Biology 103 Lake Tahoe Community College
Winter Quarter Instructor: Sue Kloss
Chapter 45: Endocrine System
The focus of this chapter is on the chemical signals that make our organ systems function in a coordinated manner.
Our overriding theme, however, is homeostasis- how chemical signals maintain an animal’s steady state.
I. The nature of chemical regulation
A. Chemical signals coordinate body functions
1. Animals rely on many chemical signals to regulate body function
a. Hormones are regulatory chemicals that travel in the blood from production site to other sites
b. Hormones are made and secreted mainly by organs in the endocrine system called glands.
c. Secretory vesicles in the endocrine cell are full of the hormone molecules (red) (Fig. 45.2)
d. Endocrine cell secretes hormone directly to the circulatory system.
e. Molecules travel directly to target cells (respond to hormone) in the blood
2. Neurosecreting cells are also hormone-secreting cells
a. specialized nerve cell that in addition to conducting nerve signals, makes and secretes hormones
b. neurosecretory cells also release hormones to the blood for transport to target cells (Fig. Fig
c. all hormone secreting cells are known as the endocrine system
d. since hormones are carried in the blood, they reach the whole body.
e. Endocrine system specializes in controlling whole body activities
1. metabolic rate
2. growth rate
f. a tiny amt. of hormone can regulate activities in an enormous number of target cells in a variety of
g. endocrine system often coordinates with nervous system, other major coordinating system
1.a few substances serve as hormones in endocrine system and as chemical signals in the
nervous system- epinephrine – fight or flight hormone and neurotransmitter
h. nervous system transmits electrical signals via nerve cells
i. when a nerve signal reaches the end of a nerve cell, it signals the release of neurotransmitters
j. neurotransmitters are chemicals that carry information from one nerve cell to another
k. or from a nerve cell to another type of cell that will react – muscle or endocrine cell
l. so nervous system relies on chemical transmitters
m. unlike endocrine system, nervous system signals do not travel in the blood
3. Why do animals need 2 types of regulatory systems?
a. timing – some endocrine responses take minutes, hours or days.
1. it takes time to make hormones
2. carry them in blood to targets
b. n.s.- split second control for pulling hand off hot stove, etc.
c. n.s. impulses are incredibly fast- not endocrine system signals, though
4. neurotransmitter = local regulator which is referred to as paracrine signaling
a. local regulator secreted into interstitial fluid, affects cells very near the point of secretion
b. hormone- target cells are typically further away.
c. Local regulators called prostaglandins are made by nearly all cells – modified fatty acids
d. Prostaglandins have a variety of functions – (fig. 45.5) platelets aggregating
e. E.g. Prostaglandins from placenta cause uterus to contract in childbirth
f. Neurotransmitters are another kind of local regulator
B. Hormones affect target cells by 2 main signaling mechanisms
1. Vertebrates make over 50 kinds of hormones
a. all involve reception – when signal molecule binds to receptor in or on target cell
b. all involve signal transduction – triggers events inside a cell
c. all involve response - change in the cell’s behavior
2. Released in the blood, hormones will contact every tissue, but only cells w/receptors will be targets
a. different responses can occur to same signal molecule, with different receptor molecules in
different organs, etc.
b. eg. Skeletal muscle and liver has Beta type epinepnrine receptors, increasing blood flow
c. smooth muscles have alpha receptors, decrease blood flow there so it can go to skeletal muscle
3. other chem.. signals called pheromones carry signals btn members of a species
4. water soluble hormone doesn’t even enter the cell (Fig. 45.3)
a. binds to receptor
b. activates relay chain of molecules final relay molecule activates protein that carries out the cell’s
response (break down of glycogen stored in liver to glucose for fast energy)
c. final relay molecule activates protein that carries out the cell’s response (break down of glycogen
stored in liver to glucose for fast energy)
1) may be activation of an enzyme
2) uptake or secre3tion of specific molecules
3) rearrangement of cytoskeleton, eg.
