Anatomy and Physiology

ENDOCRINE
SYSTEM
Table Contents

Overview of the Endocrine System

Structures of the Endocrine System
Functions of the Endocrine System
About Hormones
The Hypothalamus & The Pituitary Gland
The Pineal Gland
The Thyroid Gland
The Parathyroids
The Pancreas
The Adrenal Glands
The Gonads
Overview of the
Endocrine System
Endocrine systems, also referred to as hormone
systems, are found in all mammals, birds, fish, and many
other types of living organisms. They are made up of:
Glands located throughout the body
Hormones that are made by the glands and
released into the bloodstream or the fluid
surrounding cells
Receptors in various organs and tissues that
recognize and respond to the hormones.
Structures of the
Endocrine System
The endocrine system consists of
cells, tissues, and organs
Secrete hormones as a primary or
secondary function.
Some of these glands have both
endocrine and non-endocrine
functions.
Ex, the pancreas contains cells that
function in digestion and secrete the
hormones insulin and glucagon, and
other organs contain cells with
endocrine functions.
Functions of the Endocrine System

The Endocrine system (along with the nervous system) controls and
regulates the complex activities of the body.
Regulates the activities of the body by secreting chemical
substances (hormones) into the blood stream.
These secretions come from a variety of glands which control
various organs of the body. The key functions are:

To regulate the metabolic functions of the body.

To regulate the rate of chemical reactions in various cells.
To influence the ability of substances to transport themselves
through cell membranes.
Hormones
Divided into two major groups on the basis
of their chemical structure :
Amino acids include amines, peptides,
and proteins.
Lipids include steroids.
Hormones
Hormones play a critical role in the
regulation of physiological processes
Contribute to human reproduction,
growth, development of body tissues,
metabolism, fluid, electrolyte balance,
sleep, and many other body functions.
 The Hypothalamus and Pituitary Gland
The hypothalamus–pituitary complex can be thought of as the “command
center” of the endocrine system.
The Hypothalamus and Pituitary Gland

Be thought of as the “command center” of the endocrine system.

This complex secretes several hormones that directly produce
responses in target tissues, as well as hormones that regulate the
synthesis and secretion of hormones of other glands.
Complex coordinates the messages of the endocrine and nervous
systems.
A stimulus received by the nervous system must pass through the
hypothalamus-pituitary complex to be translated into hormones that
can initiate a response.
The Hypothalamus and Pituitary Gland
The hypothalamus is a structure of the diencephalon of the
brain located anterior and inferior to the thalamus
It has both neural and endocrine functions, producing and
secreting many hormones
Is anatomically and functionally related to the pituitary
gland (or hypophysis)
Suspended from Hypophysis by a stem called the
infundibulum (or pituitary stalk).
Hypothalamic hormones are secreted by neurons,
The Hypothalamus
The hypothalamus itself contains several types of neurons that release
different hormones
Thyrotropin-releasing hormone (TRH)
Gonadotropin-releasing hormone (GnRH)
Growth hormone-releasing hormone (GHRH)
Corticotropin-releasing hormone (CRH)
Somatostatin
Dopamine
Vasopressine
Oxytocine
The Hypothalamus
The thyrotropin-releasing hormone is a tripeptide that stimulates the release of
thyroid-stimulating hormone and prolactin from the anterior pituitary gland.
The gonadotropin-releasing hormone triggers sexual development at the onset of
puberty and maintains female and male physiology after that by controlling the
release of follicle-stimulating hormone and luteinizing hormone.
The growth hormone-releasing hormone stimulates the secretion of growth
hormone by the anterior pituitary.
The corticotropin-releasing hormone stimulates the release of
adrenocorticotropic hormone from the anterior pituitary.
Somatostatin inhibits the release of both growth hormone and thyroid-
stimulating hormone, and various intestinal hormones.
Dopamine inhibits the release of prolactin from the anterior pituitary, modulates
motor-control centers, and activates the reward centers of the brain.
The Hypothalamus
Prolactin functions mainly to promote lactation but also helps regulate
reproduction, metabolism, and the immune system.
Vasopressin and oxytocin are 2 hormones made in the hypothalamus itself
which travel in hypothalamic neurons directly to the posterior pituitary.
Vasopressin, also known as the anti-diuretic hormone or ADH, acts on the
collecting ducts in the kidneys to facilitate reabsorption of water.
Oxytocin stimulates contractions of the uterus at birth and release of milk
when an infant begins to breastfeed.
Orexin and ghrelin are known to increase appetite. So these hormones enhance
the action of the lateral hypothalamic nucleus while leptin does the reverse.
Leptin promotes the function of the ventromedial nucleus by decreasing
appetite; orexin and ghrelin antagonize its actions.
The Pituitary Gland
The pituitary gland is cradled within the sellaturcica of
the sphenoid bone of the skull.
It consists of two lobes that arise from distinct parts of
embryonic tissue:

