A compact long bone has number of …………….. systems arranged in concentric circles.

Explanation

**Step-by-Step Solution:**

1. **Understanding Compact Bone**: Compact bone, also known as cortical bone, is the dense outer layer of bone that provides strength and protection. It surrounds the medullary cavity, which contains bone marrow.

2. **Identifying the Structure**: The compact bone is made up of structural units called osteons. These are cylindrical structures that are essential for the bone's strength and functionality.

3. **Defining Osteons**: Each osteon consists of living cells called osteocytes, which are embedded in a matrix. The osteons are arranged in concentric circles around a central canal.

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