d. other hormones can turn on specific genes
5. hormones that bind to plasma membrane receptors are all synthesized from amino acids
a. amine hormones are made from single amino acids
b. peptide hormones are short chains of amino acids
c. protein hormones are made of polypeptides and may have 200 amino acids
6. Class of hormones not made from amino acids are steroid hormones, which have intracellular receptors
a. these are made of lipids made from cholesterols (Fig. 45.3b)
b. steroid hormones such as estrogen and testosterone diffuse through phospholipid membrane of
c. bind to receptor in cytoplasm
d. receptor carries out the transduction of the hormone (when activated by the hormone, it becomes a
transcription factor - a gene activator)
e. hormone receptor complex enters the nucleus and attaches to specific sites on the cell’s DNA
f. Binding to DNA activates transcription of certain genes into RNA which is translated into proteins
g. Steroid hormones always affect gene expression
h. The same hormone can have different effects in different target cells – e.g. Epinephrine causes
heart muscle to contract faster, not breakdown glycogen (Fig. 45.4)
II. Vertebrate Endocrine System
A. Overview of the vertebrate endocrine system
1. more than a dozen glands (fig. 45.6)
a. thyroid and pituitary gland are two which are endocrine specialists – main function is to secrete
hormones to blood
b. some have both endocrine and nonendocrine functions
c. pancreas contains endocrine cells that secretes 3 hormones that influence blood sugar levels
d. pancreas also has nonendocrine function in digestion
2. (Table 45.1) summarizes all the endocrine system glands and their actions
a. all vertebrate endocrine systems have 4 classes of hormone (steroids, amines, peptides and
b. only the sex organs and the adrenal cortex secrete steroid hormones (actually enter target cells)
c. most endocrine glands produce nonsteroid hormones (signal transduction)
d. thyroid is the exception, it produces 2 hormones that are amine but enter the cells as do steroid
e. Hormones have a wide range of targets
1. sex hormones produce sex characteristics all over the body
2. gastrin, secreted by the stomach, affects the stomach
3. some hormones have other glands as their targets- pituitary gland produces thyroid stimulating
hormone, which stimulates the activity of the thyroid gland
f. some organs are part of the nervous system and the endocrine system
1. hypothalamus is part of the brain and secretes many hormones that regulate other endocrine
glands, especially the pituitary
2. Pineal and thymus are endocrine glands that are not covered in other chapters, we will review
3. Pineal gland is an outgrowth of the brain that secretes melatonin, the hormone that links
environmental light conditions with daily or seasonal rhythms.
4. Melatonin is important in breeding in sheep and deer that mate in the fall, when melatonin
levels are high
5. Thymus gland lies under the breastbone in humans and is quite large during childhood
6. Thymus important in immune system- stimulating t cell development
7. It shrinks in puberty, though it maintains immune system importance throughout life
B. Hypothalamus connects nervous and endocrine systems
a. distinction btn. n.s. and endocrine system blurs esp. w/ role of hypothalamus
b. hypothalamus is part of the brain (Fig. 45.7)
c. it receives info from nerves about internal body conditions and about the external environment
d. it then responds to these conditions by sending out appropriate nervous or endocrine signals.
e. It is master control center of the endocrine system
f. It controls the pituitary which in turn controls numerous body functions
g. Pituitary- anterior (adenohypophysis) and posterior (neurohypophysis) lobes
1. posterior pituitary is composed of nervous tissue and is actually an extension of hypothalamus
2. it stores and secretes hormones made in the hypothalamus
3. anterior pituitary is composed of non-nervous glandular tissue. Unlike the posterior pituitary it
synthesizes its own hormones,.
4. several of these control the activity of other endocrine glands and are called tropic hormones
h. thalamus controls pituitary by releasing 2 hormones into the blood:
1. releasing hormone – causes pituitary to secrete hormones
2. inhibiting hormone- makes anterior pituitary stop releasing hormones
i. example of how thalamus controls pituitary (Fig. 45.9)
1. thalamus secretes releasing hormone known as TRH (TSH Releasing Hormone)
2. TRH makes anterior pituitary secrete TSH
3. TSH makes thyroid secrete thyroxine into the blood (increases metabolic rate)
4. Negative feedback control TRH
C. Hypothalamus and pituitary have multiple endocrine functions
1. hypothalamus exerts master control over endocrine system, serves as feedback center
2. uses pituitary to relay directives to other glands
3. (Fig. 45.7) set of neurosecretory cells extend from hypothalamus into posterior pituitary
4. these cells synthesize hormones oxytocin and antidiuretic hormone (ADH)
5. these hormones (blue triangles) are channeled along neurosecretory cells into the posterior pituitary.
