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Endocrine
System
Chapter 18
Two systems that directly/indirectly
control most body functions
Nervous System
Endocrine System
How do these systems work?
Nervous System
Endocrine System
Example:
Foot steps on a nail
• impulse is generated
• impulse travels to the
spinal cord
• impulses travel to the
thigh muscle
• thigh muscles contract
• foot is lifted
Example:
Eat a candy bar
• blood sugar level goes up
• pancreas are stimulated to
release hormone-insulin in
blood
• insulin helps liver and skeletal
muscle in taking out access
blood sugar and storing as
glycogen
• blood sugar level goes down
How do these systems work?
Nervous System
• Sensory system
• Transmits impulses
• Impulses go directly to
the specific target cells
• Impulses cause
contraction of muscles or
secretion from glands
• Takes a fraction of a
second to respond …
faster system
• Affect wears off within
seconds
Endocrine System
• Chemical system
• Releases hormones into blood
• Hormones are sent
everywhere but act on specific
target cells
• Hormones affect metabolism,
membrane permeability,
growth or development
• Takes minutes, hours, or days
to respond….slower system
• Affect last days, hours or
months
Endocrine and nervous systems
work together and affect each other:
Impulses may affect the release of certain
hormones.
Hormones may affect generation/conduction of
impulses.
How Cells Communicate with one another
• Direct communication: is seen between cells
of the same type which are in direct contact
with one another via gap junctions.
• Paracrines:
Local regulators, Released by body cells
Act on neighboring cells
Example: Nitric oxide (NO) secreted by
endothelial cells lining blood vessels 
causes vasodilation of the blood vessels 
• Endocrines:
True hormones
Made by endocrine cells or glands
Diffuse into blood
Travel to their specific target cells or organs
Example: Insulin released by pancreas and
act on liver and other cells as target cell.
• Synaptic communication: Neurons release
chemicals called neurotransmitters at
junctions close to target cells with specific
receptors.
Components of the
Endocrine System
Some endocrine glands secrete
hormone as their primary
function:
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Pituitary gland
Pineal gland
Thyroid gland
Parathyroid
Adrenal gland
Other endocrine glands secrete
hormones as their secondary
function:
•
•
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•
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•
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Hypothalamus
Thymus
Pancreas
Ovaries
Testis
Stomach
Kidneys
Small intestine
Functions of Hormones
• Help maintain homeostasis (water, pH,
blood sugar, temperature, minerals).
• Help regulate metabolism
(making/breaking organic chemicals,
energy production).
• Help regulate growth, development and
reproduction.
Review Time!!!
Labeling Activity: Endocrine glands of the body.
Answer the following questions to review the
material covered so far.
• What is Hoemostasis?
• Which two systems of the body maintain
homeostasis? What is their mechanism of
action?
• What is the difference between primary and
secondary endocrine organs? Give
examples of each.
• Write down the functions of hormones in the
body?
Characteristics of hormones
• Over 50 types of hormones produced by
human body
• Hormones are made by endocrine glands
• Hormones are made in very small amounts
• Hormones diffuse into blood and are
transported by blood to all body parts
• Only specific target cells or organs recognize
and respond to each hormone
• Target cells have specific receptor proteins
Classification of Hormones
• Based on their chemical structure, hormones can
be divided into three groups:
– Amino acid derivatives: also known as biogenic
amines, are derived from amino acids, e.g.
thyroid hormones, epinephrine,
norepinephrine and melatonin.
– Peptide hormones: are chains of amino acids,
e.g. TSH, LH, FSH, ADH, Oxytocin, GH and PRL.
– Lipid derivatives: are divided into two classes eicosanoids (signaling molecules, e.g.
prostaglandins) and steroid hormones (derived
from cholesterol, e.g. estrogen and androgens).
Measurement of Hormones
• Hormones are released in blood and transported
by blood.
