Download The Endocrine System

Lecture 14
The Endocrine System
Overview of the Endocrine System
 Hormone Chemistry and Action
o Hormones fall into three chemical classes:
 Steroids are lipids synthesized from cholesterol
 Monoamines are small molecules bearing an amino group
 They are synthesized from the amino acids tyrosine and
tryptophan
 Peptide hormones are chains of about 3 to 200 amino acids
 The largest peptides are proteins
o Some of these have carbohydrate chains bonded to
them, making them glycoproteins
o Hydrophilic and Hydrophobic hormones
 Some hormones are hydrophilic and mix freely with water, so they
are easily transported with blood
 Steroid hormones and thyroid hormones are hydrophobic
 They must bind to a transport protein in the blood plasma
to be carried to their target cells
o Transport proteins also protect hormones from
being broken down by enzymes
 attached hormones may circulate for hours
to weeks
 free hormones may break down in a few
minutes
o Hormones and Target Cells
 Steroid and thyroid hormones diffuse into the cell and bind to
receptors in the cytoplasm or nucleus
 Monoamines and peptides cannot enter the cell, but bind to
receptors on the surface
 The hormone acts as a first messenger to the target cell and
triggers the formation of a second messenger inside the cell
o The second messenger then stimulates metabolic
changes in the cell
 Target cells respond by
 Synthesizing new enzymes
 Activating or inhibiting preexisting hormones
 Changing metabolism
 Additional hormone terminology:
 Tropic or trophic hormones are hormones whose target
organs are other endocrine glands
o Gonadotropins are hormones whose target organs
are the gonads

Comparison of Nervous and Endocrine Systems
o Nervous System
Endocrine System
-Communicates with electrical
-Communicates with hormones
impulses and neurotransmitters
-Releases neurotransmitters at
-Releases hormones into blood for
synapses at specific target cells
general distribution throughout body
-Has relatively local, specific
-Sometimes has very general,
effects on target cells
widespread effects on many organs
-Reacts quickly to stimuli (in
-Reacts more slowly to stimuli
milliseconds)
(often taking seconds to days)
-Stops quickly when stimulus
-May continue responding after
stops
stimulus stops
-Adapts relatively quickly to
-Adapts slowly, may continue
continual stimulation
responding for days to weeks of
continual stimulation

Despite differences, there are similarities
o Some neurotransmitters and hormones produce identical effects on
some cells
 Norepinephrine and glucagons stimulate the liver to break
down glycogen and release glucose
o The nervous and endocrine systems continually regulate each other as
they coordinate the activities of organ systems
o Some neurons trigger hormone secretion, and some hormones
stimulate or inhibit neurons
The Hypothalamus and Pituitary Gland
 Anatomy
o Hypothalamus
 Shaped like a flattened funnel
 Forms the floor and walls of the third ventricle of the brain
 Regulates primitive functions ranging from water balance to sex
drive
 Many functions are carried out by way of the pituitary gland
o Pituitary Gland
 Also called (hypophysis)
 Housed in the sella turcica of the sphenoid bone
 Ovoid gland about 1.3 cm in diameter
 Composed of two structures
 Adenohypophysis (anterior lobe)
o Anterior ¾ of the pituitary
o 2 parts
 Pars distalis
 The most distal to the pituitary stalk
 Pars tuberalis




Small mass of cells adhering to the
anterior side of the stalk
o Has no nervous connection to the hypothalamus
 It’s connected by a complex of blood vessels
called the hypophyseal portal system
 Hormones are secreted by the hypothalamus
and travel to the anterior pituitary
 The hormones either trigger or inhibit the
release of pituitary hormones
Neurohypophysis (posterior lobe)
o Posterior ¼ of the pituitary
o 3 parts
 Pars nervosa
 The most distal portion
 Infundibulum
 Portion that connects the pars
nervosa to the hypothalamus
 Median eminence
 An extension of the hypothalamic
floor
o Not a true gland, but a mass of neuroglia and axons
arising from certain hypothalamic neurons
 Hormones are synthesized in the
hypothalamus, are transported to and stored
in the posterior pituitary until a nerve signal
from the hypothalmo-hypophyseal nerve
tract triggers their release
Hypothalamic Hormones
o The hypothalamus produces nine hormones
 Seven are sent through the portal system to regulate the activities
of the anterior pituitary
 In most cases, the name of the hypothalamic hormone
indicates the pituitary hormone whose secretion it
stimulates or inhibits
o “Prolactin-releasing hormone” promotes prolactin
secretion
 Gonadotropin-releasing hormones control both folliclestimulating hormones and luteinizing hormones
 Two are transported to the posterior pituitary and stored until
neurons signal their release into the blood
 Oxytocin
 Antidiuretic hormone
Anterior Pituitary Hormones
o Follicle Stimulating Hormone
 Target cells are ovaries and testes


