Which one of the following consists of a thin layer of endothelium only?

Options

A. arterioles
B. venules
C. capillaries
D. valves

Explanation

To answer the question "Which one of the following consists of a thin layer of endothelium only?", we can follow these steps:

### Step 1: Understand the Structure of Blood Vessels
Blood vessels are primarily composed of three layers:
- **Tunica Adventitia**: The outermost layer, made up of connective tissue.
- **Tunica Media**: The middle layer, consisting of smooth muscle and elastic fibers.
- **Tunica Intima**: The innermost layer, which is a thin layer of endothelium.

### Step 2: Identify the Function of Each Layer
- The **Tunica Adventitia** provides structural support and elasticity.
- The **Tunica Media** allows for the contraction and relaxation of the blood vessel.
- The **Tunica Intima** is crucial for the exchange of substances between the blood and surrounding tissues.

### Step 3: Determine Where Maximum Exchange Occurs
For maximum exchange of gases, nutrients, and waste products between blood and tissues, the blood vessel must have a very thin layer of endothelium. This is essential for efficient diffusion.

### Step 4: Identify the Type of Blood Vessel with Thin Endothelium
The blood vessels that are specialized for maximum exchange are the **capillaries**. Capillaries consist of a single layer of endothelial cells, allowing for easy transfer of materials.

### Step 5: Conclusion
Based on the above analysis, the answer to the question is **capillaries**, as they consist of a thin layer of endothelium only.

### Final Answer
**Capillaries (Option C)**

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