaries
B. Venules
C. Veins
D. Arterioles

Answer

**Step-by-Step Solution:**

1. **Understanding Blood Vessels**: There are three main types of blood vessels in the circulatory system: arteries, veins, and capillaries.

2. **Analyzing the Structure of Blood Vessels**:
- **Arteries**: These vessels have three layers in their walls:
- Tunica externa (outer layer)
- Tunica media (middle layer)
- Tunica interna (inner layer)
- **Veins**: Similar to arteries, veins also have three layers:
- Tunica externa (outer layer)
- Tunica media (middle layer)
- Tunica interna (inner layer)

3. **Identifying Capillaries**: Capillaries are different from arteries and veins. They are the smallest blood vessels and serve as the connection between arteries and veins.

4. **Structure of Capillaries**: Capillaries have walls that are only one cell thick. This single layer is known as the endothelium, which allows for efficient exchange of gases, nutrients, and waste products between blood and surrounding tissues.

5. **Conclusion**: Based on the structure of the blood vessels, the only type of blood vessel that has walls that are one cell thick is the capillary.

**Final Answer**: Capillaries have walls that are only one cell thick.

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