The endocrine system regulates metabolic processes through hormones secreted into the bloodstream. The major endocrine glands include the pituitary, thyroid, parathyroid, adrenal, pineal, thymus, pancreas, and gonads. Hormones influence target cells through second messenger systems or direct gene activation. The hypothalamus and pituitary gland regulate other endocrine glands through feedback loops to maintain homeostasis.

1. CHAPTER 3
THE ENDOCRINE SYSTEM

2. AN OVERVIEW
• The endocrine system is a major controlling
system of the body
• Influences metabolic activities by means of
hormones transported in the blood
• Responses occur more slowly but tend to last
longer than those of the nervous system
• Through hormones, it stimulates such long-
term processes as growth and development,
metabolism, reproduction, and body defense

3. • Endocrine glands are ductless, well-
vascularized glands that release hormones
directly into the blood or lymph
• Endocrine glands: pituitary, thyroid,
parathyroid, adrenal, and pineal glands
• Exocrine glands produce nonhormonal
substances, such as sweat and saliva, and have
ducts
• Some organs produce both hormones and
exocrine products (e.g., pancreas and gonads)

4. Figure 16.1
Pineal gland
Hypothalamus
Pituitary gland
Parathyroid glands
(on dorsal aspect
of thyroid gland)
Thymus
Thyroid gland
Adrenal glands
Pancreas
Ovary (female)
Testis (male)

5. • Prostaglandins are not endocrine hormones,
but they are locally acting messenger
molecules which include:
a. Autocrine - act on the cell that
released them
b. Paracrine - act on a different cell type
nearby

6. Function
• Controlling activity of specific organ or tissue in
maintaining homeostasis by secreting hormones
as in:
a. Regulator of growth and development
b. Regulating the concentration of body fluids
(water and electrolyte)
c. Metabolism of carbohydrate, protein and
lipids (nutrient)
d. Acts together with nervous system to help
the body to react to stress properly

7. HORMONES
• Chemical substances secreted by cells into
extracellular fluid (bloodstream) that regulate
the metabolic activity of other cells in the
body.
• All hormones are amino acid-based or
steroids.

8. Types of Hormones
a. Amino acid based (water soluble)
• Most hormones are amino acid-based except
thyroid hormones
• Amines, thyroxine, peptides, and proteins
b. Steroids (lipid soluble)
• Synthesized from cholesterol
• Of the hormones, only gonadal and
adrenocortical hormones are steroids

9. Regulation of Hormones Secretion
a. Negative feedback
• Response that reduces the initiating stimulus
(opposite direction)
• Important in regulating hormone levels in the
blood
b. Positive feedback
• Reinforce the initial stimulus

10. Mechanism of Hormones Action
• Hormones alters cell activity by stimulating or
inhibiting characteristics cellular processes of their
target cells
• Two main mechanism account for how a hormone
communicates with targets cells:
1. Amino acid- based hormones and
second-messenger system
2. Steroid hormones and direct gene
activation

11. Mechanisms of Hormone Action
Two mechanisms, depending on their chemical nature
1. Water-soluble hormones (all amino acid–based
hormones except thyroid hormone)
• Cannot enter the target cells
• Act on plasma membrane receptors
• Coupled by G proteins to intracellular second
messengers that mediate the target cell’s
response

12. Mechanisms of Hormone Action
2. Lipid-soluble hormones (steroid and thyroid
hormones)
• Act on intracellular receptors that directly
activate genes

13. Amino acid- based hormones and
second-messenger system
1. Hormone (first messenger) binds to receptor
2. Receptor activates G protein
3. G protein activates adenylate cyclase
4. Adenylate cyclase converts ATP to Cyclic
adenosine monophosphate (cAMP) (second
messenger)
5. cAMP (second messenger) activates protein
kinases in the cytoplasm
6. Protein kinases activated other proteins in the
cell
7. Activated proteins induce changes in the cell

