Overview of Calcium and Phosphate Homeostasis
Plasma levels of calcium (and especially ionic calcium) are tightly regulated
because of its critical role in nerve conduction and muscle contraction, intracellular
signaling, and coagulation.
In the model of calcium homeostasis, the bones serve as a
reservoir or bank for calcium that release or uptake calcium under the influence of
parathyroid hormone (PTH), vitamin D
(1, 25-dihydroxycholecalciferol) and calcitonin
to ensure that plasma levels of calcium remain between 9.0-10.5 mg /dl. (50% of
calcium is in the free, ionic form which is the physiologically active form that is tightly
regulated by parathyroid hormone.)
Failure to do so results in hypocalcemia with
possible tetany, or hypercalcemia with “painful bones, renal stones, abdominal groans
and psychic moans”.
These hormones also simultaneously regulate plasma phosphate
levels to prevent the precipitation of calcium phosphate (hydroxyapatite) in soft tissues.
The normal range for plasma phosphate is 3.0-4.5 mg/dL.
The calcium phosphate
product is a measure of the saturation potential (and likelihood of crystal precipitation) of
is considered the threshold for calcium phosphate
saturation, although in people with End Stage Renal Disease the desirable range is 42-52
Calcium and phosphate homeostasis is dependent on a balance between
absorption, storage, and excretion.
Therefore, 3 organs: the GI tract, bones, and kidneys
have important roles in maintaining this homeostasis.
Basically, excretion must equal
absorption to maintain calcium and phosphate balance in the body.
For example, a
healthy adult who has a net
small intestine absorption of 300 mg/day of calcium will
excrete 300 mg/day of calcium in the stool and urine.
(Note that it is net calcium
absorption that counts, since only ~1/3
of ingested calcium is absorbed across the
intestinal mucosa into the blood.
Average absorption ranges from 150-300 mg/day based
on an intake of 400-1500 mg/day.)
Very little calcium (just 1% of filtered calcium) is
excreted via the kidneys.
Therefore, small changes in the amount of calcium excreted by
the kidneys results in large changes in plasma calcium, and body stores of calcium.
bone, the processes in the kidneys and GI tract (especially the small intestine) that
regulate calcium and phosphate homestasis are influenced by the same three hormones:
PTH, vitamin D
, and to a lesser degree calcitonin.
Parathyroid hormone is secreted from the parathyroid glands which are usually
four in number and located on the posterior surface of the lobes of the thyroid gland.
They may also have an ectopic location in the root of the neck or superior mediastinum
because of their embryonic association with the thymus gland.
The inferior parathyroid
glands and the thymus gland are derived from the 3
pharyngeal pouch during the 7
week of the embryonic period, and then migrate inferiorly in the neck to assume their