The functions of bone(s) are (i) mechanical support of soft tissues, (ii) levers for muscle action, (iii) protection of the central nervous system, (iv) release of calcium and other ions for the maintenance of a constant ionic environment in the extracellular fluid, and (v) housing and support of hemopoiesis. The structure and amount of bone, both at the macroscopic and microscopic level, are determined by the genetic blueprint and by regulatory factors that help carry out bone functions. Genetic information is responsible for the highly conserved anatomical shape of bones and most likely for restoring that shape after fracture.
Calcium metabolism or calcium homeostasis is the mechanism by which the body maintains adequate calcium levels. Derangements of this mechanism lead to hypercalcemia or hypocalcemia, both of which can have important consequences for health. Although calcium flow to and from the bone is neutral, about five mmol is turned over a day. Bone serves as an important storage point for calcium, as it contains 99% of the total body calcium. Calcium release from bone is regulated by parathyroid hormone . Calcitonin stimulates incorporation of calcium in bone, although this process is largely independent of calcitonin.
Low calcium intake may be a risk factor in the development of osteoporosis. In one meta-analysis, the authors found that 50 out of the 52 studies that they reviewed showed that calcium intake promoted better bone balance. With a better bone balance, the risk of osteoporosis is lowered. Supplementation with vitamin D and calcium slightly improves bone mineral density, as well as decreases the risk of falls and fractures in certain groups of people.
Vitamin D is converted to calcidiol in the liver. Part of the calcidiol is converted by the kidneys to calcitriol, the biologically active form of vitamin D. It circulates as a hormone in the blood, regulating the concentration of calcium and phosphate in the bloodstream and promoting the healthy growth and remodeling of bone.
The process of bone resorption by the osteoclasts releases stored calcium into the systemic circulation and is an important process in regulating calcium balance. As bone formation actively fixes circulating calcium in its mineral form, removing it from the bloodstream, resorption actively unfixes it thereby increasing circulating calcium levels. These processes occur in tandem at site-specific locations.