Chapter 16
The Endocrine System
611
16
Te parathyroid’s glandular cells are arranged in thick,
branching cords containing scattered
oxyphil cells
and large
numbers of smaller
parathyroid cells
(Figure 16.12b). Te par-
athyroid cells secrete parathyroid hormone. Te function of the
oxyphil cells is unclear.
Te parathyroid glands were discovered by accident. Years
ago, surgeons were baffled by the observation that most patients
recovered uneventfully a±er partial (or even total) thyroid gland
removal, while others suffered uncontrolled muscle spasms and
severe pain, and subsequently died. Only a±er several such
tragic deaths were the parathyroid glands discovered and their
hormonal function identified.
Parathyroid hormone (PTH)
, or
parathormone
, the protein
hormone of these glands, is the single most important hormone
controlling calcium balance in the blood. Precise control of cal-
cium levels is critical because Ca
2
1
homeostasis is essential for
so many functions, including transmission of nerve impulses,
muscle contraction, and blood clotting.
Falling blood Ca
2
1
levels trigger P²H release, and rising
blood Ca
2
1
levels inhibit its release. P²H increases Ca
2
1
levels
in blood by stimulating three target organs: the skeleton (which
contains considerable amounts of calcium salts in its matrix),
the kidneys, and the intestine
(Figure 16.13)
. P²H release
Stimulates osteoclasts (bone-resorbing cells) to digest some
of the bony matrix and release ionic calcium and phosphates
to the blood.
Enhances reabsorption of Ca
2
1
[and excretion of phosphate
(PO
4
3–
)] by the kidneys.
Promotes activation of vitamin D, thereby increasing ab-
sorption of Ca
2
1
by intestinal mucosal cells. Vitamin D is re-
quired for absorption of Ca
2
1
from food, but first the kidneys
must convert it to its active vitamin D
3
form,
calcitriol
(1,25-
dihydroxycholecalciferol). P²H stimulates this transformation.
Homeostatic Imbalance
16.5
Hyperparathyroidism
(excess P²H) is rare and usually results
from a parathyroid gland tumor. Calcium leaches from the
bones, which so±en and deform as fibrous connective tissue re-
places their mineral salts. In
osteitis fibrosa cystica
, a severe form
of this disorder, the bones have a moth-eaten appearance on X
rays and tend to fracture spontaneously. Te resulting hyper-
calcemia (abnormally elevated blood Ca
2
1
level) has many out-
comes, but the two most notable are (1) it depresses the nervous
system, which leads to abnormal reflexes and weak skeletal
muscles, and (2) excess calcium salts precipitate in the kidney
tubules, forming kidney stones. Calcium deposits may also
form in so± tissues throughout the body and severely impair vi-
tal organ functioning, a condition called
metastatic calcification
.
Hypoparathyroidism
(P²H deficiency) most o±en follows par-
athyroid gland trauma or removal during thyroid surgery. How-
ever, an extended deficiency of dietary magnesium (required for
P²H secretion) can cause functional hypoparathyroidism. Te
resulting hypocalcemia (low blood Ca
2
1
) makes neurons more
excitable and accounts for the classic symptoms of tingling sensa-
tions,
tetany
(twitching muscle), and convulsions. Untreated, the
symptoms progress to respiratory paralysis and death.
Check Your Understanding
10.
What is the major effect of thyroid hormone? Parathyroid
hormone? Calcitonin?
11.
Name the cells that release each of the three hormones listed
above.
For answers, see Appendix H.
The Adrenal (Suprarenal)
Glands
List hormones produced by the adrenal gland, and cite
their physiological effects.
Te paired
adrenal glands
are pyramid-shaped organs perched
atop the kidneys (
ad
5
near;
renal
5
kidney), where they are
enclosed in a fibrous capsule and a cushion of fat (see Figure 16.1
Hypocalcemia
(low blood Ca
2
+
)
PTH release from
parathyroid gland
Osteoclast activity
in bone causes Ca
2+
and PO
4
3-
release
into blood
Ca
2+
reabsorption
in kidney tubule
Activation of
vitamin D by kidney
Ca
2+
absorption
from food in small
intestine
Ca
2+
in blood
Initial stimulus
Physiological response
Result
Figure 16.13
Effects of parathyroid hormone on bone, the
kidneys, and the intestine.
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