Download Maintaining Calcium Levels in the Bone

Maintaining Calcium Levels in the Bone
The body uses the bones not only for structure and protection, but also
for calcium storage. Approximately 99 percent of the body's calcium is
stored in the bone, and calcium plays an important role in most of the
body's functions. Free calcium levels must remain at a set point for
proper body functions. The extracellular levels of calcium are affected by
calcium intake from foods, excretion of calcium as waste, and the
storage and release of calcium from the bones. Hormones regulate these
processes to maintain balanced calcium levels in the extracellular fluid,
which is necessary to maintain homeostasis.
Small changes in blood calcium levels can have significant effects on
body function. For example, if extracellular calcium levels are too low,
the nervous system becomes overexcited, resulting in tetany (rigid,
locked muscles). On the other hand, if calcium levels in the body are
above normal, the nervous system becomes sluggish. Muscle activity of
the heart and gastrointestinal tract slows down.
Milk and dark green vegetables are rich in calcium, and an adequate
supply of calcium helps maintain healthy bones. When we eat calciumrich foods, some calcium is absorbed into the small intestinal wall, and
some of the calcium becomes soluble in the blood stream. However,
calcium and other divalent cations (ions that are missing two electrons)
are poorly absorbed by the small intestine. For this reason, vitamin D is
an important dietary supplement. Vitamin D increases calcium
absorption in the small intestine.
By altering the function of the osteoblasts and osteoclasts, hormones
help regulate calcium levels in the blood. There are three hormones that
control osteoblast and osteoclast activity:
•
parathyroid hormone or PTH, which increases bone resorption
by stimulating osteoclasts, leading to increased calcium release from
bone.
•
•
calcitonin, which acts in children to decrease bone resorption,
leading to less calcium entering the blood.
calcitriol, (vitamin D) which increases absorption of dietary
calcium.
Parathyroid hormone (produced in the parathyroid glands) controls
extracellular calcium by regulating how calcium is reabsorbed in the
intestines, excreted from the body, and exchanged between the
extracellular fluid and the bone. The cells of the parathyroid gland
synthesize and release parathyroid hormone in response to low blood
calcium levels. To bring blood calcium levels into the normal range, the
release of parathyroid hormone stimulates osteoclasts to reabsorb bone
mineral, therefore releasing calcium into the blood. Parathyroid
hormone also enhances calcium absorption by the intestines, and
prevents calcium loss in urine to increase calcium levels in the blood.
When parathyroid cells sense that blood calcium levels are above
normal, cellular receptors are activated and the synthesis and release of
parathyroid hormone is inhibited.
Calcitonin is produced in the thyroid gland. The function of this
hormone is to decrease bone resorption and retain calcium in the bones.
Therefore, the effects of calcitonin counteract the effects of parathyroid
hormone. When blood calcium levels are high, the thyroid releases
calcitonin into the blood. Calcitonin decreases bone resorption by
decreasing the activity of osteoclasts and decreasing the formation of
new osteoclasts. In this way, calcitonin shifts the bone balance in favor
of bone deposition, which requires the removal of calcium from the
blood and into the bone. Calcitonin also has minor effects on how the
intestines and kidney tubules handle calcium. In adult humans,
calcitonin has weak effects on the regulation of calcium levels. We know
this because if the thyroid gland is removed, calcium levels are not
adversely affected.