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Approximately 99% of the calcium in the body resides in bone as a salt and about 1% is dissolved in the extracellular fluids. When levels of plasma calcium get too high, the thyroid gland may sense the high calcium concentration and may release calcitonin hormone into the blood. The target tissue of calcitonin in children and pregnant women is bone. Calcitonin inhibits the action of osteoclasts, which break down bone, and stimulates osteoblasts, which cause bone formation. This process accelerates the uptake of calcium and phosphate into bone matrix. The osteocytes, seen here, maintain bone tissue. The net effect of calcitonin is a decrease in plasma calcium and phosphate concentrations. Calcitonin appears to be a hormone more important in children than adults. When levels of plasma calcium get too low, the parathyroid gland senses the low calcium concentration and releases parathyroid hormone. One of the target tissues of parathyroid hormone is bone. In bone, parathyroid hormone increases the number in activity of osteoclasts releasing calcium ion and phosphate into the plasma. The other target tissue of parathyroid hormone is the kidney. In the kidney, parathyroid hormone increases the uptake of calcium ion. It also inhibits reabsorption of phosphate by the kidneys causing greater phosphate excretion in the urine. Parathyroid hormone also promotes the activation of dietary vitamin D into the hormone calcitriol in the kidney. The liver is also involved in the activation of vitamin D. Calcium absorption from the small intestine can be quite variable and is influenced by calcitriol, which increases the rate of calcium ion and phosphate absorption from the gastrointestinal tract. The net result of parathyroid hormone secretion is that plasma levels of calcium increases to normal. When the plasma calcium level returns to normal, parathyroid hormones secretion slows, which is the final step in this negative feedback loop.