• Most hormones are measured by taking a blood
sample
• Some hormones are produced in excess and
excreted in urine….measured in urine sample
Secretion and Distribution of Hormones
Hormones are released in blood and transported by
blood
• Some hormones circulate freely in the blood, e.g
peptide hormones. These hormones remain
functional for a short period of time, minutes to
hours. Taken as shots/injections. Examples insulin, growth hormone.
• Thyroid hormones and steroid hormones travel in
the blood bound to a special transport proteins
and therefore remain in circulation much longer.
Can be administered orally.
Hormone Action
Hormones are made by an endocrine gland  they
travel through blood  they are recognized by
specific receptors present on the target cells  a
new protein is made or an inactive protein is
activated  protein could be an enzyme, a transport
protein, or a protein hormone
There are 2 basic mechanisms of hormone action:
• Water insoluble/lipid soluble hormone action
• Water soluble/lipid insoluble hormone action
Hormones and Intracellular receptors
Hormone diffuses out of the blood
capillary to go to its target cell
--> Lipid soluble hormone diffuses
through the phospholipid bilayer of
the plasma membrane
--> Hormone binds to the specific
cytoplasmic receptor
--> OR
--> Hormone diffuses through the
nuclear membrane to enter the
nucleus and bind to nuclear receptor
--> Hormone-receptor (HR) complex
is formed
--> HR complex binds to specific
locations on DNA
--> Specific genes are activated
--> new mRNA and proteins are
made
--> These proteins could be
enzymes or transport proteins
needed for specific hormone action
Hormones and Plasma Membrane Receptors
Hormone diffuses out of the
blood capillary to go to its target
cell --> Peptide hormones
cannot pass phospholipid
bilayer of the plasma
membrane --> Hormone binds
to specific receptor present on
plasma membrane --> Hormone
receptor (HR) complex is
formed on the outer surface of
the plasma membrane --> G
protein on the inside of the
membrane is activated --> leads
to the appearance of a second
messenger (cAMP/cGMP/Ca) -> second messenger acts as an
enzyme activator, inhibitor, or
cofactor --> results in a change
in the rate of metabolic
reactions
Hormones and Receptor Concentration
Down Regulation: Presence of a hormone triggers
decrease in number of hormone receptors. When levels
of particular hormone are high, cells become less
sensitive to it.
Up Regulation: Absence of a hormone triggers increase
in number of hormone receptors. When levels of
particular hormone are low, cells become more sensitive
to it.
Stimulation of an Endocrine Gland
There are 3 types of signals that stimulate
endocrine glands:
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Nerve impulse (Neural) stimulation
A nerve impulse stimulates an endocrine gland
Example: Stress or anxiety  generation of nerve impulses 
impulses travel to adrenal cortex  release of epinephrine and
norepinepherine  increased blood sugar, heart rate, breathing
 help the body cope with stress
Hormonal stimulation
A hormone stimulates another endocrine gland to make its
hormones
Example: Hypothalamus makes thyrotropin releasing hormone
(TRH*)  TRH stimulates anterior pituitary to make thyrotropin 
thyrotropin* stimulates thyroid gland to make thyroid hormones
* TRH and thyrotropin are called tropic hormones
Non-hormonal or other chemical stimulation (humoral stimulation)
A chemical other than a hormone stimulates an endocrine gland
Example: When blood sugar goes up  pancreas are stimulated
to produce insulin; when blood sugar level goes down  insulin
secretion goes down
Hormone Interaction
Most body functions are regulated by two or more
hormones  hormones often have to interact with
each other
• Permissive interaction
– When one hormone is required before the target cell can respond to
another hormone
– Example: Estrogen is required for the development of the uterus lining 
then progesterone maintains this lining during pregnancy
• Synergistic interaction
– When two or more hormones complement each other to produce a
greater response
– Example: Estrogen, progesterone, prolactin affect the growth of the
mammary gland
• Antagonistic interaction
– When two hormones oppose the action of each other
– Example: Insulin decreases blood sugar while glucagon increases blood
sugar
Control of Hormone Concentration
Hormones are released in short bursts
Feedback control mechanism
A sequence of events where the end response
regulates the secretion of the hormone
• Negative feedback control – most common
Where the body’s response has an opposite
affect on secretion of hormone
• Positive feedback control – not as common
Where the body’s response has the same affect
on the secretion of hormone
Review Time !!!!
