C H A P T E R 3 4
Introduction to the endocrine system
513
T
he endocrine system, in conjunction with the nervous
system, works to maintain internal homeostasis and to
integrate the body’s response to the external environ
ment. Their activities and functions are so closely related
that it is probably more correct to refer to them as the
neuroendocrine system
. However, this section deals
with drugs affecting the “traditional” endocrine system,
which includes
glands
—organised groups of special
ised cells that produce and secrete chemical messengers
called
hormones
directly into the bloodstream to com
municate within the body.
Some organs function like endocrine glands, but
they are not considered part of the traditional endocrine
system. In addition, certain hormones that influence body
functioning are not secreted by endocrine glands. For
example, prostaglandins are tissue hormones produced
in various tissues; they do not enter the bloodstream, but
exert their effects just in the area where they are released.
Moreover, neurotransmitters, such as noradrenaline and
dopamine, can be classified as hormones because they
are secreted directly into the bloodstream for dispersion
throughout the body. There also are many gastrointes
tinal (GI) hormones that are produced in GI cells and
act locally. All of these hormones are addressed in the
chapters most related to their effects.*
STRUCTURE AND FUNCTION OF THE
ENDOCRINE SYSTEM
The endocrine system provides communication within
the body and helps to regulate growth and develop
ment, reproduction, energy use and electrolyte balance.
The endocrine system is closely interconnected with the
nervous system, and the two systems work to maintain
homeostasis within the body to ensure maximum
function and adequate response to various internal and
external stressors.
Glands
The endocrine glands are collections of specialised cells
that produce hormones that cause an effect at hormone-
receptor sites. These glands do not have ducts, so they
secrete their hormones directly into the bloodstream.
There are many endocrine glands in the body. Table 34.1
lists the endocrine glands, the hormones that they produce
and the clinical effects that the hormones cause.
Hormones
Hormones are chemicals that are produced in the body
and that meet specific criteria. All hormones:
• are produced in very small amounts.
• are secreted directly into the bloodstream.
• travel through the blood to specific receptor sites
throughout the body.
• act to increase or decrease the normal metabolic
cellular processes when they react with their specific
receptor sites.
• are immediately broken down.
Hormones may act in two different ways. Some
hormones react with specific receptor sites on a cell
membrane to stimulate the nucleotide cyclic adeno
sine monophosphate (cAMP) within the cell to cause
an effect. For example, when insulin interacts with an
insulin-receptor site, it activates intracellular enzymes
that cause many effects, including changing the cell
membrane’s permeability to glucose. Hormones such as
insulin that do not enter the cell but react with specific
receptor sites on the cell membrane act very quickly—
often within seconds—to produce an effect.
Other hormones, such as oestrogen, actually enter
the cell and interact with a receptor site inside the cell
to change messenger RNA, which enters the cell nucleus
to affect cellular DNA and thereby alters the cell’s
function. These hormones that enter the cell before they
can cause an effect take quite a while to produce an
effect. The full effects of oestrogen may not be seen for
months to years, as evidenced by the changes that occur
at puberty. Because the neuroendocrine system tightly
regulates the body’s processes within a narrow range
of normal limits, overproduction or underproduction of
any hormone can affect the body’s activities and other
hormones within the system.
■■
The endocrine system and the nervous system
regulate body functions and maintain homeostasis
largely with the help of hormones. Hormones are
chemicals produced within the body that increase or
decrease cellular activity.
■■
The endocrine system regulates growth and
development, reproduction, energy use in the body
and electrolyte balance.
■■
Hormones can react with receptors on the cell
membrane to cause an immediate effect on a cell by
altering enzyme systems near the cell membrane or
they may enter the cell and react with receptor sites
on messenger RNA, which then enters the nucleus
and alters cell function.
KEY POINTS
*Gastrointestinal hormones are discussed in Part 11: Drugs
acting on the gastrointestinal system. Neurotransmitters acting
like hormones are discussed in Chapter 29: Introduction to the
autonomic nervous system. The reproductive hormones are discussed
in Chapter 39: Introduction to the reproductive system. Hormones
active in the inflammatory and immune response are discussed in
Part 3: Drugs acting on the immune system. Specific traditional
endocrine glands and hormones are discussed in Chapter 35
(hypothalamic and pituitary hormones), Chapter 36 (adrenocortical
hormones), Chapter 37 (thyroid and parathyroid hormones) and
Chapter 38 (pancreatic hormones).
