Name
CHAPTER 39
Class
Date
Endocrine System
SECTION
1 Hormones
KEY IDEAS
As you read this section, keep these questions in mind:
• What are the major functions of the endocrine system?
• Which structures produce and release hormones?
• Which cells can be affected by hormones?
• What are two ways that hormones cause changes inside a cell?
• How are hormone levels in the blood regulated?
What Is the Endocrine System?
READING TOOLBOX
Like the nervous system, the endocrine system helps
to regulate and control other systems in your body.
Remember that signals from the nervous system travel
along neurons. They move very quickly. In contrast,
signals from the endocrine system travel more slowly. In
general, endocrine signals travel in the form of chemicals
in the blood. They control changes that happen in the
body over relatively long periods of time.
The endocrine system has four main functions:
• regulate metabolism
• maintain salt, water, and nutrient balance in the blood
Summarize As you read this
section, underline the main
ideas. When you finish reading, write an outline of the
section using the ideas you
underlined.
READING CHECK
1. Compare Give two
differences between
endocrine signals and signals
from the nervous system.
• control the body’s responses to stress
• regulate growth, development, and reproduction
What Carries Endocrine Signals?
Hormones carry most signals from the endocrine
system. Hormones are substances produced in one part
of the body that cause changes in another part of the
body. These changes generally help the body maintain
homeostasis.
As these bicyclists
sweat, they lose
water and salt. In
response, their
endocrine systems
produce hormones
that help their
bodies retain water
and salt. This helps
them maintain
homeostasis.
Background
Recall that homeostasis is the
consistent internal conditions
that your body maintains.
EHHDBG@<EHL>K
2. Explain How does the
endocrine system help
maintain homeostasis when
a person sweats?
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Reader
439
Endocrine System
Name
SECTION 1
Class
Date
Hormones continued
Where Are Hormones Made?
READING CHECK
Not all cells can make hormones. Only cells that
are part of endocrine glands or endocrine tissues can
produce and secrete, or give off, hormones.
A gland is a structure that secretes hormones or
other substances. There are two main kinds of glands:
endocrine glands and exocrine glands. Endocrine
glands release hormones directly into the blood or the
fluid around cells. The main functions of endocrine
glands are to produce and secrete hormones. The figure
below shows the main endocrine glands.
3. Identify What are the
main functions of endocrine
glands?
(YPOTHALAMUS
0ITUITARYGLAND
4HYROIDGLAND
0ARATHYROID
GLANDS
EHHDBG@<EHL>K
4. List Name four endocrine
glands that are found in both
males and females.
4HYMUSGLAND
!DRENAL
GLANDS
0ANCREAS
/VARIES
ONLYINFEMALES
4ESTES
ONLYINMALES
There are many endocrine glands in the human body. The hormones that
endocrine glands secrete have many different functions.
8g^i^XVa I]^c`^c\
5. Compare Give two
differences between
endocrine glands and
exocrine glands.
Exocrine glands do not produce or secrete hormones.
They secrete nonhormonal substances, such as enzymes,
through tubelike structures called ducts. Unlike exocrine
glands, endocrine glands do not have ducts.
Some organs are both endocrine glands and exocrine
glands. For example, the pancreas acts as an exocrine
gland when it secretes digestive enzymes to the small
intestine. However, the pancreas acts as an endocrine
gland when it secretes hormones into the bloodstream.
Some organs, such as the brain, stomach, kidneys, and
liver, contain cells that release hormones. These organs
are not considered endocrine glands because their main
function is not to secrete hormones. The cells in these
organs that secrete hormones are called endocrine tissues.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Reader
440
Endocrine System
Name
SECTION 1
Class
Date
Hormones continued
How Do Hormones Affect Cells?
Recall that hormones travel through the bloodstream.
As a result, they come into contact with most cells of the
body. However, a hormone affects only certain cells. The
cells that are affected by a hormone are called target
cells.
