Endocrine online notes
Chapter 18
The Endocrine System
• The nervous and endocrine systems act as
a coordinated interlocking supersystem, the
neuroendocrine system.
• The endocrine system controls body
activities by releasing mediator molecules
called hormones.
– hormones released into the bloodstream travel
throughout the body
– results may take hours, but last longer
• The nervous system controls body actions
through nerve impulses.
NERVOUS and ENDOCRINE
SYSTEM
• The nervous system causes muscles to
contract or glands to secrete. The
endocrine system affects virtually all body
tissues by altering metabolism, regulating
growth and development, and influencing
reproductive processes.
• Parts of the nervous system stimulate or
inhibit the release of hormones.
• Hormones may promote or inhibit the
General Functions of Hormones
• Help regulate:
–
–
–
–
extracellular fluid
metabolism
biological clock
contraction of cardiac &
smooth muscle
– glandular secretion
– some immune functions
• Growth & development
• Reproduction
• Hormones have
powerful effects when
Endocrine Glands Defined
• Exocrine glands
– secrete products into ducts which empty
into body cavities or body surface
– sweat, oil, mucous, & digestive glands
• Endocrine glands
– secrete products (hormones) into
bloodstream
– pituitary, thyroid, parathyroid, adrenal,
pineal
– other organs secrete hormones as a 2nd
function
– hypothalamus, thymus,
Hormone Receptors
• Although hormones travel in blood
throughout the body, they affect only
specific target cells.
– Target cells have specific protein or
glycoprotein receptors to which hormones
bind.
• Receptors are constantly being
synthesized and broken down.
• Synthetic hormones that block the
receptors for particular naturally occurring
Circulating and Local Hormones
• Hormones that travel in blood and act on
distant target cells are called circulating
hormones or endocrines.
• Hormones that act locally without first
entering the blood stream are called local
hormones.
– Those that act on neighboring cells are called
paracrines.
– Those that act on the same cell that secreted
them are termed autocrines.
Chemical Classes of Hormones
- Overview
• Table 18.2 provides a summary of the
hormones.
• Lipid-soluble hormones include the
steroids, thyroid hormones, and nitric
oxide, which acts as a local hormone in
several tissues.
• Water-soluble hormones include the
amines; peptides, proteins, and
glycoproteins; and eicosanoids.
Hormone Transport in Blood
• Protein hormones circulate in free form in
blood
• Steroid (lipid) & thyroid hormones must
attach to transport proteins synthesized
by liver
– improve transport by making them watersoluble
– slow loss of hormone by filtration within
kidney
General Mechanisms of Hormone
Action
• Hormone binds to cell surface or receptor
inside target cell
• Cell may then
– synthesize new molecules
– change permeability of membrane
– alter rates of reactions
• Each target cell responds to hormone
differently
At liver cells---insulin stimulates glycogen
synthesis
Second Messengers
• Some hormones exert their influence by
increasing the synthesis of cAMP
– ADH, TSH, ACTH, glucagon and epinephrine
• Some exert their influence by decreasing the
level of cAMP
– growth hormone inhibiting hormone
• Other substances can act as 2nd
messengers
– calcium ions
– cGMP
• A hormone may use different 2nd
Hormonal Interactions
• The responsiveness of a target cell to a
hormone depends on the hormone’s
concentration, the abundance of the target
cell’s hormone receptors, and influences
exerted by other hormones.
• Three hormonal interactions are the
– permissive effect
– synergistic effect
– antagonist effect
Hormonal Interactions
• Permissive effect
– a second hormone, strengthens the effects of the
first
– thyroid strengthens epinephrine’s effect upon
lipolysis
• Synergistic effect
– two hormones acting together for greater effect
– estrogen & LH are both needed for oocyte
production
• Antagonistic effects
– two hormones with opposite effects
Control of Hormone Secretion
• Regulated by signals from nervous
system, chemical changes in the blood
or by other hormones
• Negative feedback control (most
common)
– decrease/increase in blood level is
reversed
• Positive feedback control
– the change produced by the hormone
causes more hormone to be released
HYPOTHALAMUS AND
PITUITARY GLAND
• The hypothalamus is the major integrating
link between the nervous and endocrine
systems.
– Hypothalamus receives input from cortex,
thalamus, limbic system & internal organs
– Hypothalamus controls pituitary gland with 9
different releasing & inhibiting hormones
• The hypothalamus and the pituitary gland
(hypophysis) regulate virtually all aspects
Anatomy of Pituitary Gland
• The pituitary gland is located in the sella
turcica of the sphenoid bone and is
differentiated into the anterior pituitary
(adenohypophysis), the posterior pituitary
(neurohypophysis), and pars intermedia
(avascular zone in between (Figures 18.5 and
18.21b).