14. Figure 16.2
Hormone (1st messenger)
binds receptor.
Receptor
activates G
protein (GS).
G protein
activates
adenylate
cyclase.
cAMP acti-
vates protein
kinases.
Adenylate
cyclase
converts ATP
to cAMP (2nd
messenger).
Receptor
G protein (GS)
Adenylate cyclase
Triggers responses of
target cell (activates
enzymes, stimulates
cellular secretion,
opens ion channel,
etc.)
Hormones that
act via cAMP
mechanisms:
Epinephrine
ACTH
FSH
LH
Inactive
protein kinase
Extracellular fluid
Cytoplasm
Active
protein
kinase
GDP
Glucagon
PTH
TSH
Calcitonin
1
2 3 4
5

15. Figure 16.2, step 1
Hormone (1st messenger)
binds receptor.
Receptor
Hormones that
act via cAMP
mechanisms:
Epinephrine
ACTH
FSH
LH
Extracellular fluid
Cytoplasm
Glucagon
PTH
TSH
Calcitonin
1

16. Figure 16.2, step 2
Hormone (1st messenger)
binds receptor.
Receptor
activates G
protein (GS).
Receptor
G protein (GS)
Hormones that
act via cAMP
mechanisms:
Epinephrine
ACTH
FSH
LH
Extracellular fluid
Cytoplasm
GDP
Glucagon
PTH
TSH
Calcitonin
1
2

17. Figure 16.2, step 3
Hormone (1st messenger)
binds receptor.
Receptor
activates G
protein (GS).
G protein
activates
adenylate
cyclase.
Receptor
G protein (GS)
Adenylate cyclase
Hormones that
act via cAMP
mechanisms:
Epinephrine
ACTH
FSH
LH
Extracellular fluid
Cytoplasm
GDP
Glucagon
PTH
TSH
Calcitonin
1
2 3

18. Figure 16.2, step 4
Hormone (1st messenger)
binds receptor.
Receptor
activates G
protein (GS).
G protein
activates
adenylate
cyclase.
Adenylate
cyclase
converts ATP
to cAMP (2nd
messenger).
Receptor
G protein (GS)
Adenylate cyclase
Hormones that
act via cAMP
mechanisms:
Epinephrine
ACTH
FSH
LH
Extracellular fluid
Cytoplasm
GDP
Glucagon
PTH
TSH
Calcitonin
1
2 3 4

19. Figure 16.2, step 5
Hormone (1st messenger)
binds receptor.
Receptor
activates G
protein (GS).
G protein
activates
adenylate
cyclase.
cAMP acti-
vates protein
kinases.
Adenylate
cyclase
converts ATP
to cAMP (2nd
messenger).
Receptor
G protein (GS)
Adenylate cyclase
Triggers responses of
target cell (activates
enzymes, stimulates
cellular secretion,
opens ion channel,
etc.)
Hormones that
act via cAMP
mechanisms:
Epinephrine
ACTH
FSH
LH
Inactive
protein kinase
Extracellular fluid
Cytoplasm
Active
protein
kinase
GDP
Glucagon
PTH
TSH
Calcitonin
1
2 3 4
5

20. Steroid hormones and direct gene
activation
1. Diffuse directly through plasma membrane
(target cells)
2. Binds with protein receptor and turns into
steroid protein complex
3. Entering nucleus to a specific Deoxyribonucleic
acid (DNA) region (activating DNA, which
initiates messenger Ribonucleic acid (RNA)
formation leading to protein synthesis)
4. Reaction between steroid-protein complex and
DNA activates genes to synthesize new proteins
and enzymes and induce changes in the cell

21. Figure 16.3
mRNA
New protein
DNA
Hormone
response
elements
Receptor-
hormone
complex
Receptor
protein
Cytoplasm
Nucleus
Extracellular fluid
Steroid
hormone
The steroid hormone
diffuses through the plasma
membrane and binds an
intracellular receptor.
The receptor-
hormone complex enters
the nucleus.
The receptor- hormone
complex binds a hormone
response element (a
specific DNA sequence).
Binding initiates
transcription of the
gene to mRNA.
The mRNA directs
protein synthesis.
Plasma
membrane
1
2
3
4
5