Target cells have specific receptor proteins that
recognize and bind to a specific hormone. These receptor
proteins can be on the cell membrane or inside the cell.
There are two main types of hormones: amino-based
hormones and cholesterol-based hormones. Aminobased hormones cannot pass through cell membranes.
Cholesterol-based hormones can. Because of this
difference, the two types of hormones affect cells in
different ways.
Type of hormone
How it works
Amino-based
hormones
Amino-based hormones bind to receptor proteins on
the surfaces of target cells. The proteins then cause
enzymes inside the cell to produce molecules called
second messengers. Second messengers activate
other enzymes within the cell.
Cholesterol-based
hormones
Cholesterol-based hormones bind to receptor proteins
inside target cells. When the hormone is bound to the
receptor protein, it can directly activate or inactivate
genes.
Amino-based hormones and cholesterol-based
hormones can produce the same effect through different
processes. For example, glucagon is an amino-based
hormone. Cortisol is a cholesterol-based hormone.
Both hormones can increase the amount of glucose in
the bloodstream. However, they produce this effect in
different ways.
Glucagon, like most amino-based hormones, binds
to proteins on the outsides of target cells. The protein
activates an enzyme, which produces a second messenger
called cAMP. The cAMP molecules activate other
enzymes, which break a compound called glycogen down
into glucose. The glucose is then released into the blood.
In contrast, cortisol passes through cell membranes
and binds to receptor proteins inside target cells. The
proteins then activate genes. The genes produce proteins
that break down fats and proteins into glucose. The
glucose is then released into the blood.
EHHDBG@<EHL>K
6. Compare How is the
location of receptor proteins
that bind to amino-based
hormones different from
that of receptor proteins that
bind to cholesterol-based
hormones?
KXcb8Yflk@k
Summarize After you read
this page, make a diagram
showing the similarities
and differences between
glucagon and cortisol. Share
your diagram with a small
group. Explain how your
diagram describes the two
hormones.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Reader
441
Endocrine System
Name
SECTION 1
Class
Date
Hormones continued
What Controls the Production of Hormones?
READING CHECK
7. Identify What are two
ways the body controls
hormone levels?
KXcb8Yflk@k
Apply Concepts In a small
group, brainstorm examples
of positive and negative
feedback that you may see in
everyday life. For example, a
thermostat that controls the
temperature of a room is an
example of a negative feedback mechanism. Share your
examples with the rest of
the class. Explain why each
is an example of a feedback
mechanism.
8g^i^XVa I]^c`^c\
8. Explain Why are insulin
and glucagon antagonistic
hormones?
The body has to have a way of controlling how much
of a hormone is in the blood at a given time. For example,
imagine what would happen if the amount of glucagon
or cortisol in the blood never changed. The body would
keep producing glucose. Eventually, the amount of
glucose in the blood would get too high, and the person
would die.
The body uses feedback mechanisms and antagonistic
hormones to control hormone levels in the blood. In a
feedback mechanism, one step in a series of events
controls a previous step in the series. Antagonistic
hormones are pairs of hormones that have opposite
effects on the body.
FEEDBACK MECHANISMS
There are two main kinds of feedback mechanisms in
the body: positive feedback and negative feedback. Most
hormone systems in the body use negative feedback.
Type of feedback
Description
Example
Positive
A change in the
body causes more
change in the
same direction.
A hormone controls the release of
eggs in women. A small amount of
the hormone causes more of the
hormone to be made, until an egg
is released.
Negative
A change in the
body prevents
more change
in the same
direction.
The thyroid gland produces several
hormones. The presence of these
hormones in the blood causes
the thyroid to stop making and
releasing them.
ANTAGONISTIC HORMONES
Antagonistic hormones work in pairs to control body
systems. For example, remember that glucagon causes
the amount of glucose in the blood to increase. A hormone called insulin is an antagonistic hormone to glucagon. Insulin causes the amount of glucose in the blood to
decrease.
