1)Vitamin D is a hormone whose precursor is produced in the skin in response to sunlight and then processed in the liver and kidney to become active Vitamin D3. Vitamin D3 increases calcium absorption from the gut.
	2) Much calcium is lost to the urine, so when you need more in the blood this is an important source.
	3) Lastly,  PTH increases osteoclastic activity to release calcium into the blood.