22. Figure 16.3, step 1
Receptor-
hormone
complex
Receptor
protein
Cytoplasm
Nucleus
Extracellular fluid
Steroid
hormone
The steroid hormone
diffuses through the plasma
membrane and binds an
intracellular receptor.
Plasma
membrane
1

23. Figure 16.3, step 2
Receptor-
hormone
complex
Receptor
protein
Cytoplasm
Nucleus
Extracellular fluid
Steroid
hormone
The steroid hormone
diffuses through the plasma
membrane and binds an
intracellular receptor.
The receptor-
hormone complex enters
the nucleus.
Plasma
membrane
1
2

24. Figure 16.3, step 3
DNA
Hormone
response
elements
Receptor-
hormone
complex
Receptor
protein
Cytoplasm
Nucleus
Extracellular fluid
Steroid
hormone
The steroid hormone
diffuses through the plasma
membrane and binds an
intracellular receptor.
The receptor-
hormone complex enters
the nucleus.
The receptor- hormone
complex binds a hormone
response element (a
specific DNA sequence).
Plasma
membrane
1
2
3

25. Figure 16.3, step 4
mRNA
DNA
Hormone
response
elements
Receptor-
hormone
complex
Receptor
protein
Cytoplasm
Nucleus
Extracellular fluid
Steroid
hormone
The steroid hormone
diffuses through the plasma
membrane and binds an
intracellular receptor.
The receptor-
hormone complex enters
the nucleus.
The receptor- hormone
complex binds a hormone
response element (a
specific DNA sequence).
Binding initiates
transcription of the
gene to mRNA.
Plasma
membrane
1
2
3
4

26. Figure 16.3, step 5
mRNA
New protein
DNA
Hormone
response
elements
Receptor-
hormone
complex
Receptor
protein
Cytoplasm
Nucleus
Extracellular fluid
Steroid
hormone
The steroid hormone
diffuses through the plasma
membrane and binds an
intracellular receptor.
The receptor-
hormone complex enters
the nucleus.
The receptor- hormone
complex binds a hormone
response element (a
specific DNA sequence).
Binding initiates
transcription of the
gene to mRNA.
The mRNA directs
protein synthesis.
Plasma
membrane
1
2
3
4
5

27. Control of Hormone Release
• Synthesis and release of most hormones are
regulated by negative feedback system
• Endocrine glands are stimulated to
manufacture and release their hormones by 3
major types of stimuli:
a. Humoral stimuli
b. Neural stimuli
c. Hormonal stimuli

28. Humoral Stimuli
• Changing blood levels of ions and nutrients
directly stimulates secretion of hormones
• Example: Ca2+ in the blood
– Declining blood Ca2+ concentration stimulates the
parathyroid glands to secrete PTH (parathyroid
hormone)
– PTH causes Ca2+ concentrations to rise and the
stimulus is removed

29. Figure 16.4a
(a) Humoral Stimulus
Capillary (low
Ca2+ in blood)
Parathyroid
glands
Thyroid gland
(posterior view)
PTH
Parathyroid
glands
1 Capillary blood contains
low concentration of Ca2+,
which stimulates…
2 …secretion of
parathyroid hormone (PTH)
by parathyroid glands*

30. Neural Stimuli
• Nerve fibers stimulate hormone release
– Sympathetic nervous system fibers stimulate the
adrenal medulla to secrete catecholamines

31. Figure 16.4b
(b) Neural Stimulus
CNS (spinal cord)
Medulla of
adrenal
gland
Preganglionic
sympathetic
fibers
Capillary
1 Preganglionic sympathetic
fibers stimulate adrenal
medulla cells…
2 …to secrete catechola-
mines (epinephrine and
norepinephrine)

