When calcium levels drop in the blood, parathyroid hormone (PTH) will signal the bones to release calcium into the bloodstream. As calcium levels fall, vitamin D will be activated to facilitate calcium absorption in the intestines. 

In addition, PTH will signal the kidneys to release less calcium in the urine. When the body has enough calcium, calcitonin signals the kidneys to release less calcium. In the opposite direction, calcitonin decreases calcium levels in the blood by preventing calcium release from bones. When the body has enough calcium, a different hormone called calcitonin works to do the opposite: it lowers calcium levels in the blood by stopping the release of calcium from bones and signaling the kidneys to rid more of it in the urine.

The body receives the calcium in two ways. By eating foods or supplements that contain calcium, or by drawing calcium from the body. If one does not eat enough calcium-containing foods, the body will remove calcium from bones. The body then “borrows” calcium from the bones and replaces it at a later time, but this does not always take place and cannot always be accomplished just by eating more calcium.