When blood calcium levels begin to drop below homeostatic levels, is released, causing calcium to be released from bones.

When blood calcium levels begin to drop below homeostatic levels, Parathyroid is released, causing calcium to be released from bones.

Explanation:

When the blood calcium level in our body begins to drop, even below the homeostatic levels then parathyroid is released that causes the calcium to be released from bones to balance the calcium level in the bone.

Parathyroid is released from the parathyroid glands in our body. There are four of these tiny glands located in our neck for the purpose of maintaining the calcium level in the body. These hormone are also known as the parathyrin.

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