Mineral Metabolism
The chem ical elem ents— exclu sive o f the com m on e le-
m ents carbon, hydrogen, nitrogen, oxygen , and sulfur—
necessary for normal structure and function o f the body
are collectively know n as
and their study as
bioinorganic chemistry.
T he m inerals can be classified
trace elements.
M acrom inerals are
present in large am ounts and include sodium , potassium ,
chloride, phosphate, calcium , and m agnesium . Trace ele -
m ents are needed in very sm all am ounts. Im provem ent
in the sensitivity o f analytical m ethods has increased the
num ber o f know n essential trace elem ents, and the list
is lik ely to grow. Certain other elem ents have no know n
b iological function and are toxic.
Iron, the central elem ent in oxygen transport and uti-
lization, is discussed in Chapter 29. Iodine, a constituent
o f thyroid horm ones, is discussed in Chapter 33. Sodium ,
potassium , and chloride, w hich are im portant for m aintain-
ing proper osm olality and ionic strength and for generating
the electrical m em brane potential, are d iscussed in Chapter
39. M ost o f this chapter is devoted to the m etabolism o f
calcium and phosphorus because o f their im portance in the
skeleton and other body system s. B ecau se o f its chem ical
and b iological relationship to calcium , m agnesium is also
covered. T he trace elem ents are surveyed w ith em phasis
on those for w hich a b iochem ical function is known.
Calcium and Phosphorus
Distribution and Function
C alcium
is the fifth m ost abundant elem ent on earth
and the principal extracellular divalent cation in the hu-
man body. A healthy, 70-kg adult contains 1 -1 .2 5 kg o f
calcium (2 5 -3 3 g/k g o f fat-free tissue), w h ile a 3.5-k g
new born contains about 25 g o f calcium . A bout
o f
body calciu m is in the skeleton as hydroxyapatite crystals.
T he rem ainder is in the extracellular fluid and is exchan ge-
able w ith that in periosteal fluid, bone-form ing surfaces,
and soft tissues. Skeletal calcium is slow ly exchangeable
with extracellular fluid calcium , and the skeleton is thus
a reservoir o f calcium . The steady-state extracellular and
periosteal fluid concentrations o f calcium depend, in large
part, on the balance betw een bone form ation and bone
resorption, w hich are regulated by horm ones.
The plasm a concentration o f calcium is kept remark-
ably constant throughout life at about 8 .8 -1 0 .3 m g/dL
(2 .2 0 -2 .5 8 m m ol/L ). The norm al serum calcium con cen -
tration is m aintained by the integrated actions o f parathy-
roid horm one (PTH ) vitam in D m etabolites, calcitonin,
and cytokines such as transform ing growth factor
. The principal target sites for these horm ones
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