ENDOCRINOLOGY
E
ENDOCRINESYSTEM
The system consists ofa collection of
hormone-producing glands. Many ofthese
glands are regulated by trophic (stimulating)
hormones secreted bythe pituitary. The
pituitary is itselfinfluenced by hormones
secreted bythe hypothalamus in the brain.
Shown here are the principal glands, with
a note on the hormones they produce.
Pancreas
This gland secretes insulin and
glucagon, which controlthe
body’s utilization ofglucose.
Adrenal cortex
When stimulated byACTH
(adrenocorticotrophic
hormone), the adrenal cortex
produces hydrocortisone,
which has widespread
effects on metabolism; it also
produces androgen hormones
and aldosterone, which
maintains blood pressure
and the body’s salt balance.
Ovaries
In females, the ovaries
produce oestrogen and
progesterone, which influence
many aspects offemale
physiology. This hormone
production is controlled by
gonadotrophin hormones,
secreted by the pituitary.
Pituitary gland
The pituitarygland secretes
hormones that stimulate
the adrenals, the thyroid,
pigment-producing skin
cells, and the gonads; it also
secretes growth hormone,
antidiuretic hormone,
prolactin, and oxytocin.
H y p o t h a l a m u s
Thyroid gland
This gland produces the
hormones thyroxine,
triiodothyronine, and calcitonin,
which stimulate metabolism,
body heat production, and bone
growth. Thyroid activityis
controlled byTSH (thyroid-
stimulating hormone), secreted
bythe pituitary.
Parathyroid glands
These glands, atthe back of
the thyroid, secrete parathyroid
hormone, which maintains the
calcium level in the blood.
Testes
In males, the testes produce
testosterone in response to
gonadotrophins secreted by
the pituitary. A combination
of gonadotrophins and
testosterone stimulates
sperm production and the
developmentofother male
characteristics.
CONTROL OF HORMONE PRODUCTION
Production oftoo much or too little hormone by a gland
is prevented by feedback mechanisms. Variations in the
blood level ofthe hormones are detected bythe part of
the brain known as the hypothalamus, which prompts the
pituitaryto modify its production ofthe appropriate trophic
(gland-stimulating) hormones accordingly.
I
The production ofhormone by the target
gland (in this illustration, the thyroid
gland) and oftrophic hormone bythe
pituitarygland is normal.
2
Ifhormone production bythe targetgland
rises, the feedback effect causes less
trophic hormone to be produced, which tends
to return the situation to normal.
3
If hormone production by the target gland
drops, the feedback lessens and more
trophic hormone is produced, which tends to
return the situation to normal.
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