Regulation and Integration of the Body
Some endocrine glands secrete their hor-
mones in direct response to changing blood levels of certain
critical ions and nutrients. Tese stimuli are called
to distinguish them from hormonal stimuli, which are
also bloodborne chemicals. (Te word
harks back to
the ancient term
to refer to various body ﬂuids.)
Humoral stimuli are the simplest endocrine controls. For ex-
ample, cells of the parathyroid glands monitor the body’s crucial
levels. When they detect a decline from normal val-
ues, they secrete parathyroid hormone (P±H)
Because P±H acts by several routes to reverse that decline,
levels soon rise, ending the stimulus for P±H re-
lease. Other hormones released in response to humoral stimuli
include insulin, produced by the pancreas, and aldosterone, one
of the adrenal cortex hormones.
estrogen causes the same cells to produce more progesterone recep-
tors, enhancing their ability to respond to progesterone.
Control of Hormone Release
Te synthesis and release of most hormones are regulated by
some type of
negative feedback mechanism
(see Chapter 1). In
such a mechanism, some internal or external stimulus triggers
hormone secretion. As levels of a hormone rise, it causes target
organ eﬀects, which then feed back to inhibit further hormone
release. As a result, blood levels of many hormones vary only
within a narrow range.
Endocrine Gland Stimuli
Tree types of stimuli trigger endocrine glands to manufacture
and release their hormones:
and binds an
enters the nucleus.
binds a speciﬁc DNA
transcription of the
gene to mRNA.
Direct gene activation mechanism of lipid-soluble hormones.
may be located in the nucleus, or in the cytoplasm as shown.