6. When released into the blood from posterior pituitary, oxytocin causes uterine muscles to contract
during childbirth and mammary glands to pump out milk during breast feeding
7. ADH helps kidney tubules retain water when body cells need it; when body has too much water,
hypothalamus responds to negative feedback, slowing release of ADH from posterior pituitary
8. Fig. 45.8 – second set of neurosecretory cells secrete releasing and inhibiting hormones (red dots) to
control anterior pituitary
9. Small blood vessels carry them from hypothalamus to anterior pituitary
10. In response to these hormones, ant. pit. Secretes many different peptide and protein hormones (blue
dots) which influence broad range of body activity
11. These (below) all control other glands (tropic hormones) (Fig. 45.8)
a. TSH – thyroid stimulating hormone
b. ACTH – adrenocorticotrophic hormone
c. FSH – follicle stimulating hormone
d. LH – leutinizing hormone
12. pituitary secretes growth hormone (GH)
13. GH promotes protein synthesis and use of body fat for energy metabolism in variety of target cells
14. In young mammals, GH promotes growth and development in all parts of body
15. If pituitary produces too much = giantism; too little = dwarfism
16. PRL = prolactin, also secreted by anterior pituitary – difft’ effects in diff’t species - evolution
a. mammals- milk production
b. birds- fat metabolism
c. amphibs- larval development
d. fish- salt and water balance
17. endorphins – body’s nat’l painkillers – similar to morphine
a. produced by brain and anterior pituitary
b. runner’s high
18. MSH – melanocyte stimulating hormone – controls pigments in skin of fishes, amphibs, reptiles
a. in mammals, controls hunger
III. Hormones and homeostasis (fig. 45.9)
A. Thyroid regulates development and metabolism
1. thyroid gland is right under your voicebox
2. thyroid hormones affect all tissues of vertebrates
3. thyroxine = T4 (4 iodine atoms)
4. triiodothyronine = T3 (3 iodine atoms)
5. T3 and T4 have same effects in target cells
a. crucial roles in development and maturation
b. turn tadpole into bullfrog
6. thyroid function very important in mammals in bone and nerve cell development
7. T3 and T4 maintain normal blood pressure, heart rate, muscle tone, digestion and reproductive
8. Throughout body, these hormones tend to increase the rate of O2 consumption and cell metabolism
9. Too much T3 and T4 – hyperthyroidism – overheat, sweaty, irritable, high blood pressure, lose weight.
10. Too little T3 and T4 – hypothyroidism – dietary disorders, or defective thyroid
11. Goiter – no iodine, too little T3 and T4 – Thyroid stim. Horm. TSH released too frequently
12. Goiter and dietary cretinism can be prevented by including iodine in diet.
13. Seawater is rich in iodine. Also incorporated into table salt.
14. Still a problem in developing nations
B. Hormones from thyroid and parathyroid maintain calcium homeostasis (Fig. 45.11)
1. calcium in blood and interstitial fluids important for many body functions.
a. nerve signal transmission from cell to cell
b. muscle contraction
c. blood clotting
d. cell transport across membranes
2. thyroid and parathyroid function in homeostasis of calcium ions (Ca2++) 9 – 11 mg of Ca in 100 ml
3. 4 parathyroid glands embedded in surface of thyroid
4. 2 peptide hormones, calcitonin from thyroid and parathyroid hormone (PTH),from parathyroids, regulate
blood calcium levels.
5. Calcitonin and PTH are antagonistic hormones bc they have opposite effects
6. Calcitonin lowers calcium level in the blood
7. PTH increases it (Fig. 45.11 = feedback loops)
C. Pancreatic hormones manage cellular fuel (Fig. 45.12)
1. Pancreas produces 2 hormones that play large role in managing body’s energy supply.
2. Insulin is protein hormone produced by islet cells. There are several distinct types of islet cells
3. Only beta cells produce insulin
4. Alpha cells secrete a peptide hormone called glucagon
5. Insulin and glucagon control homeostatic balance between amount of the cellular fuel glucose in blood
and glycogen, a polymer, stored in the liver (Fig. 26.8 feedback loop)
D. Diabetes – body no longer produces enough insulin, or cells don’t respond to it. Body can still absorb it
during digestion, though, so blood sugar level gets really high.