The posterior pituitary (neurohypophysis) is

neural tissue
The anterior pituitary (also known as the
adenohypophysis) is glandular tissue that
develops from the primitive digestive tract.
Anterior Pituitary

Originates from the digestive tract in the embryo and

migrates toward the brain during fetal development.
There are three regions:

The pars distalis is the most anterior,

The pars intermedia is adjacent to the posterior pituitary
The pars tuberalis is a slender “tube” that wraps the
infundibulum.
Anterior Pituitary
The anterior pituitary does manufacture hormones.
The secretion of hormones from the anterior pituitary
is regulated by two classes of hormones.
These hormones—secreted by the hypothalamus—are
the releasing hormones that stimulate the secretion
of hormones from the anterior pituitary and the
inhibiting hormones that inhibit secretion.
Anterior Pituitary
The anterior pituitary produces 6 hormones.
Growth Hormone
Also called somatotropin
Regulates the growth of the human body, protein
synthesis, and cellular replication.
Its primary function is anabolic; it promotes protein
synthesis and tissue building through direct and indirect
mechanisms
GH levels are controlled by the release of GHRH and GHIH
(also known as somatostatin) from the hypothalamus.
Thyroid-Stimulating Hormone
Also called thyrotropin.
it triggers the secretion of thyroid hormones by the
thyroid gland.
In a classic negative feedback loop, elevated levels of
thyroid hormones in the bloodstream then trigger a drop
in the production of TRH and subsequently TSH.
Adrenocorticotropic Hormone
Also called corticotropin.
Stimulates the adrenal cortex to secrete corticosteroid
hormones such as cortisol.
Come from a precursor molecule known as pro-
opiomelanocortin (POMC) which produces several
biologically active molecules when cleaved, including ACTH,
melanocyte-stimulating hormone, and the brain opioid
peptides known as endorphins.
A variety of stressors can also influence its release
Follicle-Stimulating Hormone & Luteinizing Hormone

Also called gonadotropins.

Hormones that control the development and regulation of
the reproductive system
Much of the development of the reproductive system occurs
during puberty and is marked by the development of sex-
specific characteristics in adolescents.
Follicle-Stimulating Hormone & Luteinizing Hormone

The gonadotropins include two glycoprotein hormones:

Follicle-stimulating hormone (FSH) stimulates the
production and maturation of sex cells, or gametes,
including ova and sperm, and also promotes follicular
growth; these follicles then release estrogens in ovaries.
Luteinizing hormone (LH) triggers ovulation, as well as the
production of estrogens and progesterone by the ovaries. LH
stimulates production of testosterone by the testes.
Prolactin (PRL)

Promotes lactation (milk production) during pregnancy by

stimulates the mammary glands
Only during pregnancy do prolactin levels rise in response to
prolactin-releasing hormone (PRH) from the hypothalamus.
In non-pregnant females, prolactin secretion is inhibited by
prolactin-inhibiting hormone (PIH)
Intermediate Pituitary: Melanocyte-Stimulating Hormone