These two hormones act to control the level of glucose in the blood. When glucose levels are high, the body
secretes insulin, which lowers glucose levels. When glucose levels are low, the body secretes glucagon, which
increases glucose levels.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Reader
442
Endocrine System
Name
Class
Date
Section 1 Review
SECTION VOCABULARY
antagonistic hormone a hormone that
counteracts the effect of another hormone
endocrine gland a ductless gland that secretes
hormones into the blood
feedback mechanism a cycle of events in which
information from one step controls or affects a
previous step
hormone a substance that is made in one cell or
tissue and that causes a change in another cell
or tissue located in a different part of the body
second messenger a molecule that is generated
when a specific substance attaches to a
receptor on the outside of a cell membrane,
which produces a change in cellular function
target cell a specific cell to which a hormone is
directed to produce a specific effect
1. Identify What are the four major functions of the endocrine system?
2. Describe Which cells can produce and secrete hormones?
3. Explain Hormones move in the blood, so they come into contact with many
different cells. Why don’t they affect every cell they come into contact with?
4. Describe Fill in the blank spaces in the flowchart below to describe how amino-
based hormones affect cells.
The
causes enzymes in the cell
to produce molecules called
An amino-based hormone binds to a
on
the outside of a target cell.
.
The other enzymes cause a change
in the cell.
The
activates other enzymes within
the cell.
5. Compare How is positive feedback different from negative feedback?
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Reader
443
Endocrine System
Name
CHAPTER 39
Class
Date
Endocrine System
SECTION
2 Major Endocrine Glands
KEY IDEAS
As you read this section, keep these questions in mind:
• Which two glands control the endocrine system?
• Which glands regulate metabolism?
• How is the adrenal gland involved in responding to stress?
• Which glands and hormones regulate reproduction?
READING TOOLBOX
Define As you read this
section, underline words you
don’t know. When you figure
out what the words mean,
write the words and their
definitions in your notebook.
What Controls the Endocrine System?
Remember that endocrine glands produce and secrete
hormones. Two of these glands—the hypothalamus and
the pituitary gland—work with the nervous system to
control the endocrine system. Both the hypothalamus and
the pituitary gland are located in the brain.
The hypothalamus affects both the nervous
system and the endocrine system. It controls
feelings of hunger and thirst. It also produces
hormones that affect the pituitary gland and
other parts of the body.
Hypothalamus
Anterior
pituitary
Posterior pituitary
EHHDBG@<EHL>K
1. Compare Give one
difference between the
anterior pituitary gland and
the posterior pituitary gland.
The anterior pituitary gland produces
several important hormones. Some
of them, called tropins, control
hormone production in other glands.
The others affect body processes,
such as cell growth and division.
The posterior pituitary gland
does not produce hormones.
However, it does store and release
hormones that are made by the
hypothalamus. These hormones
affect other parts of the body, such
as the kidneys.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Reader
444
Endocrine System
Name
Class
SECTION 2
Date
Major Endocrine Glands continued
Which Glands Regulate Metabolism?
Remember that metabolism is all of the chemical
reactions that occur in the body. The thyroid gland,
parathyroid glands, pancreas, and pineal gland control
metabolism. Each has a different function.
Gland
Location
Role
Thyroid
throat
produces hormones that control metabolic
rate, protein production, growth, development,
and reproduction
Parathyroid
glands
throat,
below the
thyroid
produce a hormone that increases the amount
of calcium in the blood
Pancreas
abdomen
produces hormones that control the level of
sugar in the blood
Pineal gland
brain
produces the hormone that controls the sleep
cycle
8g^i^XVa I]^c`^c\
2. Infer A person has
Type I diabetes. His body
cannot control the amount
of sugar in his blood. Which
gland do you think Type I
diabetes affects? Explain your
answer.
How Does the Endocrine System Respond
to Stress?
