Which hormone increases blood `Ca^(++)` levels and which decrease it both together regulate the level in blood plasma ?

Explanation

**Step-by-Step Solution:**

1. **Identify the Hormone that Increases Blood Calcium Levels:**
- The hormone that increases blood calcium levels is known as Parathyroid Hormone (PTH).
- PTH is secreted by the parathyroid glands, which are located behind the thyroid glands in the neck.
- This hormone is released when blood calcium levels drop below normal, helping to restore calcium levels to their normal range.

2. **Identify the Hormone that Decreases Blood Calcium Levels:**
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