deep in the brain, releases MELATONIN, a hormone
associated with sleep cycles. Eastern traditions
have long viewed the pineal gland as the meta-
physical “third eye,” an energy pathway by which
the brain communicates directly with the external
environment. Western researchers are now dis-
covering this perception may have tangible scien-
tific substance. The pineal gland is located near
the optic nerve, which appears to convey input
about external light and dark to the pineal gland.
Though researchers do not yet fully understand
the mechanisms through which this occurs, they
do know that melatonin secretion increases with
darkness and decreases with lightness, apparently
to facilitate the circadian cycle of sleep and wake-
fulness. Though as yet melatonin is the only iden-
tified hormone the pineal gland produces,
researchers believe the pineal gland has additional
functions and continue to study its role in the
body.
Hormonal rhythms, cascades, and feedback
loops The structures of the endocrine system
function in tight synchronization with one
another. Some hormonal processes are cyclic,
under the control of the body’s circadian rhythm.
Others are “on demand,” with physiologic events
triggering the release of hormones.
For example, the hypothalamus releases CORTI-
COTROPIN-RELEASING HORMONE (CRH) on a regular
cycle that begins a sharp spike a few hours before
daybreak and peaks a few hours later, dropping off
over the daylight hours to trough in the early
evening. The release of CRH initiates a cascade of
hormonal responses that accelerate metabolism in
preparation for the body’s heightened level of
activity during waking hours: CRH stimulates the
pituitary gland to release ADRENOCORTICOTROPIC
HORMONE (ACTH), which subsequently stimulates
the adrenal cortex to release cortisol. Cortisol then
initiates numerous metabolic actions throughout
the body.
Correspondingly, hormonal activity from the
thyroid gland, islets of Langerhans, and gastroin-
testinal tract accelerates, instigating further cas-
cades of hypothalamic-pituitary-adrenal activity.
As the flow of CRH diminishes, the hormonal cas-
cade slows. Feedback mechanisms also come into
play. Cortisol reaches a certain level in the blood
circulation, signaling the hypothalamus to stop
releasing CRH. The body’s metabolic activity
begins to drop off. By nightfall the CRH level
reaches its lowest point, and the body is metaboli-
cally ready for rest. The pineal gland’s release of
melatonin similarly follows, and may in fact estab-
lish, the body’s circadian rhythm.
Other hormonal cycles follow different pat-
terns. The hormonal cascades of puberty, for
example, continue during the period of growth
during which the secondary sex characteristics
emerge—typically a range between the ages of 11
to 12 and 18 to 20. A woman’s MENSTRUAL CYCLE
repeats approximately every 28 days. The hor-
mones of PREGNANCYfollow a precise schedule.
Additional hormonal activity occurs in response to
physiologic needs in integration with routine hor-
monal cycles. Feedback loops regulate such activ-
ity, with the endocrine system responding to
stimuli from other body systems.
During intense physical exercise, for example,
the hypothalamus releases ADH in response to
hormonal signals from the kidneys (renin release)
and barosensory signals from the cardiovascular
system, stimulating the adrenal glands to release
aldosterone, epinephrine, and norepinephrine to
readjust fluid volume, electrolyte balance, heart
rate, BREATHINGrate, and blood pressure. The vari-
ous physiologic changes that occur then signal the
hypothalamus to stop releasing ADH. Such
changes may take the form of rising levels of hor-
mones in the blood or events that indicate the
body’s needs are being met, such as increased
blood volume and elevated blood pressure. Some
hormones, such as ADH, are stimulatory; they ini-
tiate activity. Other hormones, such as somato-
statin, are inhibitory; they stop activity.Health and Disorders of the Endocrine System
Some endocrine structures are more active early
in life, then recede to maintenance roles later in
life. The thymus, for example, establishes the
foundation of the immune system in early to mid-
dle childhood and subsequently shrinks in size
and function at puberty to take a background,
supportive role in immune function. Other
endocrine structures become active at puberty
such as the ovaries (female) or testes (male),
known collectively as the gonads or sex glands.
The sex glands establish the body’s SECONDARY SEX-The Endocrine System 101