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Physiologic Calcium Homeostasis

- Discussion:
    - calcium homeostasis is the summation of regulatory feedback systems involved in the control of plasma Ca concentrations;
    - despite the small quantity of ionized calcium (as compared to the amount of calcium in bone), calcium is maintained in blood at a closely 
          regulated concentration;
    - rate of passive Ca fluxes lies in cellular processes that influence and regulate calcium efflux from bone;
    - two main hormonal regulators of calcium homeostasis are 1,25 Vit D and parathyroid hormone;
    - calcium homeostasis allows rapid but controlled changes in calcium efflux, thru a negative feedback system between plasma calcium ion 
          levels and parathyroid hormone secretion;
    - as a part of the feedback system active osteoblasts are capable of controlling extracellular environment needed for  mineralization, while
          also participating in control of plasma Ca concentrations;
    - if adequate dietary calcium is not available, calcium is first released from bone surfaces;
          - if dietary calcium deficiency continues, parathyroid hormone secretion rises, & rates of bone resorption are increased through
                 production & activity of osteoclasts;
          - these cells digest all components of bone w/ the effect of adding calcium to extracellular fluid

Calcium flux and endogenous calcium content in isolated mammalian growth-plate chondrocytes, hyaline-cartilage chondrocytes, and hepatocytes.