A fascinating interplay of pathways and signals are involved in calcium regulation. As research and new technologies evolve, new aspects are elucidated in these complex mechanisms. Various animal models have been used in order to understand the cellular responses and the hormonal regulation (Eckermann 2008).
Moe (2008) states "normal homeostatic regulation maintains serum levels, intracellular levels, and optimal mineral content in bone", summarizing the intricacy of structures and organs that contribute to "normal" levels. The major organs involved in the regulation of its metabolism are the parathyroid glands, the kidneys, the skeleton, and the gut (Wysolmerski & Insogna 2007). Our body obtains the necessary amount of calcium from two major sources: diet (through intestinal absorption) and bones (through calcium mobilization). Foods that include a high amount of calcium are diary milk, cheese yogurt, and green vegetables. The amount of calcium people need varies: young people need a high intake, so that growth and development are assured. Older women need calcium supplementation in order to prevent osteoporosis. Calcium is absorbed in the very beginning of the small bowel: duodenum and the first portion of the jejunum. The intestinal absorption is regulated by Vitamin D, and its mobilization is tightly regulated by various hormones and signaling mechanisms, with a continuous remodeling of calcium storage. For example, if calcium is not adequately obtained from diet, a series of hormonal mechanisms are set off in order to get the necessary amount from bones; if this process is sustained for a long period of time, bones weaken.
The kidneys are also important organs which regulate calcium metabolism and keep its blood levels (Choudhury & Levy 2007). In the different portions of the kidney, calcium is reabsorbed, so that equilibrium is achieved in the blood. Additionally, in both the proximal and distal nephrons, filtrated calcium is passively or actively absorbed into the bloodstream obtaining adequate levels in the urine. Those mechanisms can be modified with different drugs, which are used in certain conditions such as renal calculi. To review the impact of kidney disease in calcium metabolism, Hamdy (2007) has published an article where he explains the role of chronic kidney disease and the major disturbances seen in bone and mineral metabolism.
A review of calcium metabolism would not be complete without addressing bone processes. Bone undergoes remodeling in all life stages, but certainly this process is faster in younger individuals, and more significant in trabecular bone than cortical bone. Normal bones are formed by different substances. Most of the body's calcium is stored in the bones, along with phosphorus and, mostly, as calcium phosphate salts. The great majority (almost 99%) of the calcium is found in the skeleton and approximately 1% moves freely from and into the bones (Matos 2008). Two principal cells regulate bone formation and destruction: osteoclasts and osteoblasts. Osteoclasts resorb bone and are formed by numerous monocytes that fuse and take shape of a multinucleated cell. On the other hand, osteoblasts deposit new bone and arise from mesenchymal cells. The remainder component of bone