32. Hormonal Stimuli
• Hormones stimulate other endocrine organs
to release their hormones
– Hypothalamic hormones stimulate the release of
most anterior pituitary hormones
– Anterior pituitary hormones stimulate targets to
secrete still more hormones
– Hypothalamic-pituitary-target endocrine organ
feedback loop: hormones from the final target
organs inhibit the release of the anterior pituitary
hormones

33. Figure 16.4c
(c) Hormonal Stimulus
Hypothalamus
Thyroid
gland
Adrenal
cortex
Gonad
(Testis)
Pituitary
gland
1 The hypothalamus secretes
hormones that…
2 …stimulate
the anterior
pituitary gland
to secrete
hormones
that…
3 …stimulate other endocrine
glands to secrete hormones

34. MAJOR ENDOCRINE ORGANS
a. Hypothalamus
b. Pituitary gland
c. Thyroid gland
d. Parathyroid glands
e. Adrenal glands
f. Pancreas
g. Pineal gland
h. Thymus gland
i. Gonads (ovaries, testes)

35. Figure 16.1
Pineal gland
Hypothalamus
Pituitary gland
Parathyroid glands
(on dorsal aspect
of thyroid gland)
Thymus
Thyroid gland
Adrenal glands
Pancreas
Ovary (female)
Testis (male)

36. Hypothalamus
• A neuroendocrine organ
• The hypothalamus:
– Link the nervous system to the endocrine system
via the pituitary gland (hypophysis)
– Synthesizes two hormones (oxytoxin and ADH)
that it exports to the posterior pituitary for
storage and later release
– Regulates the hormonal output of the anterior
pituitary via releasing and inhibiting hormones

38. Pituitary Gland (Hypophysis)
• Attached to hypothalamus by the
infundibulum within the sphenoid bone
• Divided into 2 lobes:
a. Posterior lobe (neurohypophysis)
–Store hormones from hypothalamus -
oxytocin and antidiuretic hormone (ADH)

39. b. Anterior lobe (adenohypophysis)
– Influenced by hypothalamic hormone
– Growth hormone (GH), prolactin (PRL),
adrenocorticotropic hormone ( ACTH), thyroid-
stimulating hormone (TSH), follicle-stimulating
hormone ( FSH) and luteinizing hormone ( LH)
– ACTH, TSH, FSH and LH are tropic hormones
(regulate other endocrine gland)

41. Anterior Pituitary Hormones
a. Growth hormone (GH)
• An anabolic and protein-conversing hormone
that promotes total body growth
• It important effect is on skeletal muscles and
bones
• Promotes protein synthesis and encourages
use of fats for fuel

42. Homeostatic Imbalances of Growth
Hormone
• Hypersecretion
– In children results in gigantism
– In adults results in acromegaly
• Hyposecretion
– In children results in pituitary dwarfism

43. Anterior Pituitary Hormones
b. Prolactin (PRL)
• Stimulates production of breast milk
(lactation)
• Regulation of PRL release
– Primarily controlled by prolactin-inhibiting
hormone (PIH) (dopamine)
• Blood levels rise toward the end of pregnancy
• Suckling stimulates PRH release and promotes
continued milk production

44. Anterior Pituitary Hormones
c. Adrenocorticotropic hormone (ACTH)
– Stimulates the adrenal cortex to release its
hormones (mineralocorticoids, glucocorticoids
and gonadocorticoids)

45. Anterior Pituitary Hormones
d. Thyroid-stimulating hormone (TSH)
• Stimulates the thyroid gland to release thyroid
hormones (thyroxine and triiodothyronine)
that are primarily responsible for regulation of
metabolism.

