This image shows how the thyroid and parathyroid glands (upper left cropped off inset with yellow/orange background. Thyroid is a salmon pink butterfly, parathyroids are the embedded tiny yellow beans) signal the osteoclasts and osteoblasts to control the level of calcium in the blood (lower right, purple swiss-cheesed deep purple vessel). If levels are low, osteoclasts break down bone which releases calcium as a result. If levels are high, osteoblasts absorb excess calcium, trapping it in the bone matrix.
In our previous image, we saw how bisphosphonate can influence the activities of such cells. Calcium is yet another chemical that cells use to understand the environment and what it calls for to be in a normal state. What we ultimately want to take away is that cells depend on signals which determine their course of action, much like how we depend on a growling stomach to let us know we are hungry and should find something to eat.