Final answer:
PTH, which is released from the parathyroid glands when blood calcium levels are low, increases blood calcium by stimulating activities in the bones, kidneys, and intestines that lead to calcium being released into the bloodstream or absorbed from the diet.
Step-by-step explanation:
When blood calcium levels are low, PTH stimulates the parathyroid glands. PTH stands for parathyroid hormone, which is secreted by these glands to regulate blood calcium levels. The parathyroid glands produce PTH and release it in response to low blood calcium levels to increase those levels by acting on the skeleton, the kidneys, and the intestine.
In the skeleton, PTH stimulates osteoclasts and inhibits osteoblasts, which allows for the release of calcium from the bones into the bloodstream. In the kidneys, PTH enhances the reabsorption of calcium, and in the intestines, it indirectly causes increased absorption of dietary calcium through the synthesis of calcitriol (active form of vitamin D).