Effector organs
Absorption
Calcium enters the body in a normal diet and is absorbed across the intestinal brush border membrane. Calbindin is a vitamin D-dependent calcium-binding protein inside intestinal epithelial cells actively transports calcium into the body.
Excretion
The kidney excretes 250 mmol a day in pro-urine, and resorbs 245 mmol, leading to a net loss in the urine of 5 mmol/d. In addition to this, the kidney processes Vitamin D into calcitriol, the active form that is most effective in assisting
intestinal absorption. Both processes are stimulated by parathyroid hormone.
The role of bone
Although calcium flow to and from the bone is neutral, about 5 mmol is turned over a day. Bone serves as an important storage point for calcium, as it contains 99% of the total body calcium. Calcium release from bone is regulated by
parathyroid hormone. Calcitonin stimulates incorporation of calcium in bone, although this process is largely independent of calcitonin. Low calcium intake may also be a risk factor in the development of osteoporosis.
