Chapter 6 The Endocrine System • MHR 169Endocrine glandsare ductless glands that
secrete hormones directly into the bloodstream,
whereas exocrine glandsrelease their secretions
through ducts or tubes (as shown in Figure 6.2).
Examples of exocrine glands are sweat glands,
salivary glands, and tear (lacrimal) glands.
Hormones produced by the endocrine glands
influence the activity of every organ and tissue in
the body. The term “hormone” was introduced in
1908 by British physiologist Ernest Henry Starling.
He identified hormones as “chemical messengers,”
substances that carry instructions or signals to one
or more distant organs or tissues in the body.
These signals instigate some type of specialized
biochemical process in the target organ. For
example, the pancreas produces hormones that
can stimulate the liver to convert sugar to glycogen,
or glycogen to sugar, depending on the body’s
immediate needs.
Very small quantities of each type of hormone
are produced and secreted into the blood. The
concentration of a hormone substance in blood
may be no more than 10 −^12 mol/L (which can be
compared to a drop of oil in a swimming pool full
of water). However, the potency (or impact) of
hormones is magnified many times by their ability
to affect key metabolic processes in the target cells.
Figure 6.2(A) Endocrine glands do not have ducts; they
produce and secrete hormones directly into the bloodstream.
(B) Exocrine glands have ducts and secrete sweat, milk,
digestive enzymes, and other materials.
The adrenal gland has been the focus of many
experimental breakthroughs in the field of
endocrinology. One of the founders of
endocrinology, a British country doctor named
George Oliver, was among the first to demonstrate
the physiological action of an extract of endocrine
tissue. In 1894, he discovered that a preparation of
tissue from the adrenal gland could raise blood
pressure in test subjects.
In 1897, American scientist John Jacob Abel
discovered adrenaline, the first hormone molecule
to be isolated from an endocrine gland extract.
Following Abel’s discovery, Jokichi Takamine, a
Japanese-born chemist working in the United
States, independently isolated the same hormone
molecule, which he named “adrenaline.” With
Takamine’s assistance, a U.S. pharmaceutical
company soon began to mass-produce and sell
adrenaline as a treatment for a number of ailments.
At the time, the company was unaware that their
product was actually a mixture of two hormones,
adrenaline and noradrenaline. Both hormones are
produced by the adrenal gland.
Hormones such as adrenaline secreted by the
adrenal gland come into contact with virtually all
cells and tissues as they circulate throughout the
body. However, they trigger a response only in cells
(such as liver cells) that have specific receptor sites
for the hormone. The combining of a hormone, such
as adrenaline, with a specific receptor on the outer
membrane of a liver cell sets off a cascade of
chemical reactions, such as the conversion of
glycogen to sugar.Factors in Hormone
Production and Function
In general, hormone production increases or
decreases in response to changing metabolic needs
of the body, such as fluid balance, and other factors
such as infection, physical injury, and emotional
stress. Hormone levels are also regulated by the
activity of the nervous system and other endocrine
glands. Hormone-secreting cells contain receptor
molecules that are sensitive to regulatory hormones
from other sources in the body. For example,
thyroid-stimulating hormone molecules produced
by the pituitary gland bind to receptors on cells of
the thyroid gland. This action stimulates synthesis
of a hormone called thyroxine, which you will
study in greater detail later in this chapter.
The impact of a specific hormone on the activity
of target tissues is a function of the rate of hormoneblood
vesselblood
vesselglandductsecretionglandsecretionEndocrine gland
Exocrine gland