role of hormones

Parathyroid Hormone -increases the levels of calcium in the blood
Does the following:
1) causes release of calcium from the bones where it is stored
2) Enchances the reabsorbtion of calcium from renal tubules (distal tubules and the thick ascending limb) so that, not as much calcium is exerted through urination.
3) It enhances the absorption of calcium in the intestine (by increasing the production of activated vitamin D.)
http://en.wikipedia.org/wiki/Parathyroid_hormone#Regulation_of_serum_calcium If you want more details, it is explained nicely in wikipedia.


Calcitonin
It can be considered as the "opposite" of Parathyroid Hormone

Wikipedia (added some explanations):
calcitonin lowers blood Ca2+ levels in four ways:
1)Inhibits Ca2+ absorption by the intestines
2)Inhibits osteoclast activity in bones (this means that, it inhibits calcium release from the bones to the bloodstream)
3)Stimulates osteoblastic activity in bones. (this means that, it enchants calcium deposition to the bones, thus reducing blood levels of calcium)
4)Inhibits renal tubular cell reabsorption of Ca2+ allowing it to be excreted in the urine
http://en.wikipedia.org/wiki/Calcitonin#Effects


Initially, I only knew about these two enzymes, but with a short search I found another enzyme that apparently plays a role at calcium regulation:

Calcitriol (also commonly referred as 1,25-dihydroxyvitamin D3)

Wikipedia:
Calcitriol increases blood calcium levels ([Ca2+]) by promoting absorption of dietary calcium from the gastrointestinal tract and increasing renal tubular reabsorption of calcium, thus reducing the loss of calcium in the urine. Calcitriol also stimulates release of calcium from bone by its action on the specific type of bone cells referred to as osteoblasts, causing them to release RANKL, which in turn activates osteoclasts
http://en.wikipedia.org/wiki/Calcitriol#Function

The functions of calcitriol are pretty much the same as PTH (increases release of calcium from the bones, reabsorbtion from renal tubules and absorption in the intestine)