The cells in the zone between the pituitary lobes secrete a

hormone known as melanocyte-stimulating hormone
Formed by cleavage of the pro-opiomelanocortin (POMC)
precursor protein
Local production of MSH in the skin is responsible for
melanin production in response to UV light exposure.
Posterior Pituitary
Is actually an extension of the neurons of the paraventricular
and supraoptic nuclei of the hypothalamus.
The cell bodies of these regions rest in the hypothalamus, but
their axons descend as the hypothalamic–hypophyseal tract
within the infundibulum and end in axon terminals that
comprise the posterior pituitary
The posterior pituitary gland does not produce hormones but
rather stores and secretes hormones produced by the
hypothalamus.
Posterior Pituitary
The paraventricular nuclei produce the hormone oxytocin,
whereas the supraoptic nuclei produce ADH.
These hormones travel along the axons into storage sites in the
axon terminals of the posterior pituitary.
In response to signals from the same hypothalamic neurons, the
hormones are released from the axon terminals into the
bloodstream.
Oxytocin
Stimulates uterine contractions and dilation of the cervix
when fetal development is complete,
Is necessary for the milk ejection reflex (commonly referred
to as “let-down”) in breastfeeding people.
Is thought to contribute to parent–newborn bonding, known
as attachment.
Is also thought to be involved in feelings of love and
closeness, as well as in the sexual response.
Antidiuretic Hormone (ADH)
The target cells of ADH are located in the tubular cells of the
kidneys.
The osmoreceptors signal the posterior pituitary to release
antidiuretic hormone (ADH).
Monitoring blood osmolarity by osmoreceptors
Its effect is to increase epithelial permeability to water,
allowing increased water reabsorption.
Particularly sensitive to the concentration of sodium ions and
other solutes.
Antidiuretic Hormone (ADH)
ADH is controlled by a negative feedback loop. As blood
osmolarity decreases, the hypothalamic osmoreceptors sense
the change and prompt a corresponding decrease in the
secretion of ADH.
ADH is also known as vasopressin because, in very high
concentrations,=> constriction of blood vessels => increases
blood pressure by increasing peripheral resistance.
The Pineal Gland
Recall that the hypothalamus, part of the
diencephalon of the brain, sits inferior and
somewhat anterior to the thalamus.
Inferior but somewhat posterior to the
thalamus
A tiny endocrine gland whose functions
are not entirely clear.
The pinealocyte cells that make up the
pineal gland are known to produce and
secrete the amine hormone melatonin,
which is derived from serotonin.
The secretion of melatonin may influence
the body’s circadian rhythms, the dark-
light fluctuations that affect not only
sleepiness and wakefulness, but also
appetite and body temperature.
Melatonin Secretion
The Thyriod Gland
The Thyriod Gland
A butterfly-shaped organ is anterior to the trachea
and inferior to the larynx.
The medial region, called the isthmus
Each thyroid lobe is embedded with parathyroid
glands on their posterior surfaces.
Composed mostly of thyroid follicles.
Central cavity filled with a sticky fluid called colloid
Surrounded by a wall of epithelial follicle cells, the
colloid is the center of thyroid hormone production
Production is dependent on the hormones’ essential
and unique component: iodine.
Thyriod Follicle Cells
STAGES OF SYNTHESIS OF THYROID HORMONES
Synthesis of thyroid hormones occurs in stages:
Thyroglobulin synthesis
Iodide trapping
Oxidation of iodide
Transport of iodine into the follicular cavity
Iodination of tyrosine
Coupling reactions.
Synthesis of Thyroid Hormones
Release of Thyroid Hormones
Thyroid hormone (T3 and T4) affects every cell and all the organs, mainly
responsible for controlling the speed of your body’s metabolism by:

Regulating the rate at which your body uses calories (energy). This
affects weight loss or weight gain and is called the metabolic rate.
Slowing down or speeding up your heart rate.
Raising or lowering your body temperature.
Influencing the speed at which food moves through your digestive tract.
Affecting brain development.
Controlling the way your muscles contract.
Managing skin and bone maintenance by controlling the rate at which
your body replaces dying cells (a normal process).
Calcitonin
Produced by the parafollicular cells (also called C cells) that stud the tissue between
different follicles.
Response to a rise in blood calcium levels. It appears to have a function in decreasing
blood calcium concentrations by:

Inhibiting the activity of osteoclasts, bone cells that release calcium into the
circulation by degrading bone matrix
Increasing osteoblastic activity
Decreasing calcium absorption in the intestines
Increasing calcium loss in the urine

These functions are usually not significant in maintaining calcium homeostasis, so the
importance of calcitonin is not entirely understood.
PARATHYROID GLANDS
 PARATHYROID GLANDS

The parathyroid glands are tiny, round structures usually

found embedded in the posterior surface of the thyroid gland
A thick connective tissue capsule separates the glands from
the thyroid tissue.
Most people have four parathyroid glands, but occasionally
there are more in tissues of the neck or chest.
The primary functional cells of the parathyroid glands are the
chief cells.
The major hormone involved in the regulation of blood
calcium levels.
PARATHYROID GLANDS

Produce and secrete PTH

A peptide hormone
In response to low blood calcium levels
stimulating osteoclasts
Inhibits osteoblasts
Increased reabsorption of Ca Mg in the
kidney tubules
Initiates the production of the steroid
hormone calcitriol (also known as 1,25-
dihydroxyvitamin D)
 THE ENDOCRINE PANCREAS