• Pea-shaped, 1/2 inch gland found in sella
turcica of sphenoid
– Infundibulum attaches it to brain
• Anterior lobe = 75%
•
Anterior Pituitary Gland
(Adenohypophysis)
The blood supply to the anterior pituitary is
from the superior hypophyseal arteries.
• Hormones of the anterior pituitary and the
cells that produce the:
– Human growth hormone (hGH) is secreted by
somatotrophs.
– Thyroid-stimulating hormone (TSH) is secreted by
thyrotrophs.
– Follicle-stimulating hormone (FSH) and luteinizing
hormone (LH) are secreted by gonadotrophs.
– Prolactin (PRL) is secreted by lactrotrophs.
– Adrenocorticotrophic hormone (ACTH) and melanocytestimulating hormone (MSH) are secreted by
Feedback
• Secretion of anterior pituitary gland
hormones is regulated by hypothalamic
regulating hormones and by negative
feedback mechanisms (Figure 18.6, Table
18.3).
•
Human Growth
Hormone and
Insulin-like Growth
Human growth Factors
hormone (hGH) is the most
plentiful anterior pituitary hormone.
• It acts indirectly on tissues by promoting
the synthesis and secretion of small
protein hormones called insulin-like growth
factors (IGFs).
– IGFs stimulate general body growth and
regulate various aspects of metabolism.
– Various stimuli promote and inhibit hGH
production (Figure 18.7).
Human Growth Hormone
• Produced by somatotrophs
• target cells synthesize insulinlike
growth
– common target cells are liver, skeletal
muscle, cartilage and bone
– increases cell growth & cell division by
increasing their uptake of amino acids &
synthesis of proteins
– stimulate lipolysis in adipose so fatty acids
used for ATP
– retard use of glucose for ATP production
so blood glucose levels remain high
enough to supply brain
Thyroid Stimulating Hormone (TSH)
• Hypothalamus regulates thyrotroph cells
• Thyrotroph cells produce TSH
• TSH stimulates the synthesis & secretion of
T3 and T4
• Metabolic rate stimulated
Follicle Stimulating Hormone (FSH)
• Releasing hormone
from
hypothalamus
controls
gonadotrophs
• Gonadotrophs release
follicle stimulating
hormone
• FSH functions
– initiates the formation of
follicles within the ovary
– stimulates follicle cells to
Luteinizing Hormone (LH)
• Releasing hormones from hypothalamus
stimulate gonadotrophs
• Gonadotrophs produce LH
• In females, LH stimulates
– secretion of estrogen
– ovulation of 2nd oocyte from ovary
– formation of corpus luteum
– secretion of progesterone
• In males, LH stimulates the interstitial cells of
the testes to secrete testosterone.
Prolactin (PRL)
• Prolactin (PRL), together with
other hormones, initiates and
maintains milk secretion by
the mammary glands.
– Hypothalamus regulates
lactotroph cells
– Lactotrophs produce prolactin
– Under right conditions,
prolactin
causes milk
production
• Suckling reduces levels of
Adrenocorticotrophic Hormone
• Adrenocorticotrophic
hormone (ACTH)
controls the production
and secretion of
hormones called
glucocorticoids by the
cortex of the adrenal
gland.
– Hypothalamus releasing
hormones stimulate
corticotrophs
– Corticotrophs secrete
Melanocyte-Stimulating Hormone
• Melanocyte-stimulating hormone (MSH)
increases skin pigmentation although its
exact role in humans is unknown.
– Releasing hormone from hypothalamus
increases MSH release from the anterior
pituitary
– Secreted by corticotroph cells
• Function not certain in humans (increase
skin pigmentation in frogs )
Posterior Pituitary Gland
(Neurohypophysis)
• Although the posterior pituitary gland does
not synthesize hormones, it does store
and release two hormones.
– Hormones made by the hypothalamus and
stored in the posterior pituitary are oxytocin
(OT) and antidiuretic hormone (ADH).
– The neural connection between the
hypothalamus and the neurohypophysis is via
the hypothalamohypophyseal tract (Figure
18.8).
Oxytocin
• Two target tissues both involved
in neuroendocrine reflexes
• During delivery
– baby’s head stretches cervix
– hormone release enhances uterine
muscle contraction
– baby & placenta are delivered
• After delivery
– Oxytocin stimulates contraction of
the uterus and ejection (let-down)
of milk from the breasts.
• Nursing a baby after delivery
stimulates oxytocin release, promoting
uterine contractions and the expulsion
of the placenta (Clinical Application).
