Calcium tends to be regulated by 2 hormones primarily: calcitonin and parathyroid hormone. Each of these is released in response to either an increase or decrease in serum calcium levels. These are actually very important due to the effects of an improper calcium balance! See the link below to see those effects. I leave them out, though to avoid distracting ourselves! I will say, though, that these hormones' importance is reflected by the narrow range of calcium levels compatible with life: 9-10.2 mg/dl!
Let's start with calcitonin. This hormone is produced by C cells in the thyroid gland and it is secreted in response to increasing serum calcium levels. Calcitonin primarily cuts down on bone reabsorption into the blood and it increases calcium secretion in urine at the kidneys. Consequently, phospate is also excreted in urine and retained in bone with these actions of calcitonin. Therefore, calcitonin also causes hypophosphatemia.
Parathyroid hormone (PTH) generally has the opposite effect. In bone, PTH tends to stimulate osteoclasts to increase resorption of calcium (and phosphate) into the blood. PTH also stimulates the kidney to retain calcium from the urine, though it actually also stimulates more secretion of phosphate. This increase in phosphate secretion will actually allow more calcium to stay in serum.
PTH also acts in a very important way in the kidney by stimulating the final step in the conversion of vitamin D to its active form: 1a,25(OH)2D3. This active form will cause increased calcium absorption from the intestine. Therefore, PTH indirectly also causes absorption of calcium from the intestine.
So in summary:
- Calcitonin: decrease bone resorption of calcium, increase calcium excretion in kidney
- PTH: increase bone resorption of calcium, decrease calcium excretion in kidney, act through 1a,25(OH)2D3 to increase calcium absorption from the intestine.
I hope that helps!