Answer
Two factors are important in the continuous process of bone remodeling. These are, (1) the regulation of calcium (Ca++) concentration in the blood--controlled by a hormone feed-back loop involving parathyroid hormone and calcitonin--, and (2), responses of bone to gravitationl and mechanical stress.
Parathormone (PTH) is the chief hormone in this process, but calcitonin and cacitriol play minor roles. When blood calcium falls below the homeostatic range, PTH release is triggered from the parathyroid glands. This hormone (PTH) targets the osteoclsts and increases their rate of bone matrix resorption. This releases and raises the blood level of calcium. Parathormone also has other actions: it acts on the kidneys to increase calcium resorption, and promotes the production of calcitriol which increases the absorption of dietary calcium. When blood calcium rises to normal levels, this shuts off the release of parathormone from the prarathyroid glands. Calcitonin is secreted by the parafollicular cells of the thyroid glands. This hormone slows down removal of calcium from bones when blood levels of calcium are low.
Work Step by Step
Gravitational pull and mechanical stress keep bones strong in the areas where the stressors act.
The importance of gravitational pull is demonstrated by the weakening and demineralization of bones of astronauts who stay for long periods in low gravitational environments above the Earth
Weight lifting and carrying of loads, stress bones and keep them strong. Bones also grow in response to the loads or stresses put on them, for example, where muscles pull on them, and where weights bear down on them or pull on them.
