bone resorption
Bone resorption is the dissolving of bone, the demolition phase of skeletal life. Think of bone as the body's calcium warehouse: when blood calcium runs short, the body opens the warehouse by breaking down a little bone and releasing its mineral into the bloodstream.
Osteoclasts carry out resorption by acidifying a sealed pocket against the bone surface and digesting its mineral and collagen. The process is driven hormonally: PTH ramps it up when calcium is low, and it is amplified by RANKL signaling; sex steroids like estrogen restrain it. Resorption is the fast, on-demand way to raise blood calcium, working far quicker than adjusting gut absorption.
Some resorption is healthy and necessary, clearing old bone for renewal and supplying emergency calcium. The trouble comes when it persistently outruns bone formation, as in osteoporosis, hyperparathyroidism, or prolonged immobility, leaving bones thin and fragile. Markers of resorption, such as collagen breakdown products in blood or urine, let clinicians gauge how fast bone is being lost.
During long bed rest after surgery, the lack of weight-bearing accelerates bone resorption, and calcium spills into the urine, sometimes forming kidney stones.
Immobility speeds resorption and dumps calcium into urine.
Confusingly, in bone the word resorption means breakdown and release, not reabsorption as in the kidney; the two are unrelated processes that happen to share a similar word.