182
U N I T 2
Integrative Body Functions
feedback regulation.
35,37
It stimulates the absorption of
calcium, and to a lesser extent phosphorus, from the
intestine; it increases calcium and phosphorus reabsorp-
tion by the renal tubules; and it inhibits PTH synthesis
by the parathyroid glands. The role of PTH and vitamin
D on the skeleton is discussed further in Chapters 42
and 43.
Disorders of Calcium Balance
Calcium is the major divalent cation in the body.
Approximately 99% of body calcium is found in bone,
where it provides strength and stability for the skeletal
system and serves as an exchangeable source to main-
tain ECF calcium levels.
36,37
Most of the remaining cal-
cium (approximately 1%) is located in the ICF, and only
0.1% to 0.2% (approximately 8.5 to 10.5 mg/dL [21
to 26 mmol/L]) of the remaining calcium is present in
the ECF.
Extracellular calcium exists in three forms: (1) pro-
tein bound, (2) complexed, and (3) ionized (Fig. 8-14).
Approximately 40% of serum calcium is bound to
plasma proteins, mainly albumin. Another 10% is com-
plexed (i.e., chelated) with substances such as citrate,
phosphorus, and sulfate.
36
The remaining 50% of serum
calcium is present in the ionized form. Only the ion-
ized form of calcium (Ca
++
) is free to leave the vascu-
lar compartment and participate in cellular functions.
Since most of the protein-bound calcium combines with
albumin, total serum calcium is significantly altered by
serum albumin levels. As a general rule, a decrease in
serum albumin of 1.0 g/dL below normal will decrease
total serum Ca
++
by 0.8 mg/dL.
3
Ionized Ca
++
levels are
inversely affected by the pH of the blood. For example,
when the arterial pH increases in alkalosis, more cal-
cium becomes bound to protein.
3
Although the total
serum calcium remains unchanged, the ionized portion
decreases.
Ionized Ca
++
serves a number of functions. It partici-
pates in many enzyme reactions; exerts an important
effect on membrane potentials and neuronal excitabil-
ity; is necessary for contraction in skeletal, cardiac, and
smooth muscle; participates in the release of hormones,
neurotransmitters, and other chemical messengers; influ-
ences cardiac contractility and automaticity by way of
slow calcium channels; and is essential for blood coagu-
lation (see Chapter 12).
Regulation of Calcium Balance
Calcium enters the body through the gastrointestinal
tract, is absorbed from the intestine under the influence
of vitamin D, excreted by the kidney, and stored in bone.
Approximately 35% of dietary calcium is absorbed
from the duodenum and upper jejunum; the remainder
is eliminated in the stool.
2
Calcium is stored in bone and excreted by the
kidney. The ionized form of Ca
++
is filtered from the
plasma into the glomerulus and then selectively reab-
sorbed back into the blood. The distal convoluted
Parathyroid
glands
Bone
Intestine
Increased calcium
absorption
Release of
calcium
Kidney
Decreased calcium
elimination and increased
phosphate elimination
Feedback
Increased
serum calcium
Activated
vitamin D
Decreased serum
calcium
Parathyroid
hormone
FIGURE 8-13.
Regulation of serum calcium concentration by
parathyroid hormone.
Bone
99%
ICF
(~1%)
Free 50%
Complexed 10%
Protein
bound 40%
ECF
(0.1–0.2%)
FIGURE 8-14.
Distribution of body calcium between the
bone and the intracellular fluid (ICF) and extracellular fluid
(ECF) compartments.The percentages of free, complexed, and
protein-bound calcium in extracellular fluids are indicated.
