596
UNIT 3
Regulation and Integration of the Body
16
Humoral Stimuli
Some endocrine glands secrete their hor-
mones in direct response to changing blood levels of certain
critical ions and nutrients. Tese stimuli are called
humoral
stimuli
to distinguish them from hormonal stimuli, which are
also bloodborne chemicals. (Te word
humoral
harks back to
the ancient term
humor
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
blood Ca
2
1
levels. When they detect a decline from normal val-
ues, they secrete parathyroid hormone (P±H)
(Figure 16.4a)
.
Because P±H acts by several routes to reverse that decline,
blood Ca
2
1
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:
humoral
(hu
9
mer-ul),
neural
, and
hormonal stimuli
.
mRNA
New protein
DNA
Receptor
binding
region
Receptor-
hormone
complex
Plasma
membrane
Receptor
protein
Cytoplasm
Nucleus
Extracellular
fluid
Steroid
hormone
1
2
3
4
5
The steroid
hormone diffuses
through the
plasma membrane
and binds an
intracellular
receptor.
The receptor-
hormone complex
enters the nucleus.
The receptor-
hormone complex
binds a specific DNA
region.
Binding initiates
transcription of the
gene to mRNA.
The mRNA
directs protein
synthesis.
Figure 16.3
Direct gene activation mechanism of lipid-soluble hormones.
Receptors
may be located in the nucleus, or in the cytoplasm as shown.
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