The parathyroid glands and
the
parafollicular cells
of the
thyroid
gland
are
thought
to
work together to control blood
calcium levels
(Figure 9.11).
The calcium level in the blood
must be maintained within nar-
row limits for proper functioning
of nerves and muscles, particu-
larly the heart muscle, vascular
smooth muscle, and intestinal
smooth muscles.
When the parathyroid glands
are
damaged,
which
happens
most often during surgery on the
thyroid, the circulating levels of
PTH
fall.
Hypoparathyroidism
leads to a fall in blood calcium
levels, which
especially affects
nerve and muscle cells. It can of-
ten be treated with calcium and
vitamin D supplements.
Location o f th e p arath y ro id g lan d s • Figure 9.1 0
The parathyroid glands are small glands embedded
in the posterior side of the thyroid gland.
Left superior
parathyroid gland
Esophagus
Left inferior
parathyroid gland
Parathyroid
glands (behind
thyroid gland)
Thyroid gland
Right superior
parathyroid gland
Right inferior
parathyroid gland
Trachea
Trachea
Posterior view
The activity o f calcitonin and p arath y ro id h o rm o n e • Figure 9.11
Calcitonin and parathyroid hormone
act in concert to regulate blood
calcium levels.
r
Q Calcitriol stimulates the
intestine to absorb calcium
from food into the blood,
thereby increasing blood
calcium levels.
>
k
^
PTH stimulates the
kidneys to release
calcitriol.
The primary target tissues for PTH,
calcitonin, and calcitriol are
V
_____
:______;______
J
)
k
V
The thyroid’s parafollicular
Q The parafollicular cells
Q PTH stimulates osteoclasts in bone Q Calcitonin inhibits osteoclast
to release calcium into the blood.
activity in bone and lowers
PTH acts on kidney to decrease
blood calcium levels,
excretion of calcium into the urine.
V
_________
S
Endocrine Glands Regulate Key Body Functions
265
PROCESS DIAGRAM
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