Bone and calcium homeostasis
Bone is the major storage site for calcium in the body, and move-ment of calcium into and out of bone helps determine blood calcium\ levels, which is critical for normal muscle and nervous system function. Calcium (Ca2+) moves into bone as osteoblasts build new bone and out of bone as osteoclasts break down bone (figure 6.9). When osteoblast and osteoclast activity is balanced, the movements of calcium into and out of a bone are equal.
When blood calcium levels are too low, osteoclast activity increases, osteoclasts release calcium from bone into the blood, and blood calcium levels increase. Conversely, if blood calcium levels are too high, osteoclast activity decreases, osteoblasts remove calcium fromthe blood to produce new bone, and blood calcium levels decrease.
Calcium homeostasis is maintained by three hormones: para-thyroid hormone (PTH) from the parathyroid glands, vitamin Dfrom the skin or diet, and calcitonin (kal-si-tō′\nin) from thethyroid gland. PTH and vitamin D are secreted when blood cal-cium levels are too low and calcitonin is secreted when blood calcium levels are too high.
PTH works through three simultaneous mechanisms to increase blood calcium levels.
1.PTH indirectly stimulates osteoclasts to break down bone, which releases stored calcium into the blood.
2.PTH stimulates the kidney to take up calcium from the urine and return it to the blood.
3.PTH stimulates the formation of active vitamin D, which, in turn, promotes increased calcium absorption from the small intestine.
PTH and vitamin D, therefore, cause blood calcium levels to increase, maintaining homeostatic levels. Decreasing blood calcium levels stimulate PTH secretion.
Calcitonin works to decrease blood calcium levels by inhibit-ing osteoclast activity. Even in the absence of osteoclast activity, osteoblast activity continues, removing calcium from the blood and depositing it into the bone. Thus, calcitonin maintains homeo-static blood calcium levels by decreasing calcium levels that are too high. In summary, PTH, vitamin D, and calcitonin work together to keep blood calcium levels within the homeostatic range.