• suckling & hearing baby’s cry
stimulates milk ejection
ADH
• Antidiuretic hormone stimulates water
reabsorption by the kidneys and arteriolar
constriction.
• The effect of ADH is to decrease urine
volume and conserve body water.
• ADH is controlled primarily by osmotic
pressure of the blood (Figure 18.9).
THYROID GLAND - Overview
• The thyroid gland is located just below the
larynx and has right and left lateral lobes
(Figure 18.10a).
• Histologically, the thyroid consists of the
thyroid follicles composed of follicular
cells, which secrete the thyroid hormones
thyroxine (T4) and triiodothyronine (T3),
and parafollicular cells, which secrete
calcitonin (CT) (Figures 18.10b and
18.13c).
Formation, Storage, and Release
of Thyroid Hormones
• Thyroid hormones are synthesized from
iodine and tyrosine within a large
glycoprotein molecule called thyroglobulin
(TGB) and are transported in the blood by
plasma proteins, mostly thyroxine-binding
globulin (TBG).
• The formation, storage, and release steps
include
– iodide trapping,
– synthesis of thyroglobulin,
– oxidation of iodide,
•
PARATHYROID
GLANDS
The parathyroid glands are embedded on the
posterior surfaces of the lateral lobes of the
thyroid
– principal cells produce parathyroid hormone
– oxyphil cells … function is unknown (Figure
18.13).
• Parathyroid hormone (PTH) regulates the
homeostasis of calcium and phosphate
• increase blood calcium level
• decrease blood phosphate level
– increases the number and activity of osteoclasts
– increases the rate of Ca+2 and Mg+2 from
reabsorption from urine and inhibits the
Blood Calcium
• Blood calcium level directly controls the
secretion of calcitonin and parathyroid
hormone via negative feedback loops that
do not involve the pituitary gland (Figure
18.14).
• Table 18.7 summarizes the principal
actions and control of secretion of
parathyroid hormone.
Adrenal Cortex
• The adrenal cortex is divided into three
zones, each of which secretes different
hormones (Figure 18.15).
– The zona glomerulosa (outer zone)
• secretes mineralocorticoids.
– The zona fasciculata (middle zone)
• secretes glucocorticoids.
– The zona reticularis (inner zone)
• secretes androgens.
Mineralocorticoids
• 95% of hormonal activity due to
aldosterone
• Functions
– increase reabsorption of Na+ with Cl- ,
bicarbonate and water following it
– promotes excretion of K+ and H+
• Hypersecretion = tumor producing
aldosteronism
– high blood pressure caused by retention of
Glucocorticoids
• 95% of hormonal activity is due to
cortisol
• Functions = help regulate metabolism
– increase rate of protein catabolism &
lipolysis
– conversion of amino acids to glucose
– stimulate lipolysis
– provide resistance to stress by making
nutrients available for ATP production
– raise BP by vasoconstriction
– anti-inflammatory effects reduced (skin
cream)
• reduce release of histamine from mast cells
Androgens from Zona
Reticularis
• Small amount of male hormone produced
– insignificant in males
– may contribute to sex drive in females
– is converted to estrogen in postmenopausal
females
Adrenal Medulla
• Chromaffin cells receive direct
innervation from sympathetic nervous
system
– develop from same tissue as postganglionic
neurons
• Produce epinephrine & norepinephrine
• Hormones are sympathomimetic
– effects mimic those of sympathetic NS
– cause fight-flight behavior
• Acetylcholine increase hormone
PANCREATIC ISLETS
• The pancreas is a flattened organ located
posterior and slightly inferior to the
stomach and can be classified as both an
endocrine and an exocrine gland (Figure
18.18).
• Histologically, it consists of pancreatic
islets or islets of Langerhans (Figure
18.19) and clusters of cells (acini)
(enzyme-producing exocrine cells).
Cell Types in the Pancreatic Islets
•
•
•
•
Alpha cells (20%) produce glucagon
Beta cells (70%) produce insulin
Delta cells (5%) produce somatostatin
F cells produce pancreatic
polypeptide
Ovaries and Testes
• Ovaries
– estrogen, progesterone, relaxin & inhibin
– regulate reproductive cycle, maintain
pregnancy & prepare mammary glands for
lactation
• Testes
– produce testosterone
– regulate sperm production & 2nd sexual
characteristics
• Table 18.10 summarizes the hormones
Pineal Gland
• Small gland attached to 3rd ventricle of
brain
• Consists of pinealocytes & neuroglia
• Melatonin responsible for setting of
biological clock
• Jet lag & SAD treatment is bright light