Calcium is the most abundant mineral in your body. Ninety-nine percent of your calcium stores are sequestered in your bones and teeth, with only 1 percent found in your soft tissues and bloodstream. However, the small amount of calcium located outside your skeleton is very important to your health, and your body works very hard to maintain serum calcium concentrations within a very narrow range. When your calcium level falls too low, several mechanisms come into play to maintain normalcy, or “homeostasis.”
Functions
Calcium acts as a cofactor for many enzymes and other functional proteins in your body, including those that are responsible for blood clotting and for making glucose available for your muscle cells. Your heart, muscles and nerves cannot function properly unless your serum calcium level is normal. Calcium homeostasis is so important that your body will remove calcium from your skeleton – a process that can eventually weaken your bones – to provide enough of this vital mineral for other physiologic functions.
Parathyroid Hormone
Your parathyroid glands are small nests of tissue that are found in your neck, just behind your thyroid gland. The cells in your parathyroid glands contain receptors that can sense changes in calcium concentrations in your bloodstream. When your calcium level falls below a normal range, the receptors are triggered and your parathyroid glands secrete parathyroid hormone, or PTH, which stimulates your kidneys to produce calcitriol, an activated form of vitamin D-3. PTH and calcitriol then cooperate to increase your serum calcium level.
Vitamin D Conversion Enhanced
Whenever you are exposed to sunlight, cholesterol-like molecules in your skin are converted to cholecalciferol, or vitamin D-3. Cholecalciferol is converted to calcitriol in a series of chemical reactions that occur in your liver and kidneys. The final conversion to calcitriol is enhanced by PTH, which is released in response to low blood calcium levels. Thus, low calcium levels ultimately increase your kidneys’ production of calcitriol.
Homeostasis Attained
One of calcitriol’s most important effects is to increase calcium absorption from your intestine. In addition, calcitriol and PTH work together to reduce the loss of calcium through your urine and to mobilize calcium from your bones. Collectively, these actions rapidly increase your blood calcium level. Once serum calcium concentrations return to normal, PTH secretion is “switched off,” calcitriol production slows, absorption of calcium from your intestine and kidneys decreases and skeletal calcium mobilization ceases. Like many other metabolic processes, calcium homeostasis continues day and night without our conscious thought.
References
- “Staying Healthy with Nutrition – The Complete Guide to Diet and Nutritional Medicine: Calcium”; Elson M. Haas, M.D.; 2006
- Linus Pauling Institute at Oregon State University; Calcium; Jane Higdon, Ph.D.; April 2003
- Colorado State University: Pathophysiology of the Endocrine System: Vitamin D (Calcitriol)
