ENDOCRINE SYSTEM -LECTURE 1
The endocrine system and the nervous system work together to coordinate
functions of the body systems. Nervous control is faster, happening in
milliseconds and uses chemical messengers called neurotransmitters. Endocrine
responses are longer to begin from hours, months and even years and have longer
effects and use hormones as the chemical messengers. Hormones are mediator
molecules that are released in one part of the body and regulate the activity of cells
in other parts of the body. Most enter the blood stream and bind to receptors on
target cells. Endocrine responses function to regulate long acting body changes,
such as changes in metabolic activity, changes which adapt animal to environment
and help to provide homeostasis.
There are two types of glands in the body-exocrine and endocrine. Exocrine
glands such as the sweat glands secrete their products through a duct that carry
them into body cavities, into the lumen of an organ or onto the surface of the body.
Endocrine glands secrete their products into the interstitial fluid surrounding the
secretory cells of the gland and from there they enter capillaries flow through the
blood until they reach a target organ.
Endocrine glands include the pituitary, thyroid, parathyroid, adrenal and pineal
glands. There are other organs and tissues that are not exclusively endocrine but
have cells that secrete hormones. These include: the hypothalamus, thymus,
pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart,
adipose tissue and the placenta.
Endocrine organs are small, unimpressive and widely scattered through the body.
They secrete hormones, chemical messengers produced in one part of body and
transported by circulatory system to distant target tissues, where effects are
exerted. In order for a hormone to have an effect it must bind to receptor cells on a
target organ. Hormones exert prolonged effects at low concentrations.
Concentration reflects the rate of release, speed of inactivation and rate of
removal half-life which can be a fraction of a minute to 30 minutes or longer.
Eventually these hormones are inactivated by the liver and excreted by the kidneys.
Circulating and Local Hormones
Most hormones of the endocrine system are circulating. They reach their target
organs by flowing through the circulatory system. Local hormones are released
and act on nearby cells without entering the bloodstream. These are termed
paracrines. Those that act on the same cell that secreted them are called
autocrines. Local hormones are rapidly inactivated.
Chemical Classes of Hormones
Hormones fall into two broad classes-those soluble in lipids and those soluble in
water. These two classes exert their effects differently. Lipid soluble hormones
include steroid hormones, thyroid hormones and nitric oxide. Steroid hormones
are derived from cholesterol and include the gonadal hormones-estrogen,
progesterone, testosterone, and adrenocortical hormones-cortisol. Steroids
hormones, are transported bound to blood transport proteins. Thyroid hormone is
made by adding iodine molecules to tyrosine. Water soluble hormones include
amines, peptide and protein hormones as well as the eicosanoid hormones. Amine
hormones are modified amino acids; epinephrine and norepinehrine are examples
and are often called catecholamines. Peptide/protein hormones range from short
chains of amino acids-3-49-peptides to polypeptides-50 -200 amino acids. Some
have sugars attached and are called glycoproteins. These include growth hormone,
insulin, prolactin, oxytocin. Eicosanoids are derived from arachidonic acid, a fatty
acid and include prostaglandins which raise blood pressure, mediate uterine
contractions, enhance blood clotting and leukotrienes which are signaling
chemicals found during times of inflammation and allergic reactions. Some do not
include these as true hormones since they are localized and not circulating.
Mechanisms of Hormone Action
A hormone announces its arrival to a target organ by binding to a receptor.
Receptors for lipid soluble hormones are found inside the cell while those for
water soluble ones are found on the cell membrane of target cells.
Direct Gene Activation-Lipid Soluble Hormone
For hormones that are lipid soluble the mechanism of action is termed direct gene
activation. Since these hormones are lipid soluble they can diffuse into target cells.
Direct gene activation occurs in several steps. First, the hormone diffuses from the
blood into a cell. The hormone binds to and activates receptrs within the cytosol or
the nuclues of the target cell. Binding of hormone to receptor forms a receptorhormone complex. This complex alters gene expression, i.e. it turns genes on or
off. As the Dna is transcribed, new mRNA is made. This leaves the nucleus and
enters the cytosol where new protein is made. This new protein alters the cell’s
activity. Proteins or exaple may be enzymes that promote metabolic activities of
other proteins.
Water Soluble Hormones- Second-Messenger System
Protein, peptide and amine hormones cannot pass through the plasma membrane.
They must bind to receptors on the cell membrane of target cells and exert their
effects through an intracellular second messenger. The hormone is the first
messenger; it binds to a plasma membrane receptor and causes the production of a
second messenger inside the cell. One common second messenger is cAMP (cyclic
AMP). The first step of the second messenger mechanism is fro the hormone to
diffuse through the blood and gind to a receptor on the cell membrane. This in turn
activates a G protein, a type of regulatory molecule. The activated G protein
activates adenlyate cyclase, an effector enzyme. Activated adenlyate cyclase
conversts ATP to the second messenger-cAMP. cAMP activates one or more
protein kinase molecules. Protein kinase is an enzyme that phosphorylates or adds
phosphate groups to proteins. Activated protein kinase phosphorylates cellular
proteins; this activates some proteins and inhibits others which allows for a variety
of reactions in one cell. cAMP is degraded rapidly by phosphodiesterase. Its action
is brief and no extracellular controls are needed to stop its action.