• Labeling Activity: three mechanisms of hypothalamic control
over endocrine function.
Answer the following questions to review the material covered
so far.
• Name the types of hormones based on their structure.
• Differentiate between the types of hormones based on their
solubility, transportation in blood and administration in the
body?
• Discuss the two mechanism of hormone action.
• Differentiate between up and down regulation.
• Differentiate between the three types of hormone
interaction.
• Describe the three types of signals that stimulate endocrine
secretions.
• Which hormone diffuses through the plasma membrane and
binds to its receptors in the cytoplasm?
Things to learn for each gland
• Location and Anatomy
• Hormone/s secreted
• For each hormone, know
– Its target tissue and the response of the tissue to the
hormone
– Regulation of the hormone – i.e. what controls its
release (chemical, neurons, or hormone)
– Effects of Hyposecretion/Hypersecretion
Hypothalamus – Pituitary Gland
Hypothalamus
• Links nervous system with endocrine system
• Is anatomically and physiologically connected to pituitary
gland by infundibulum
Pituitary Gland (hypophysis)
Made of two lobes:
• Anterior pituitary (adenohypophysis)
An endocrine tissue
Makes a variety of hormones
Regulated by releasing and inhibitory tropic hormones made
by hypothalamus
• Posterior pituitary (neurohypophysis)
A nervous tissue
Does not make hormones
Houses axon terminals of neurosecretory cells located in
hypothalamus
Stores and releases hypothalamic hormones in response to
nerve impulses
Anterior Pituitary Hormones
Hypothalamus secretes specific
releasing hormones (RH)
and inhibiting hormones (IH)
• Hormones travel via
circulatory system –
hypothalamo-hypophyseal
portal system.
• anterior pituitary is
stimulated or inhibited to
release its hormones
• hormones are circulated
throughout the body.
Anterior Pituitary Hormones
1. Human Growth Hormone (hGH)
• Secretion:
- Most actively secreted in early childhood
years
- Its secretion goes down during puberty
• Stimulates:
Growth of body cells, especially bone,
cartilage and muscle cells
Protein synthesis, fat metabolism, blood
sugar level, ATP production
• Regulation: chemical control
Decreased blood sugar level
(hypoglycemia)  stimulates GHRH
secretion by hypothalamus  stimulates
anterior pituitary  increased hGH
secretion
Increased blood sugar level (hyperglycemia)
 stimulates GHIH secretion by
hypothalamus  inhibits anterior pituitary 
decreased hGH secretion
Growth Hormone (contd)
• Clinical application:
- Dwarfism: hyposecretion of hGH during early
childhood years
- Giantism: hypersecretion of hGH during early
childhood years
- Acromegaly: hypersecretion of hGH in an adult
after most long bones
have sealed  thicker jaw bone and forehead,
larger hands, feet, tongue, lips
- Diabetogenic effect of hGH: hypersecretion
of hGH  symptoms similar to
diabetes mellitus (increased blood sugar)
Anterior Pituitary Hormones
2. Thyroid Stimulating Hormone (TSH) – Thyrotropin
• Tropic hormone
• Stimulates:
- Thyroid gland  increased secretion of thyroid hormones, T3
and T4 increased metabolism
• Regulation:
- Decreased metabolism  stimulates Thyrotropin Releasing
Hormone (TRH) secretion by hypothalamus  increased TSH
secretion by anterior pituitary
- Increased metabolism  decreased TRH secretion by
hypothalamus  decreased TSH secretion by anterior pituitary
• Clinical application:
- Causes metabolic disorders due to its affect on thyroid
hormone secretion
Anterior Pituitary Hormones
Gonadotropins
• Tropic hormones:
3.
