502
UNIT THREE
parathyroid glands secrete a hormone that regulates the con-
centrations of calcium and phosphate ions in the blood.
Structure of the Glands
Each parathyroid gland is a small, yellowish brown structure
covered by a thin capsule of connective tissue. The body
of the gland consists of many tightly packed secretory cells
closely associated with capillary networks
(f g. 13.25)
.
Parathyroid Hormone
The parathyroid glands secrete a protein,
parathyroid hor-
mone
(PTH), or
parathormone
(see F
g. 13.4
c
). This hormone
increases blood calcium ion concentration and decreases
blood phosphate ion concentration through actions in the
bones, kidneys, and intestines.
The extracellular matrix of bone tissue contains a con-
siderable amount of calcium phosphate and calcium car-
bonate. PTH stimulates bone resorption by osteoclasts and
inhibits the activity of osteoblasts (see chapter 7, p. 201).
As bone resorption increases, calcium and phosphate ions
are released into the blood. At the same time, PTH causes
the kidneys to conserve blood calcium ions and to excrete
more phosphate ions in the urine. PTH also indirectly stimu-
lates absorption of calcium ions from food in the intestine by
infl uencing metabolism of vitamin D.
Vitamin D (cholecalciferol) is synthesized from dietary
cholesterol, which intestinal enzymes convert into provi-
tamin D (7-dehydrocholesterol). This provitamin is largely
stored in the skin, and exposure to the ultraviolet wave-
lengths of sunlight changes it to vitamin D. Some vitamin D
also comes from foods.
PRACTICE
24
Where is the thyroid gland located?
25
Which hormones of the thyroid gland aF
ect carbohydrate
metabolism, the mobilization of lipids, and protein synthesis?
26
What substance is essential for the production of thyroxine and
triiodothyronine?
27
How does calcitonin in±
uence the concentrations of blood
calcium and phosphate ions?
13.7
PARATHYROID GLANDS
The
parathyroid glands
(par
ah-thi
roid glandz) are on the
posterior surface of the thyroid gland, as
f
gure 13.24
shows.
Usually there are four of them—a superior and an inferior
gland associated with each of the thyroid’s lateral lobes. The
FIGURE 13.21
Cretinism is due to an underactive thyroid gland
during infancy and childhood.
FIGURE 13.22
Graves disease may cause the eyes to protrude
(exopthalmia).
FIGURE 13.23
An iodine de²
ciency causes simple (endemic) goiter
and results in high levels of TSH.
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