Calcium homeostasis is one of your body's highest priorities. Although 99 percent of your body's calcium is found in your bones, the other 1 percent is so critical to indispensable life processes that it is regulated by a system of complex mechanisms, according to Kathleen Mahan, M.S., and Sylvia Escott-Stump, M.A., in "Krause's Food, Nutrition, & Diet Therapy." Your body regulates a critical balance of blood calcium with vitamin D and two hormones, calcitonin and parathormone. These hormones act on your intestines, bones and kidneys as three mechanisms to control your blood calcium level.
Maintaining Balance
Falling blood calcium signals your parathyroid glands to secrete parathormone. Parathormone stimulates the activation of vitamin D and together they stimulate calcium reabsorption in your kidneys and stimulate osteoclast cells to release calcium into your blood by breaking down bone cells. Vitamin D enhances calcium absorption in your intestines. These actions raise your blood calcium levels and inhibit the further secretion of parathormone.
Intestines
Calcium absorption occurs in your intestines. When your body needs your intestines to absorb less calcium, it uses the hormone calcitonin from your thyroid to signal your intestines to slow down the rate of calcium absorption. Your body uses vitamin D to enhance calcium absorption from your intestines.
Bones
Your bones provide a vital service as a reservoir to store excess calcium when it is in plentiful supply in your blood and as a source of calcium when your blood calcium level is low. Specialized cells, called osteoblasts, build new bone by taking up calcium from your blood when blood calcium is too high. Conversely, other specialized cells, called osteoclasts, break down bone and release calcium into your blood when more calcium is needed to regulate muscle contractions, clot blood, transmit nerve impulses, secrete hormones, activate certain enzyme reactions or maintain normal blood pressure. Your thyroid signals your bones when to hold onto more calcium and your parathyroid signals your bones when to release more calcium.
Kidneys
Reabsorption of calcium occurs in your kidneys. When signaled by calcitonin from your thyroid, your kidneys abruptly stop reabsorbing calcium. It takes a combination of vitamin D, activated by calcitonin from your thyroid, along with the hormone parathormone, secreted by your parathyroid, working together to stimulate your kidneys to begin reabsorption of calcium again.
References
- "Krause's Food, Nutrition, & Diet Therapy"; Kathleen Mahan, M.S., R.D. and Sylvia Escott-Stump, M.A., R.D.; 2000
- "Understanding Nutrition, Ninth Edition"; Eleanor Whitney, Ph.D. and Sharon Rolfes, M.S., R.D.; 2002
- MayoClinic.com: Hypercalcemia: Causes