The pancreas is a long, slender organ, most of which is

located posterior to the bottom half of the stomach
It is primarily an exocrine gland, secreting a variety of
digestive enzymes, the pancreas has an endocrine function.
Its pancreatic islets—clusters of cells formerly known as the
islets of Langerhans—secrete the hormones glucagon, insulin,
somatostatin, and pancreatic polypeptide (PP).
 CELLS AND SECRETIONS OF THE
PANCREATIC ISLETS
The alpha cell produces the hormone glucagon and makes up approximately 20%of each
islet. Glucagon plays an important role in blood glucose regulation; low blood glucose levels
stimulate its release.
The beta cell produces the hormone insulin and makes up approximately 75% of each islet.
Elevated blood glucose levels stimulate the release of insulin.
The delta cell accounts for 4% of the islet cells and secretes the peptide hormone
somatostatin. Recall that somatostatin is also released by the hypothalamus (as GHIH), and
the stomach and intestines also secrete it. An inhibiting hormone, pancreatic somatostatin
inhibits the release of both glucagon and insulin.
The PP cell accounts for about 1% of islet cells and secretes the pancreatic polypeptide
hormone. It is thought to play a role in appetite, as well as in the regulation of pancreatic
exocrine and endocrine secretions.
 REGULATION OF BLOOD
GLUCOSE LEVELS BY INSULIN
AND GLUCAGON
Glucose is required for cellular respiration and is the preferred fuel for all body
cells.
The body derives glucose from the breakdown of the carbohydrate-containing
foods and drinks we consume.
Glucose not immediately taken up by cells for fuel can be stored by the liver and
muscles as glycogen, or converted to triglycerides and stored in the adipose
tissue.
Hormones regulate both the storage and the utilization of glucose as required.
Receptors located in the pancreas sense blood glucose levels, and subsequently
the pancreatic cells secrete glucagon or insulin to maintain normal levels.
 GLUCAGON

Stimulates the liver to convert its stores of glycogen back into glucose. This
response is known as glycogenolysis. The glucose is then released into the
circulation for use by body cells.
It stimulates the liver to take up amino acids from the blood and convert them into
glucose. This response is known as gluconeogenesis.
It stimulates lipolysis, the breakdown of stored triglycerides into free fatty acids
and glycerol. Some of the free glycerol released into the bloodstream travels to the
liver, which converts it into glucose. This is also a form of gluconeogenesis.
The activity of glucagon is regulated through a negative feedback mechanism;
rising blood glucose levels inhibit further glucagon production and secretion.
 INSULIN
The primary function of insulin is to facilitate the uptake of glucose into body cells.
Red blood cells, as well as cells of the brain, liver, kidneys, and the lining of the
small intestine, do not have insulin receptors on their cell membranes and do not
require insulin for glucose uptake.
Although all other body cells do require insulin if they are to take glucose from the
bloodstream, skeletal muscle cells and adipose cells are the primary targets of
insulin.
The presence of food in the intestine triggers the release of gastrointestinal tract
hormones such as glucose-dependent insulinotropic peptide (previously known as
gastric inhibitory peptide).
 INSULIN
Appears to activate a tyrosine kinase receptor, triggering the phosphorylation of
many substrates within the cell.
These multiple biochemical reactions converge to support the movement of
intracellular vesicles containing facilitative glucose transporters to the cell
membrane.
Insulin triggers the rapid movement of a pool of glucose transporter vesicles to the
cell membrane, where they fuse and expose the glucose transporters to the
extracellular fluid. The transporters then move glucose by facilitated diffusion into
the cell interior.
 INSULIN
Reduces blood glucose levels by stimulating glycolysis, the metabolism
of glucose for generation of ATP.
Stimulates the liver to convert excess glucose into glycogen for
storage, and it inhibits enzymes involved in glycogenolysis and
gluconeogenesis.
Promotes triglyceride and protein synthesis. The secretion of insulin is
regulated through a negative feedback mechanism.
The Adrenal Glands
The adrenal glands are wedges of glandular and
neuroendocrine tissue adhering to the top of the
kidneys by a fibrous capsule
Have a rich blood supply and experience one of the
highest rates of blood flow in the body
They are served by several arteries branching off the
aorta, including the suprarenal and renal arteries.
Blood flows to each adrenal gland at the adrenal
cortex and then drains into the adrenal medulla.
Adrenal hormones are released into the circulation
via the left and right suprarenal veins.
 The Adrenal Glands
Consists of an outer cortex of glandular tissue and an inner medulla of nervous tissue.
The cortex itself is divided into three zones: the zona glomerulosa, the zona fasciculata, and
the zona reticularis. Each region secretes its own set of hormones.
One of the major functions of the adrenal gland is to respond to stress.
 The Adrenal Glands
The adrenal cortex The adrenal medulla
A component of the hypothalamic- Is neuroendocrine tissue composed
pituitary-adrenal (HPA) axis of postganglionic sympathetic
Secretes steroid hormones for the nervous system (SNS) neurons.
regulation of the long-term stress Extension of the autonomic
response, blood pressure and nervous system, which regulates
volume, nutrient uptake and homeostasis in the body.
storage, fluid and electrolyte The sympathomedullary (SAM)
balance, and inflammation. pathway involves the stimulation
The HPA axis involves the of the medulla by impulses from
stimulation of hormone release of the hypothalamus via neurons
ACTH from the pituitary by the from the thoracic spinal cord.
hypothalamus. The medulla is stimulated to
ACTH stimulates the adrenal cortex secrete the amine hormones
to produce the hormone cortisol. epinephrine and norepinephrine.
Adrenal Cortex
Consists of multiple layers of lipid-storing cells that occur in 3 structurally
distinct regions. Each of these regions produces different hormones.