4.
Follicle Stimulating Hormone (FSH)
Luteinizing Hormone (LH)
• Stimulate:
Ovaries
- increased secretion of female hormones (estrogen, progesterone)
- stimulate follicular development and ovulation
Testis
- increased secretion of male hormones (testosterone, inhibin)
- stimulate sperm formation
• Regulation:
- Decreased sex hormones  increased Gonadotropin Releasing
Hormone (GnRH) secretion by hypothalamus  increased FSH and LH
secretion by anterior pituitary  increased sex hormones by ovaries
and testis
- Increased sex hormones  decreased (GnRH) secretion by
hypothalamus  decreased FSH and LH secretion by anterior pituitary
 decreased sex hormones by ovaries and testis
• Clinical application:
- Cause reproductive disorders due to their affect on secretion of sex
hormones
Anterior Pituitary Hormones
5. Adrenocorticotropic Hormone (ACTH)
• Tropic hormone
• Stimulates:
- Cortex of adrenal gland
- Secretion of cortisol
• Regulation:
– Corticotropin Releasing Hormone (CRH) secretion by
hypothalamus
 increased ACTH secretion by anteriorpituitary
 increased cortisol secretion by adrenal cortex
Anterior Pituitary Hormones
6. Prolactin (PRL)
– Stimulates:
- mammary gland
- milk production
7. Melanocyte Stimulating Hormone
(MSH) Stimulates:
- melanin synthesis by melanocytes
- Darkening of color in the skin
Posterior Pituitary Hormones
Hypothalamus has neurosecretory cells
 make two hormones (ADH and OT)
 hormones are transported through the axons that from the
hypothalamic-hypophyseal tract
 hormones are stored in axon terminals
 hormones are released when a nerve impulse is transmitted
by hypothalamus to the axon terminals
Posterior Pituitary Hormones
Oxytocin (OT)
• Secretion:
- Stimulated by nerve impulses
• Stimulates:
– Smooth muscle contraction in mammary duct 
release of milk
– Smooth muscle contraction in the wall of the uterus 
delivery of the baby
• Regulation:
– Positive feedback system
• Clinical application:
– Low OT during delivery  Pitocin (commercial form of
OT) shot is given to induce labor
Posterior Pituitary Hormones
Antidiuretic Hormone (ADH)
• Secretion: Stimulated by water levels in blood
• Stimulates: Water reabsorption at the distal convoluted tubule
and collecting duct
• Regulation:
– Low water intake  hypothalamus detects osmolarity of
blood  neurosecretory cells in hypothalamus generate
impulses  stimulates release of ADH from axon terminals
located in posterior pituitary  ADH stimulates water
reabsorption in kidneys  less urine formed  water is
conserved
– High water intake  hypothalamus detects osmolarity of
blood  neurosecretory cells in hypothalamus do not
generate impulses  less ADH released from axon
terminals located in posterior pituitary  less water
reabsorption in kidneys  more urine formed
• Clinical application: Diabetes insipidus: hyposecretion of ADH 
symptoms similar to diabetes mellitus (large amounts of urine
formed, dehydration, thirst)
Summary of Pituitary Hormones
Review Time!!!!
We have covered a lot of important information in the last few
sections. Make sure you spend enough time reviewing this
material. Assess your understanding by attempting these
questions:
• Name the two lobes of the pituitary gland?
• Describe the relationship between hypothalamus and anterior vs
posterior pituitary gland.
• Name the hormones made by the hypothalamus and released by
posterior pituitary gland. What is the effect of these hormones
in the body.
• Name the hormones of anterior pituitary gland and their target
tissues.
• What is diabetogenic effect of growth hormone?
