346
Chapter 11
SECTION E
Endocrine Control of Growth
One of the major functions of the endocrine system is to con-
trol growth. At least a dozen hormones directly or indirectly
(e.g., hypophysiotropic hormones) play important roles in con-
trolling growth. This complex process is infl uenced by genet-
ics, endocrine function, and a variety of environmental factors,
including nutrition. The growth process involves cell division
and net protein synthesis throughout the body, but a person’s
height is determined specifi cally by bone growth, particularly
of the vertebral column and legs. We fi rst provide an overview
of bone and the growth process before describing the roles of
hormones in determining growth rates.
Bone Growth
Bone is a living tissue consisting of a protein (collagen) matrix
upon which calcium salts, particularly calcium phosphates,
are deposited. A growing long bone is divided, for descriptive
purposes, into the ends, or
epiphyses,
and the remainder, the
shaft.
The portion of each epiphysis in contact with the shaft is
a plate of actively proliferating cartilage, the
epiphyseal growth
plate
(
Figure 11–26
).
Osteoblasts,
the bone-forming cells at
the shaft edge of the epiphyseal growth plate, convert the carti-
laginous tissue at this edge to bone while cells called
chondro-
cytes
simultaneously lay down new cartilage in the interior of
the plate. In this manner, the epiphyseal growth plate remains
intact (indeed, actually widens) and is gradually pushed away
from the center of the bony shaft as the shaft lengthens.
Linear growth of the shaft can continue as long as the
epiphyseal growth plates exist, but ceases when the growth
plates are converted to bone as a result of hormonal infl uences
at puberty. This is known as
epiphyseal closure
and occurs
at different times in different bones. Thus, a person’s
bone
age
can be determined by x-raying the bones and determining
which ones have undergone epiphyseal closure.
As shown in
Figure 11–27
, children manifest two peri-
ods of rapid increase in height, the fi rst during the fi rst two
years of life and the second during puberty. Note that increase
in height is not necessarily correlated with the rates of growth
of specifi c organs.
The pubertal growth spurt lasts several years in both sexes,
but growth during this period is greater in boys. In addition,
boys grow more before puberty because they begin puberty
approximately two years later than girls. These factors account
for the differences in average height between men and women.
fi ght-or-fl ight response
343
stress
342
SECTION D CLINICAL TERMS
Addison’s disease
344
adrenal insuffi ciency
344
allergy
343
arthritis
343
atherosclerosis
344
chronic fatigue syndrome
344
Cushing’s disease
344
Cushing’s Syndrome
344
hypertension
344
hypotension
344
immunosuppression
345
osteoporosis
345
primary adrenal
insuffi ciency
344
secondary adrenal
insuffi ciency
344
tuberculosis
344
SECTION D REVIEW QUESTIONS
1. Diagram the CRH-ACTH-cortisol pathway.
2. List the physiological functions of cortisol.
3. Defi ne stress, and list the functions of cortisol during stress.
4. List the major effects of activation of the sympathetic nervous
system during stress.
5. Contrast the symptoms of adrenal insuffi ciency and Cushing’s
Syndrome.
II. Epinephrine is secreted from the adrenal medulla during
stress, in response to stimulation from the sympathetic
nervous system. The norepinephrine from sympathetic neuron
terminals, combined with the circulating epinephrine, prepare
the body for stress in several ways. These include increased
heart rate and heart pumping strength, increased ventilation,
increased shunting of blood to skeletal muscle, and increased
generation of fuel sources that are released into the blood.
Psychological Stress and Disease
I. Chronic psychological stress can negatively affect health by
contributing to the progression or development of several
diseases, among them hypertension and atherosclerosis. Of
greater concern is that chronic psychological stress may cause
immunosuppression, and this in turn has been postulated to
contribute to the advent of certain types of cancer.
Additional Clinical Examples
I. Adrenal insuffi ciency may result from adrenal destruction
(primary adrenal insuffi ciency, or Addison’s disease) or from
hyposecretion of ACTH (secondary adrenal insuffi ciency).
II. Adrenal insuffi ciency is associated with decreased ability to
maintain blood pressure (due to loss of aldosterone) and blood
sugar. It may be fatal if untreated.
III. Cushing’s Syndrome is the result of chronically elevated
plasma cortisol concentration. When the cause of the increased
cortisol is secondary to an ACTH-secreting pituitary tumor,
the condition is known as Cushing’s disease.
IV. Cushing’s Syndrome is associated with hypertension, high blood
sugar, redistribution of body fat, obesity, and muscle and bone
weakness. If untreated, it can also lead to immunosuppression.
SECTION D KEY TERMS
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