Hormones of the Zona Glomerulosa

The most superficial region
Produces a group of hormones collectively referred to as mineralocorticoids
These hormones are essential for fluid and electrolyte balance
Aldosterone is the major mineralocorticoid.
It is important in the regulation of the concentration of Na+ and K+ ions in
urine, sweat, and saliva.
Its secretion is prompted when CRH from the hypothalamus triggers ACTH
release from the anterior pituitary.
Adrenal Cortex Hormones of the Zona Glomerulosa
Aldosterone is also a key component of the renin-angiotensin-
aldosterone system (RAAS) in which specialized cells of the kidneys
secrete the enzyme renin in response to low blood volume or low blood
pressure.
Renin then catalyzes the conversion of the blood protein
angiotensinogen, produced by the liver, to the hormone angiotensin I.
Angiotensin I is converted in the lungs to angiotensin II by angiotensin-
converting enzyme (ACE). Angiotensin II has three major functions:

Initiating vasoconstriction of the arterioles, decreasing blood flow

Stimulating kidney tubules to reabsorb NaCl and water, increasing
blood volume
Signaling the adrenal cortex to secrete aldosterone, the effects of
which further contribute to fluid retention, restoring blood
pressure and blood volume
Adrenal Cortex Hormones of the Zona Fasciculata

The intermediate region of the adrenal cortex

The cells of the zona fasciculata produce hormones called glucocorticoids (role in
glucose metabolism.)
The most important of these is cortisol, some of which the liver converts to
cortisone.
A glucocorticoid produced in much smaller amounts is corticosterone.
In response to long-term stressors, the hypothalamus secretes CRH, which in turn
triggers the release of ACTH by the anterior pituitary.
Their overall effect is to inhibit tissue building while stimulating the breakdown of
stored nutrients to maintain adequate fuel supplies.
In conditions of long-term stress, for example, cortisol promotes the catabolism of
glycogen to glucose, the catabolism of stored triglycerides into fatty acids and
glycerol, and the catabolism of muscle proteins into amino acids.
Adrenal Cortex Hormones of the Zona Reticularis

The deepest region of the adrenal cortex

Produces small amounts of a class of steroid sex hormones called
androgens
The androgens produced in the zona reticularis supplement the
gonadal androgens.
Converted in the tissues to testosterone or estrogens.
The main source of estrogens becomes the androgens produced by the
zona reticularis.
Adrenal Medulla
Releases hormones in response to acute, short-term stress mediated
by the sympathetic nervous system (SNS).
The medullary tissue is composed of unique postganglionic SNS
neurons called chromaffin cells, and produces the neurotransmitters
epinephrine (also called adrenaline) and norepinephrine (or
noradrenaline).
Epinephrine is produced in greater quantities—approximately a 4 to 1
ratio with norepinephrine—and is the more powerful hormone.
Derived from the amino acid tyrosine, they are chemically classified as
catecholamines.
Adrenal Medulla
The secretion of medullary epinephrine and norepinephrine is
controlled by a neural pathway that originates from the hypothalamus
in response to danger or stress (the SAM pathway).
Both epinephrine and norepinephrine signal
Liver and skeletal muscle cells to convert glycogen into glucose,
Increased blood glucose levels
Increase the heart rate, pulse, and BP to prepare the body to fight the perceived
threat
Dilates the airways, raising blood oxygen levels.
Vasodilation
Vasoconstriction to blood vessels serving less essential organs such as the
gastrointestinal tract, kidneys, and skin, and downregulates some components of
the immune system.
Dry mouth, loss of appetite, pupil dilation, and a loss of peripheral vision.
Gonadal and Placental Hormones

This section briefly discusses the hormonal

role of the gonads—the testes and ovaries—
which produce the sex cells (sperm and ova)
and secrete the gonadal hormones.
The primary hormone produced by the testes
is testosterone
The primary hormones produced by the
ovaries are estrogens, which include estradiol,
estriol, and estrone.
 Thanks

REF:
https://openstax.org/books/anatomy-and-physiology-
2e/pages/17-introduction