503
CHAPTER THIRTEEN
Endocrine System
A negative feedback mechanism operating between the
parathyroid glands and the blood calcium ion concentration
regulates secretion of PTH
(f g. 13.27)
. As the concentration
of blood calcium ions rises, less PTH is secreted; as the con-
centration of blood calcium ions drops, more PTH is released.
The opposite effects of calcitonin and PTH maintain
calcium ion homeostasis. This is important in a number of
physiological processes. For example, as the blood calcium
ion concentration drops (hypocalcemia), the nervous sys-
tem becomes abnormally excitable, and impulses may be
triggered spontaneously. As a result, muscles, including
the respiratory muscles, may undergo tetanic contractions,
and the person may suffocate. In contrast, an abnormally
high concentration of blood calcium ions (hypercalcemia)
depresses the nervous system. Consequently, muscle con-
tractions are weak, and refl
exes are sluggish.
Table 13.9
lists
parathyroid disorders.
The liver changes vitamin D to hydroxycholecalciferol,
which is carried in the bloodstream or stored in tissues.
When PTH is present, hydroxycholecalciferol can be changed
in the kidneys into an active form of vitamin D (dihydroxy-
cholecalciferol), which controls absorption of calcium ions
from the intestine
(f
g. 13.26)
.
FIGURE 13.24
The parathyroid glands are embedded in the
posterior surface of the thyroid gland.
Posterior view
Trachea
Esophagus
Parathyroid
glands
Thyroid
gland
Pharynx
Secretory cells
Capillaries
FIGURE 13.25
Light micrograph of the parathyroid gland (540×).
Liver
Controls absorption of
calcium in intestine
Dihydroxycholecalciferol
(active form of vitamin D)
Intestinal enzymes
Also obtained directly
from foods
Ultraviolet light in skin
Kidney
Stimulated by PTH
Hydroxycholecalciferol
Foods
Ca
+2
Ca
+2
Ca
+2
Vitamin D
(Cholecalciferol)
Cholesterol
Provitamin D
FIGURE 13.26
Mechanism by which PTH promotes calcium
absorption in the intestine.
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