47. Anterior Pituitary Hormones
e. Gonadotropic hormones
i. Follicle-stimulating hormone (FSH)
– Beginning at puberty, stimulates follicle
development and estrogen production by female
ovaries, promotes sperm production in male
ii. Luteinizing hormone (LH)
– Beginning at puberty, stimulates ovulation and
stimulates ovarian to produce estrogen and
progesterone, stimulates the male’s testes to
produce testosterone

48. Posterior Pituitary Hormones
a. Oxytoxin
• Stimulates powerful uterine contractions (trigger
labor and delivery of infant) and causes milk
ejection in the nursing woman
• Also promote sexual arousal
b. Antidiuretic hormone (ADH) or vasopressin
• Causes kidney tubule cells to reabsorb and
conserve body water and increased blood
pressure by constricting blood vessels

50. Thyroid Gland
• Located on the trachea, just inferior to the
larynx (in the anterior throat)
• Thyroid hormone (TH) includes thyroxine (T4)
and triiodotyronine (T3), which increase the
rate of cellular metabolism
• Calcitonin produced by parafollicular (C) cells
in response to high blood calcium levels. It
causes calcium to be deposited in bones

52. Parathyroid Glands
• 4 small glands located posterior/dorsal aspect
of the thyroid gland
• Low blood levels of calcium stimulate the
release parathyroid hormone (PTH)
• PTH causes bone calcium to be liberated into
the blood, the intestine to increase calcium
absorption from food and the kidneys to
increase calcium reabsorption

54. Adrenal (Suprarenal) Glands
• Lies on top of kidneys
• Divided into:
a. Adrenal cortex
b. Adrenal medulla

57. Adrenal cortex
a. Mineralocorticoids (aldosterone)
• Regulate sodium ion (Na+) and potassium ion
(K+) reabsorption by the kidneys
• Their release is stimulated by low Na+ and/or
high K+ levels in blood
b. Glucocorticoids (cortisol)
• Enable the body to resist long-term stress by
increasing blood glucose levels and depressing
the inflammatory response.
c. Gonadocorticoids/Sex hormones (androgens)
• Responsible for sex drive in female

58. Adrenal medulla
• Adrenal medulla hormones produce
catecholamines (epinephrine and
norepinephrine) in response to sympathetic
nervous system stimulation. Its hormones
enhance and prolong the effects of the ‘fight-
or-flight’ response to short-term stress

59. Pancreas
• Located behind stomach
• Composed of both endocrine and exocrine gland cells
• Hormones produced from pancreatic islets (islets of
Langerhans) containing alpha (α) cells (glucagon) and
beta (β) cells (insulin)
• Insulin is released when the blood levels of glucose are
high. It increases the rate of glucose uptake and
metabolism by body cells; stimulates glycogen
formation
• Glucagon is released when blood levels of glucose are
low, stimulates the liver to release glucose to the blood

61. Pineal Gland
• Located in the diencephalon/ third ventricle of
the brain (epithalamus)
• Releases melatonin, which acts as biological
clock; reproductive behaviour; affects daily
biological rhythms such as body temperature,
sleep and appetite

63. Thymus Gland
• Located deep to sternum
• Large and conspicuous in infant and children
• Diminishes in size throughout adulthood
• Its hormones, thymosins, thymic factor, and
thymopoietins, are important to the normal
development of the immune responses
(thymosin promotes maturation of T
lymphocytes, important in body defense)

65. Gonads
Ovaries
• Ovaries located in abdominopelvic cavity
• Ovaries release:
a. Estrogens
– Release of estrogens by ovarian follicles begins at
puberty (FSH)
– Estrogens stimulate maturation of female reproductive
organs and female secondary sex characteristics
– With progesterone, they cause the menstrual cycle
b. Progesterone
– Release in response to LH, works with estrogens
establishing the menstrual cycle

67. Gonads
Testes
• Testes located in the scrotum
• Testes begin to produce testosterone at
puberty in response to LH stimulation
• Testosterone promotes maturation of the
male reproductive system, male secondary sex
characteristics, and production of sperm by
the testes