resorption is when osteoclasts break down bone. your bones are full of calcium, so when your bone is broken down, calcium is released into the blood, thus raising Ca2+ level of the blood.
recall the function of the parathyroid hormone? inc. blood calcium level, remmeber that the parathyroid hormone will also increase osteoclastic activity while calcitonin has the antagonistic role. Reduced osteoclastic activity and thus less calcium levels. 🙂