The Endocrine System
353
the bone-forming cells. They secrete collagen to form a sur-
rounding matrix, which then becomes calcifi ed
(mineraliza-
tion).
Once surrounded by calcifi ed matrix, the osteoblasts
are called
osteocytes.
The osteocytes have long cytoplasmic
processes that extend throughout the bone and form tight
junctions with other osteocytes.
Osteoclasts
are large, mul-
tinucleated cells that break down (resorb) previously formed
bone by secreting hydrogen ions, which dissolve the crystals,
and hydrolytic enzymes, which digest the osteoid.
Throughout life, bone is constantly remodeled by the
osteoblasts and osteoclasts working together. Osteoclasts
resorb old bone, and then osteoblasts move into the area and
lay down new matrix, which becomes mineralized. This pro-
cess depends in part on the stresses that gravity and muscle
tension impose on the bones, stimulating osteoblastic activity.
Many hormones, as summarized in
Table 11–7
, and a variety
of autocrine/paracrine growth factors produced locally in the
bone also play a role. Of the hormones listed, only parathy-
roid hormone is controlled primarily by plasma calcium con-
centration. Nonetheless, changes in the other listed hormones
have important infl
uences on bone mass and plasma calcium
concentration.
Kidneys
As you will learn in Chapter 14, the kidneys fi lter the blood
in order to eliminate soluble wastes. In the process, cells in
the tubules that comprise the functional units of the kidneys
recapture most of the necessary solutes that got fi ltered, to
minimize their loss in the urine. Thus, the urinary excretion
of calcium is the difference between the amount fi ltered and
the amount recaptured (reabsorbed). The control of calcium
excretion is exerted mainly on reabsorption. Reabsorption
decreases when plasma calcium concentration increases, and
increases when plasma calcium decreases.
The hormonal controllers of calcium also regulate
phosphate balance. Phosphate, too, is subject to a combina-
tion of fi ltration and reabsorption, with the latter hormonally
controlled.
Gastrointestinal Tract
The absorption of such solutes as sodium and potassium from
the gastrointestinal tract normally approximates 100 percent.
In contrast, a considerable amount of ingested calcium is
not absorbed from the intestine and simply leaves the body
along with the feces. Moreover, the active transport system
that achieves calcium absorption is under hormonal control.
Thus, large regulated increases or decreases can occur in the
amount of calcium absorbed from the diet. Hormonal control
of this absorptive process is the major means for regulating
total-body calcium balance, as we see next.
Hormonal Controls
The two major hormones that regulate plasma calcium concen-
tration are parathyroid hormone and 1,25-dihydroxyvitamin D.
A third hormone, calcitonin, plays a limited role, if any.
Parathyroid Hormone
Bone, kidneys, and the gastrointestinal tract are subject, directly
or indirectly, to control by a protein hormone called
para-
thyroid hormone (PTH),
produced by the
parathyroid
glands.
These endocrine glands are in the neck, embedded
in the posterior surface of the thyroid gland, but are distinct
from it (
Figure 11–31
). Parathyroid hormone production is
controlled by the extracellular calcium concentration acting
directly on the secretory cells via a plasma membrane calcium
receptor.
Decreased
plasma calcium concentration
stimulates
parathyroid hormone secretion, and an increased plasma cal-
cium concentration does just the opposite.
Table 11–7
Summary of Major Hormonal
Infl uences on Bone Mass
Hormones that favor bone formation and increased bone mass
Insulin
Growth hormone
Insulin-like growth factor I (IGF-1)
Estrogen
Testosterone
Calcitonin
Hormones that favor increased bone resorption and decreased
bone mass
Parathyroid hormone (chronic elevations)
Cortisol
Thyroid hormones (T
4
and T
3
)
Figure 11–31
The parathyroid glands. There are usually four parathyroid glands
embedded in the posterior surface of the thyroid gland.
Pharynx (posterior view)
Thyroid gland
Esophagus
Parathyroid glands
Trachea
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