Blood Calcium Level

The level of ionized calcium in the blood must be maintained within a narrow range to perform calcium's regulatory functions. When the diet is low in calcium, the bones release enough calcium into the bloodstream to meet the body's needs.

Although the amount of calcium outside bones and teeth is relatively small, it is required for a number of basic regulatory functions including:

• Contraction and relaxation of muscle (including normal heart beat)
• Coagulation of blood
• Transmission of nerve impulses
• Activation of enzyme reactions
• Stimulation of hormone secretions
• Integrity of intracellular cement substances

Even during severe dietary calcium deficiency, blood calcium levels vary only slightly within the normal range. To do this, the body regulates blood calcium in three ways. First, the kidneys can decrease urinary calcium excretion. Second, the body can adjust the efficiency of dietary calcium absorption. Third, the body can withdraw calcium from bones. The body activates these processes by three main hormones:

Parathyroid Hormone (PTH)

PTH is the primary regulator of blood calcium levels, responding rapidly to minor changes in calcium levels. When calcium levels drop below normal range (hypocalcemia), PTH is released to decrease urinary calcium excretion and mobilize calcium from bone.

Calcitriol

This hormonal form of vitamin D is produced by the kidneys in response to stimulation by the parathyroid hormone. Calcitriol (1,25(OH)2 D) increases calcium absorption from the intestine and limits calcium excretion when blood calcium levels are low.

Calcitonin

An increase in blood calcium stimulates calcitonin, a hormone synthesized in the thyroid gland. This hormone lowers blood calcium by inhibiting bone resorption. Calcitonin's physiological function as a regulator of blood calcium levels is relatively minor compared to the actions of PTH and calcitriol.