Think about how you feel when someone scares
you. Your heart beats faster. You may breathe harder.
Your hands may start to sweat or shake. These are all
reactions to stress. Two glands above your kidneys, the
adrenal glands, control responses like these.
Each adrenal gland has two parts: the adrenal medulla
and the adrenal cortex. The adrenal medulla controls
short-term responses to stress, which generally occur
quickly and do not last very long. The adrenal cortex
controls long-term responses to stress, which generally
happen slowly and last a longer time.
READING CHECK
3. Identify Which glands
control the body’s responses
to stress?
Adrenal
cortex
Adrenal
gland
EHHDBG@<EHL>K
Adrenal
medulla
Kidney
4. Label On the figure, label
the functions of the adrenal
cortex and the adrenal
medulla.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Reader
445
Endocrine System
Name
SECTION 2
Class
Date
Major Endocrine Glands continued
HORMONES PRODUCED BY ADRENAL GLANDS
READING CHECK
5. Describe What does
epinephrine do?
The adrenal medulla produces epinephrine and
norepinephrine. Epinephrine increases heart rate and
blood flow to the muscles and brain. It also causes the
liver to produce more glucose so that the muscles have
more energy available. Norepinephrine increases blood
pressure to make sure muscles get enough oxygen and
nutrients. This results in the “fight-or-flight” response
humans feel when they are under sudden stress.
The adrenal cortex produces several different hormones called corticosteroids. These hormones increase
blood pressure and glucose levels. This helps a person’s
body respond to severe or long-term stress. However, if
the stress lasts too long, corticosteroids can make the
person more likely to get sick.
What Hormones Control Reproduction?
8g^i^XVa I]^c`^c\
6. Compare Give one
similarity and one difference
between gonadotropins and
sex hormones.
There are two main types of hormones that control
reproduction: gonadotropins and sex hormones.
Gonadotropins are produced by the pituitary gland. They
control the functions of the gonads—the ovaries and the
testes. They cause the gonads to mature during puberty.
They also cause the gonads to produce sperm and egg
cells.
Sex hormones are produced by the gonads. There are
three types of sex hormones: androgens, estrogens, and
progesterone.
Type of hormone
Description
Effects
Androgens
male sex hormones;
mainly produced by
the testes
control the development
and function of the male
reproductive system; causes
the development of male
characteristics, such as facial
hair
Estrogens
female sex
hormones; mainly
produced by the
ovaries
work with progesterone to
control the development
and function of the female
reproductive system
Progesterone
female sex
hormone; produced
by the ovaries and
by the placenta
during pregnancy
works with estrogens to
control the female reproductive
system; causes changes in
the uterus to prepare it for
pregnancy
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Reader
446
Endocrine System
Name
Class
Date
Section 2 Review
SECTION VOCABULARY
androgen a type of hormone that regulates
the sexual development of males and that
stimulates development of secondary sex
characteristics in males
epinephrine a hormone that is released by the
adrenal medulla and that rapidly stimulates
the metabolism in emergencies, decreases
insulin secretion, and stimulates pulse and
blood pressure; also called adrenaline
estrogen a hormone that regulates the sexual
development and reproductive function of
females
norepinephrine a chemical that is both a
neurotransmitter produced by the sympathetic
nerve endings in the autonomic nervous
system and a hormone secreted by the
adrenal medulla to stimulate the functions
of the circulatory and respiratory systems
(abbreviation, NE)
progesterone a steroid hormone that is secreted
by the corpus luteum of the ovary, that
stimulates changes in the uterus to prepare for
the implantation of a fertilized egg, and that is
produced by the placenta during pregnancy
1. Identify Which two glands control the endocrine system?
2. Describe What are four functions of the thyroid gland?
3. Compare Give one similarity and one difference between epinephrine and
norepinephrine.
4. Explain How can corticosteroids be both helpful and harmful to a person?
5. Describe What do androgens do?
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Reader
447
Endocrine System