How is plasma calcium held constant? Milieu interieur of calcium.

Extracellular or plasma calcium ion concentration is held constant at 5 mg/dl through the actions of parathyroid hormone (PTH), vitamin D, and calcitonin, on their target organs, kidney and bone. The thresholds of renal tubular calcium reabsorption and bone resorption and formation are both set at 5 mg/dl. The set point of PTH secretion is also set at 5 mg/dl plasma calcium ion. Therefore, the sensing system (parathyroid cell) and the effectors, kidney and bone, are all set to maintain plasma calcium at 5 mg/dl, perhaps through membrane-bound calcium sensor proteins. The effectiveness of this system depends upon the presence of bone remodeling, which allows a swift shift of plasma calcium from and to bone in response to PTH and calcitonin, respectively. In this regard, directing hematopoiesis to bone marrow that provides bone resorbing osteoclasts is critical. It is likely that this shift of hematopoiesis occurs through evolution at the transition from the aquatic to the terrestrial life, and this event is directed by expression of "homing molecule" in bone marrow stromal cells. This brief review provides a factual and conceptual framework of the current understanding of the milieu interieur of the calcium ion.