2) Through the action of osteoclasts,

Osteoclasts are the type of cells which are responsible for the absorption of calcium from bones.

Explanation:

Osteoclasts is special type of cells which is released by parathyroid gland when the blood have low levels of calcium. Osteoclasts absorb calcium from the bones and add this calcium into the blood to increase the calcium level. If calcium is present in very high amount so this calcium is stored in the bones of the body and extracted when needed by the body.