In the ovaries:
 It stimulates the development of the eggs in the follicles
that contain them
 It stimulates secretion of ovarian hormones
 In the testes, it stimulates the production of sperm
o Luteinizing Hormone
 In females, it stimulates ovulation (the release of the egg)
 It’s named for the fact that after ovulation, the follicle is
called the corpus luteum.
 It also stimulates the corpus luteum secrete progesterone which is
important to pregnancy
 In males, it stimulates the testes to produce testosterone
o Thyroid-stimulating Hormone
 It stimulates the growth of the thyroid gland and the secretion of
the thyroid hormone
o Adrenocorticotropic Hormone
 It helps regulate the body’s response to stress
 It stimulates the adrenal cortex (outer layer of the adrenal gland) to
secrete glucocorticoids, which are important in glucose, fat, and
protein metabolism
o Prolactin
 After a woman gives birth, it stimulates the mammary glands to
secrete milk
 In males, it makes the testes more sensitive to luteinizing hormone,
which enhances secretion of testosterone
o Growth Hormone
 It promotes mitosis and cellular differentiation, thus promoting
widespread tissue growth
Posterior Pituitary Hormones
o Antidiuretic Hormone
 It increases water retention by the kidneys, reduces urine volume,
and helps prevent dehydration
o Oxytocin
 In childbirth, it stimulates labor contractions
 In lactating mothers, it stimulates the flow of milk from the
mammary glands to the nipple
 It surges in both sexes during sexual arousal and orgasm and may
play a role in emotional bonding
Other Endocrine Glands
 The Pineal Gland
o Found attached to the roof of the third ventricle beneath the posterior end
of the corpus callosum
o It produces melotonin
 Melatonin secretion rises in the dark and occurs at a low level in
daylight






Its secretion fluctuates seasonally with changes in day
length
Melatonin levels are elevated in seasonal affective disorder
and premenstrual syndrome
The Thymus
o Gland in the mediastinum superior to the heart, behind the sternal
manubrium
o It is a site of maturation of T lymphocytes, which are critical for immune
defense
o It secretes several hormones that stimulate the development of other
lymphatic organs and regulate the development and activity of the T
lymphocytes
The Thyroid Gland
o Gland composed to two lobes that lie adjacent to the trachea, immediately
below the larynx
o The two lobes are joined together by a bridge of tissue called the isthmus,
which crosses in front of the trachea
o The gland is composed mostly of sacs called thyroid follicles
 Thyroid follicles are lined by simple cuboidal epithelium of
follicular cells
 Follicular cells secrete thyroid hormone
o Technically, thyroid hormone consists of 2
hormones, thyroxine and triiodothyronine
 Thyroid hormone increases the concentration and activities
enzymes that make ATP and the activity of sodiumpotassium pumps
o This influences the rate at which the body consumes
calories
o Between thyroid follicles are C cells (parafollicular cells)
 C cells secrete calcitonin
 Calcitonin inhibits the bone-resorbing activity of
osteoclasts
 This lowers blood calcium levels
The Parathyroid Glands
o These are small ovoid glands that adhere to the posterior side of the
thyroid gland
o Chief cells of the parathyroid secrete parathyroid hormone
 Parathyroid hormone raises blood calcium
 It promotes intestinal calcium absorption
 It inhibits urinary calcium excretion
 It stimulates osteoclasts to resorb bone
The Adrenal Glands
o Attached to the top medial surfaces of the kidneys
o Hormones of the adrenal medulla
 Epinephrine and norepinephrine




Mimic the arousing effects of the sympathetic nervous
system
o Hormones of the Adrenal Cortex
 Mineralocorticoids
 Steroids that control electrolyte balance
 Aldosterone is a hormone that influence blood volume, and
blood pressure by influencing sodium and potassium in the
kidneys
 Glucocorticoids
 Stimulate fat and protein catabolism and the release of fatty
acids and glucose in the blood
 Help the body adapt to stress and repair damaged tissues
 Sex steroids
 These are weak androgens and smaller amounts of
estrogens
 Androgens stimulate the sex drive and growth of pubic and
axillary hair in both sexes
The Pancreatic Islets
o Most of the pancreas is an exocrine digestive gland
o Scattered among those cells are pancreatic islets which secrete hormones
 Alpha cells secrete glucagons
 Glucagon secretion rises between meals when blood
glucose falls
 In the liver, it stimulates the breakdown of glycogen and
the synthesis of glucose, and the release of glucose into the
blood
 In adipose tissue, it stimulates fat catabolism
 Beta cells secrete insulin
 Insulin secretion rises during and immediately after a meal
 It stimulates cells to absorb nutrients and store or
metabolize them
 It promotes the synthesis of glycogen, fat, and protein
The Gonads
o Both exocrine and endocrine glands
o Exocrine products are eggs or sperm
o Endocrine products are gonadal hormones
 Granulosa cells of the ovaries produce estrogen
 Interstitial cells of the testes produce testosterone
Endocrine Cells in Other Organs
 Heart, skin, liver, kidneys, stomach, small intestine, and placenta also contain
cells that produce hormones