Now, when the blood calcium level drops, when it's relatively low, the parathyroid gland releases the parathyroid hormone. And this basically increases the concentration of calcium inside our blood as a result of three different things. So it basically does three important things. Firstly, it increases the activity rate of osteoclasts and decreases the activity of osteoblasts. So osteoblasts are those cells inside our bone that build the bone. It builds bone matrix by taking calcium from the blood and using that calcium to build the matrix.