Regulation and Integration of the Body
occurs when one hormone opposes the action of
another hormone. For example, insulin, which lowers blood
glucose levels, is antagonized by glucagon, which raises blood
glucose levels. How does antagonism occur? Antagonists
may compete for the same receptors, act through diﬀerent
metabolic pathways, or even, as noted in the progesterone-
estrogen interaction at the uterus, cause down-regulation of
the receptors for the antagonistic hormone.
Check Your Understanding
Name the two major chemical classes of hormones. Which
class consists entirely of lipid-soluble hormones? Name
the only hormone in the other chemical class that is lipid
Consider the signaling mechanisms of water-soluble and
lipid-soluble hormones. In each case, where are the receptors
found and what is the ﬁnal outcome?
What are the three types of stimuli that control hormone
For answers, see Appendix H.
The Pituitary Gland
Describe structural and functional relationships between
the hypothalamus and the pituitary gland.
Discuss the structure of the posterior pituitary, and describe
the effects of the two hormones it releases.
List and describe the chief effects of anterior pituitary
Securely seated in the sella turcica of the sphenoid bone, the tiny
ĭ-sis; “to grow under”),
secretes at least eight hormones. Usually said to be the size and
shape of a pea, this gland is more accurately described as a pea
on a stalk. Its stalk, the funnel-shaped
the gland to the hypothalamus superiorly as shown in
Hypothalamus and Pituitary Interactions
In humans, the pituitary gland has two major lobes. One lobe
is neural tissue and the other is glandular.
(lobe) is composed largely of neu-
ral tissue such as pituicytes (glia-like supporting cells) and
nerve ﬁbers. It releases
by neurons) received ready-made from the hypothalamus.
Consequently, this lobe is a hormone-storage area and not a
true endocrine gland that manufactures hormones. Te pos-
terior lobe plus the infundibulum make up the region called
ĭ-sis), a term commonly
used (incorrectly) to indicate the posterior lobe alone.
ĭ-sis), is composed of glandular tissue (
gland). It manufactures and releases a number of hormones
on pp. 602–603).
Half-Life, Onset, and Duration
of Hormone Activity
Hormones are potent chemicals, and they exert profound ef-
fects on their target organs even at very low concentrations.
Hormones circulate in the blood in two forms—free or bound
to a protein carrier. In general, lipid-soluble hormones (steroids
and thyroid hormone) travel in the bloodstream attached to
plasma proteins. Most others circulate without carriers.
Te concentration of a circulating hormone in blood at any
time reﬂects (1) its rate of release, and (2) the speed at which
it is inactivated and removed from the body. Some hormones
are rapidly degraded by enzymes in their target cells. However,
most hormones are removed from the blood by the kidneys or
liver, and the body excretes their breakdown products in urine
or, to a lesser extent, in feces. As a result, the length of time for a
hormone’s blood level to decrease by half, referred to as its
, varies from a fraction of a minute to a week. Water-soluble
hormones have the shortest half-lives.
How long does it take for a hormone to have an eﬀect? It var-
ies. Some hormones provoke target organ responses almost im-
mediately, while others, particularly steroid hormones, require
hours to days before their eﬀects are seen. Additionally, some
hormones are secreted in a relatively inactive form and must be
activated in the target cells.
Te duration of hormone action is limited, ranging from 10
seconds to several hours, depending on the hormone. Eﬀects
may disappear rapidly as blood levels drop, or they may persist
for hours even at very low levels. Because of these many varia-
tions, hormonal blood levels must be precisely and individually
controlled to meet the continuously changing needs of the body.
Interaction of Hormones at Target Cells
Understanding hormonal eﬀects is a bit more complicated than
you might expect because multiple hormones may act on the
same target cells at the same time. In many cases the result of
such an interaction is not predictable, even when you know the
eﬀects of the individual hormones. Here we will look at three
types of hormone interaction—permissiveness, synergism, and
is the situation in which one hormone can-
not exert its full eﬀects without another hormone being
present. For example, reproductive system hormones largely
regulate the development of the reproductive system, as we
might expect. However, thyroid hormone is also necessary
(has a permissive eﬀect) for normal
reproductive structures. Lack of thyroid hormone delays re-
occurs when more than one hormone produces
the same eﬀects at the target cell and their combined eﬀects
are ampliﬁed. For example, both glucagon (produced by the
pancreas) and epinephrine cause the liver to release glucose
to the blood. When they act together, the amount of glucose
released is about 150% of what is released when each hor-
mone acts alone.