• What are tropic hormones? Give examples.
Thyroid Gland
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Located anterior to trachea and inferior to larynx
Made of two lobes connected by isthmus
Each lobe consists of numerous sacs called follicles
Follicles store large amounts of hormones
Has a rich blood supply for quick delivery of hormones in the
blood stream
Secretes:
Thyroid hormones - T3 and T4 for maintaining metabolism
Calcitonin (CT) for maintaining blood calcium level
Thyroid Gland Hormones – Thyroid Hormones
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T3 – Triiodothyroxine:
tyrosine with 3 iodines
T4 – Thyroxine: tyrosine with
4 iodines
Made by follicular cells
Modified and stored inside the
follicles with the help of
thyroglubulin (TGB)
Function:
- Regulate carbohydrate and
lipid metabolism
- Affect growth and
development
- Increase blood pressure,
heart beat, body temperature,
and GI tract movement
Clinical application
• Cretinism: hyposecretion of T3 and T4 in infants
 physical and mental retardation, yellow skin,
fat pads, protruding tongue, enlarged belly;
hormone therapy
• Myxedema: hyposecretion of T3 and T4 in adults
 low metabolism, lethargy, increased weight,
decreased body temperature and heart rate,
enlarged heart
• Exophthalmic goiter/Graves disease:
hypersecretion of T3 and T4 in adults  high
metabolism, decreased weight, increased body
temperature and heart rate, bulging eyes
Thyroid Gland Hormones – Calcitonin (CT)
Calcitonin (CT)
• Made by parafollicular cells (C Cells) located
between the follicles
• Function:
- Decreases blood calcium
- Increases bone calcium
Parathyroid Gland
• Four tiny glands on the posterior surface of thyroid
gland
• Made of two types of cells:
Chief cells – secrete parathyroid hormone (PTH)
Oxyphil cells – function unknown
PTH - Increases blood calcium
- Decreases bone calcium
Calcium Regulation
• Calcitonin and PTH are
antagonistic hormones
• High blood levels of
Ca+2 stimulate
Calcitonin
• Low blood levels of
Ca+2 stimulate PTHPTH
Review time
Assess your understanding of the Thyroid and Parathroid
hormones by attempting these questions:
• Name the hormones of thyroid gland?
• Which trace element plays an important role in the
structure of thyroid hormones?
• Which thyroid hormone/s are under the regulation of
anterior pituitary?
• Name the hormone/s secreted by the follicular cells and
their function in the body.
• Describe the location of parathyroid glands?
• Increased blood calcium levels will stimulate the secretion
of ________ hormone by the _______ gland.
Adrenal Gland
• Located above the kidneys
• Consists of:
- Capsule
- Adrenal cortex
- Adrenal medulla
Adrenal Gland
Capsule – Connective tissue protective layer
Adrenal cortex – Made up of three zones, secrete three types
of hormones
Adrenal medulla – Secretes two types of hormones
Adrenal Cortex Hormones
Secreted by zona
glomerulosa
Mineralocorticoids: Mostly
aldosterone
•Regulate minerals
• Reabsorption of Na+,
Cl-, HCO3- and water
• Excretion of K+ and
H+
Affects water level in
the body
•Clinical application:
• Hypersecretion causes
Na+ and water
retention 
hypertension
Adrenal Cortex Hormones
Secreted by zona
fasciculata Glucocorticoids:
Mostly cortisol
Functions:
- Increases blood glucose level
- Causes vasoconstriction 
increases BP
- Has anti-inflammatory effects
Clinical application:
Addison’s disease: hyposecretion of
aldosterone and cortisol 
hypoglycemia, lower BP, dehydration
Cushing’s disease: hypersecretion of
cortisol  redistribution of fat, spindle
legs, moon face, pendulous abdomen
Adrenal Cortex Hormones
• Secreted by zona
reticularis
• Androgens:
• Insignificant in males
(main source is
testis)
• Main source in
females
• Clinical application:
Hirsuitism: hypersecretion
of androgens 
excessive hair growth
Adrenal Medulla Hormones
Epinephrine and
norepinephrine:
• Similar structure and
function
• Released in response to
sympathetic nerve
• Involved in “Fight or flight
response”
• Increase heart rate, BP,
breathing rate , blood
sugar, metabolism, and
muscle contraction
Pancreas
• Located inferiorly and posteriorly to stomach
• Consists of Head, Body and Tail
• Endocrine
– 1% of the tissue
– produces hormones that diffuse into blood
• Exocrine
– 99% of the tissue
– produces digestive enzymes that are released through a
pancreatic duct into small intestine
Pancreas
Islet of Langerhans/Pancreatic Islets - are the endocrine portion
of the pancreas. This region is made up of 3 types of cells:
•Alpha cells - secrete glucagon, which stimulates liver cells to
release glucose --> causes blood glucose to rise.
