Page 303 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Calcium and Phosphate Metabolism precipitate in solutions, and the deposition of
calcium phosphate salts occurs. The salts are
Calcium, particularly ionized calcium (Ca ), chiefly deposited in the bone, but can also pre-
2+
plays a central role in the regulation of numer- cipitate in other organs. The infusion of
ous cell functions (! pp. 36, 62ff., 192, 276). phosphate leads to a decrease in the serum cal-
Calcium accounts for 2% of the body weight. cium concentration since calcium phosphate
Ca. 99% of the calcium occurs in bone while 1% accumulates in bone. Conversely, hypo-
is dissolved in body fluids. The total calcium phosphatemia leads to hypercalcemia (Ca 2+ is
conc. in serum is normally 2.1–2.6 mmol/L. Ca. released from bone).
Hormonal control. Calcium and phosphate
50% of it is free Ca 2+ (1.1–1.3 mmol/L) while ca. homeostasis is predominantly regulated by
Hormones and Reproduction sites on protein molecules also rises with the mainly affect three organs: the intestines, the
10% is bound in complexes and 40% is bound to
parathyroid hormone and calcitriol, but also by
proteins (mainly albumin; ! p. 178). Calcium
protein binding increases as the pH of the
calcitonin to a lesser degree. These hormones
blood rises since the number of Ca
binding
2+
kidneys and the bone (! B and D).
pH. The Ca
2+
Parathyrin or parathyroid hormone (PTH) is
conc. accordingly decreases in al-
kalosis and rises in acidosis (by about
a peptide hormone (84 AA) secreted by the
sensors in cells of the
2+
0.21 mmol/L Ca
parathyroid glands. Ca
2+
per pH unit). Alkalosis (e.g.,
parathyroid glands regulate PTH synthesis and
due to hyperventilation) and hypocalcemia
2+
concentration of ionized Ca
The calcium metabolism is tightly regulated
(! p. 36). More
to ensure a balanced intake and excretion of
PTH
is
bloodstream
the
secreted
into
11 (see below) can therefore lead to tetany. secretion in response to changes in the plasma
Ca
whenever the Ca
2+
2+
conc. falls below normal
provides
(! A). The dietary intake of Ca
2+
around 12–35 mmol of Ca 2+ each day (1 mmol (hypocalcemia). Inversely, PTH secretion
= 2 mEq = 40 mg). Milk, cheese, eggs and decreases when the Ca 2+ level rises (! D, left
2+
“hard” water are particularly rich in Ca . panel). The primary function of PTH is to nor-
When calcium homeostasis is maintained, malize decreased Ca conc. in the blood (! D).
2+
most of the ingested Ca 2+ is excreted in the This is accomplished as follows: (1) PTH acti-
feces, while the remainder is excreted in the vates osteoclasts, resulting in bone breakdown
urine (! p. 178). When a calcium deficiency and the release of Ca 2+ (and phosphate) from
exists, up to 90% of the ingested Ca 2+ is ab- the bone; (2) PTH accelerates the final step of
sorbed by the intestinal tract (! A and p. 262). calcitriol synthesis in the kidney, resulting in
2+
increased reabsorption of Ca from the gut; (3)
2+
Pregnant and nursing mothers have higher Ca re-
quirements because they must also supply the fetus in the kidney, PTH increases calcitriol synthe-
2+
or newborn infant with calcium. The fetus receives sis and Ca reabsorption, which is particularly
2+
ca. 625 mmol/day of Ca 2+ via the placenta, and important due to the increased Ca supply re-
nursed infants receive up to 2000 mmol/day via the sulting from actions (1) and (2). PTH also inhib-
breast milk. In both cases, the Ca 2+ is used for bone its renal phosphate reabsorption (! p. 178), re-
formation. Thus, many women develop a Ca 2+ defi- sulting in hypophosphatemia. This, in turn,
ciency during or after pregnancy. stimulates the release of Ca from the bone or
2+
Phosphate metabolism is closely related to cal- prevents the precipitation of calcium
cium metabolism but is less tightly controlled. phosphate in tissue (solubility product; see
The daily intake of phosphate is about 1.4 g; above).
0.9 g of intake is absorbed and usually excreted Hypocalcemia occurs due to a deficiency (hypo-
by the kidneys (! p. 178). The phosphate con- parathyroidism) or lack of efficiency (pseudohypo-
centration in serum normally ranges from parathyroidism) of PTH, which can destabilize the
0.8–1.4 mmol/L. resting potential enough to produce muscle spasms
Calcium phosphate salts are sparingly and tetany. These deficiencies can also lead to a sec-
soluble. When the product of Ca 2+ conc. times ondary calcitriol deficiency. An excess of PTH (hyper-
phosphate conc. (solubility product) exceeds a parathyroidism) and malignant osteolysis overpower
290 the Ca 2+ control mechanisms, leading to hypercal-
certain threshold, calcium phosphate starts to cemia. The long-term elevation of Ca results in cal-
2+
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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