E. Adrenal glands mobilize responses to stress
1. 2 adrenal glands sit above your kidneys. (Fig. 45.13) signals come via spinal cord
2. central portion is medulla, outer is cortex. Composed of different cells and produce difft hormones
3. allow your body to deal w/ stress
4. adrenal medulla produces “fight or flight” hormones – rapid response
5. heart beats faster, hands sweat, skin gets goosebumps – can be same response in extreme pleasure
6. 2 amine hormones secreted by medulla- epinephrine (adrenaline) and norepinephrine (nonadrenaline)
7. glycogen broken down to glucose
8. increase in blood pressure, heart rate, metabolism, change in blood flow patterns.
9. Adrenal cortex responds to chemical signals in the blood rather than from spinal cord; slower, longer-
lasting stress response.
10. ACTH secretes family of steroid hormones called corticosteroids – some are glucocorticoids, some are
mineralcorticoids which increase salts and therefore blood pressure
11. High levels of glucocorticoids help suppress inflammatory response – pain is useful to a person.
F. gonads secrete sex hormones
1. gonads or sex glands – ovaries in women, testes in men secrete sex hormones and produce gametes
2. all women and men have androgens, estrogens and progestins in diff/t amts.
3. estrogen= maintain female reproductive system, and contribute to smaller body size, higher voice,
breasts and wider hips of women.
4. Progestins= preparing uterus to support embryo
5. Androgens= devel and maintenance of male reproductive system
6. Males have a high androgen:estrogen, main androgen = testosterone
7. High androgens stimulate fetus into being male and not female
8. Also lower voice, higher muscle mass, facial hair.
9. Synthesis of sex hormones is regulated by hypothalamus and anterior pituitary.
10. Releasing factor from hypothalamus causes anterior pituitary to secrete FSH and LH
11. These stimulate ovaries and testes to synthesize and secrete the sex hormones, among other effects.
Ch. 45 Lesson Objectives
1. Compare the response times of the two major systems of internal communication: the nervous system and the
2. Explain how neurosecretory cells, epinephrine, and control of day/night cycles illustrate the integration of the
endocrine and nervous systems.
3. Describe the organization of a stimulus, receptor, control center, efferent signal, and effector in a simple endocrine
4. Describe an example of a negative feedback loop in an endocrine pathway involved in maintaining homeostasis.
5. Explain why the neurohormone pathway that regulates the release of milk by a nursing mother is an example of
6. List the three major classes of molecules that function as hormones in vertebrates.
7. Name the three key events involved in signaling by vertebrate hormones.
8. Explain what changes may be triggered by a signal transduction pathway initiated by the binding of a water-soluble
hormone to a receptor in the plasma membrane of a target cell.
9. Discuss how and why different target cells exposed to the same hormone may respond in different ways.
10. Describe the nature and location of intracellular receptors for hormones that pass easily through cell membranes.
Explain how their role compares to the signal-transduction pathway noted above, and describe the changes they are
likely to trigger within the target cell.
11. Explain the role of local regulators in paracrine signaling. Describe the diverse functions of cytokines, growth
factors, nitric oxide, and prostaglandins.
12. Explain how the hypothalamus and pituitary glands interact and how they coordinate the endocrine system.
13. Describe the location of the pituitary. List and explain the functions of the hormones released from the anterior and
14. Explain the role of tropic hormones in coordinating endocrine signaling throughout the body. Distinguish between
releasing hormones and inhibiting hormones.
15. List the hormones of the thyroid gland and explain their roles in development and metabolism. Explain the causes
and symptoms of hyperthyroidism, hypothyroidism, and goiter.
16. Note the location of the parathyroid glands and describe the hormonal control of calcium homeostasis.
17. Distinguish between alpha and beta cells in the pancreas and explain how their antagonistic hormones (insulin and
glucagon) regulate carbohydrate metabolism.
18. Distinguish between type I diabetes mellitus and type II diabetes mellitus.
19. List the hormones of the adrenal medulla, describe their functions, and explain how their secretions are controlled.
20. List the hormones of the adrenal cortex and describe their functions.
21. List the hormones of three categories of steroid hormones produced by the gonads. Describe variations in their
production between the sexes. Note the functions of each category of steroid and explain how secretions are controlled.