There is a tiny amount in the blood stream which is responsible for important functions such as muscle contraction, maintenance of the heartbeat, and transmission of nerve impulses. We constantly lose calcium from our bloodstream through urine, sweat, and feces. It is renewed with calcium from bone. In this process, bones continuously lose calcium. This bone calcium must be replaced from food.
Calcium needs change throughout life.
Up until the age of 30 or so, we consume more calcium than we lose. Adequate calcium intake during childhood and adolescence is especially important. The loss of too much calcium can lead to soft bones or osteoporosis. How rapidly calcium is lost depends, in part, on the kind and amount of protein you eat as well as other diet and lifestyle choices. Protein from animal products is much more likely to cause calcium loss than protein from plant foods.