•Beta cells - secrete insulin, which stimulates glucose uptake by
body cells and increased conversion of glucose to glycogen in
liver.
•Delta cells - secrete peptide hormone similar to GHIH, which
regulates the secretion of insulin and glucagon.
Pancreas – Insulin and Glucagon Regulation
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•
Low blood glucose stimulates release of glucagon
High blood glucose stimulates secretion of insulin
Diabetes mellitus
• Type I – childhood diabetes
- decrease in beta cells  decrease in insulin secretion  high blood
glucose (hyperglycemia)  excretion of glucose (glucosuria) and large
amounts of water (polyuria)  low blood glucose (hypoglycemia) and
dehydration  excessive thirst.
- Treatment – insulin shots
•
Type II – Non-insulin dependent diabetes
- Obese or older adults  fewer insulin receptors on target cells 
lesser response  symptoms as above
- Treatment – diet, exercise, insulin shots
•
Hyperinsulinism
- Overdoze of insulin  sudden drop in glucose  coma and death
Role of Insulin in blood Sugar
Homeostasis
Role of Glucagon in blood Sugar
Homeostasis
Review time!!!
Assess your understanding of the Thyroid and Parathroid
hormones by attempting these questions:
• Describe the location of adrenal gland. Identify the two regions of
an adrenal gland?
• List the three zones of adrenal cortex.
• What effect will elevated cortisol levels have on blood glucose
levels?
• Which hormone/s are responsible for the fight or flight response
of the body?
• Identify the types of cells in the pancreatic islet and the
hormones produced by each.
• The secretion of which hormone lowers blood glucose
concentration.
• Describe diabetes mellitus. Identify the two types of diabetes
mellitus.
Ovaries and Testis
• Primary function: Gamete formation – eggs and sperms
• Secondary function: Hormone production
- Ovaries produce estrogen, progesterone, relaxin, inhibin
- Testis produce testosterone, inhibin
• Regulation:
Hypothalamus secretes GnRH  anterior pituitary
secretes gonadotropins (FSH and LH)  stimulate
ovaries (follicular and oocyte development, hormone
production) and testis (sperm and hormone production)
Pineal Gland
• Located posterior to thalamus
• Has neuroglial cells and secretory cells
• Secretes hormone, melatonin
- secretion is regulated by light impulses
- affects secretion of gonadotropins
- regulates internal cycle
Thymus
• Large organ in infants but atrophied as adult
• 2 lobed organ located in mediastinum – superior to the
heart
• Primary function – maturation of T lymphocytes
• Secondary function – secretion of hormone thymosin to
aid maturation process
Other Endocrine Glands
Stomach:
- secretes gastrin to regulate secretion of gastric juices
Small intestine:
- secretes secretin, cholecystokinin that stimulate pancreas
and gall bladder but inhibit stomach
Placenta:
- secretes several hormones to support pregnancy
Kidneys:
- secrete erythropoietin to stimulate RBC production by red
bone marrow