Bone resorption

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Bone resorption is the process by which osteoclasts break down bone and release the minerals, resulting in a transfer of calcium from bone fluid to the blood. The osteoclasts are multi-nucleated cells that contain numerous mitochondria and lysosomes. These are the cells responsible for the resorption of bone. Attachment of the osteoclast to the osteon begins the process. The osteoclast then induces an infolding of its cell membrane and secretes collagenase and other enzymes important in the resorption process. High levels of calcium, magnesium, phosphate and products of collagen will be released into the extracellular fluid as the osteoclasts tunnel into the mineralized bone. During childhood, bone formation exceeds resorption, but as the aging process occurs, resorption exceeds formation.

Calcium-sensing membrane receptors in the parathyroid gland monitor calcium levels in the extracellular fluid. Low levels of calcium stimulates the release of Parathyroid hormone (PTH) from chief cells of the parathyroid gland. PTH stimulates active resorption of calcium in the thick ascending limb and distal convoluted tubule in the kidney. PTH also increases the number and activity of osteoclasts to release calcium from bone, and upregulates the enzyme 1-alpha-hydroxylase present in kidney cells, which increases the production of Vitamin D. Vitamin D, in turn, increases the amount of calcium absorption from the small intestine. The net result is to increase levels of calcium in the blood.