Arterioles, venules, capillaries, endothelium.

Explanation

### Step-by-Step Solution:

1. **Definition of Arterioles**:
- Arterioles are small diameter blood vessels that branch out from arteries and lead into capillaries. They have muscular walls composed of one or two layers of smooth muscle. This muscular structure allows them to contract and relax, playing a crucial role in regulating blood flow and vascular resistance.

2. **Definition of Venules**:
- Venules are very small blood vessels in the microcirculation that collect blood from capillary beds and transport it to larger blood vessels known as veins. Venules typically range from 7 micrometers to 1 millimeter in diameter.

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