Control of Hormone Secretion
Hormonal blood levels vary within a narrow range. Secretions are strictly
controlled. Secretions are regulated by: 1) nervous system, 2) hormones of 3)
changes in blood chemistry called humoral stimuli. Neural stimuli means that
nerve fibers stimulate hormone release. A classic example is the milk-let down
reflex in cows. There is a 6-8 minute delay between time of initial udder
stimulation-suckling, manual stimulation and full milk release. Udder stimulated
neural signals travel along afferent nerves to spinal cordbrainrelayed to
secretory neurons of posterior pituitaryoxytocin released into capillary bed
surrounding posterior pituitaryblood flows through venous system to heart
lungsback to heartarterial systemuddersmilk let down. Stress
sympathetic nervous systemadrenal medullanorepinephrine & epinephrine.
Hormonal stimuli refers to glands releasing hormones in response to other
hormones. Releasing hormones from the hypothalamusanterior pituitary
stimulates release of anterior pituitary hormones. Tropic hormones from the
anterior pituitaryovaries to secrete hormones. Humoral Stimuli directs
endocrine glands to secrete hormones as a direct response to changing blood levels
of ions and nutrients. For example the parathyroid gland monitors Ca++
concentration, when low parathyroid hormone releasedcauses Ca++ to
increase. Insulin and aldosterone are also released by humoral stimuli.
Endocrine Glands
Hypothalamus
The hypothalamus is the major link between the nervous system and the endocrine
system. It is only recently realized as the major gland in the body. It is responsible
for secreting nine hormones some which control the release of other hormones
from the pituitary gland. Together the hypothalamus and the pituitary play
important roles in virtually all aspects of growth, development, metabolism and
homeostasis.
Pituitary gland or Hypophysis-hi-pof-I-sis
The pituitary gland is located in the sella turcica of sphenoid bone. It is about the
size and shape of a pea and is attached to the hypothalamus by the infundibulum.
The pituitary has 2 anatomically and functionally separate parts-adenohypothesis
or anterior lobe and the neurohypophyesis or posterior lobe. The anterior lobe
consists of two parts: pars distalis and the pars tuberalis; the posterior lobe is
divided into the pars nervosa and the pars intermedia. The pars intermedia
atrophies during fetal development and is not a separate lobe in adults.
Anterior pituitary hormones are stimulated to be released by hormones made in the
hypothalamus called releasing hormones and are suppressed by hormones made in
the hypothalamus called inhibiting hormones. The posterior lobe stores hormones
made in the hypothalamus.
The connection between the hypothalamus and the anterior pituitary is via the
hypophyseal portal system. In a portal system blood flows from one capillary
network into a portal vein and then to a secondary capillary network before
returning to the heart. Arterial blood gets to the pituitary by hypophyseal branches
of the internal carotid arteries. Veins leave via dural sinuses. The anterior lobe is
connected vascularly by a primary capillary plexus. The infundibulum connects to
hypophyseal portal veins via secondary capillary plexus in the anterior lobe.
Primary and secondary plexi make up hypophyseal portal system. Blood
transports releasing factors from hypothalamus directly to the anterior pituitary
gland.
Neurosecretory cells in the hypothalamus made RH (releasing hormones) and IH
(inhibiting hormones). These enter the hypophyseal portal system and go directly
to the adenohypophysis. These stimulate the pituitary to make tropins or tropic
hormones-tropic=turn on. These stimulate other endocrine glands or tissues to
function.
Types of Cells in the Anterior Pituitary
There are several types of cells in the anterior pituitary which make a variety of
trophins. Somatotrophs make growth hormone or somatorophin which stimulate
cells to secrete insulin-like growth factors, which in turn simulates general growth
and regulate metabolism. Thyrotrophs secrete TSH-thyroid stimulating hormone
or thyrotropin which controls secretions from the thyroid gland. Gonadotrophs
secrete (GnRH) gonadotrophins. There are two types-FSH and LH, follicle
stimulating hormone and leutinizing hormone which act on the gonads to stimulate
production of hormones and to influence maturation of oocytes and sperm.
Lactotrophs stimulate prolactin which stimulate the mammary glands to make
milk. Corticotrophs make ACTH or corticotrophin which stimulate the adrenal
cortex to make glucocorticoids such as cortisol. Cells in the parts intermedia
secrete MSH-melanocyte stimulating hormone.
Control of Secretion of Anterior Pituitary Hormones
There are two ways to regulate hormone release from the anterior pituitary. One
way is via secretion of releasing hormones (five) and inhibiting hormones (two)
from the hypothalamus. The second way is via negative feedback. Endocrine
glands are often regulated by negative feedback mechanisms. In this mechanism
substances produced by target organs are transported by blood back to the anterior
pituitary and/or the hypothalamus to decrease the secretion from these glands and
in so doing stop the production of the hormone from the endocrine gland. As an
example, LH secreted by the pituitary stimulates follicle to produce estrogen;
estrogen travels back to the pituitaryinhibits release of LH.
Anterior Pituitary Hormones
Growth Hormone
Growth hormone is made by somatotrophs which are the most numerous type of
cell in the anterior pituitary. These cells make hGH-human growth hormone which
has an anabolic effect. They promote protein synthesis and secretion of IGFsInsulin like growth factors or somatomedins. Target cells for growth hormone are
found in the liver, skeletal muscle, cartilage, bone and other tissues. IGFs cause
cells to grow and to multiply by increasing the uptake of amino acids into cells and
accelerating protein synthesis. They decrease protein breakdown. Overall these
functions increase the growth rate of the skeleton and skeletal muscles during
childhood and teenage years. In adults IGFs help maintain muscle and bone mass
and help to promote healing and tissue repair. IGFs encourags use of fats for fuel
conserving glucose; lipolyis. They influence carbohydrate metabolism by
decreasing glucose uptake (glucose sparing effect).
Somatotrphs release bursts of growth hormone every few hours, especially during
sleep. Secretion is regulated by 2 hypothalamic hormones GHRF-stimulates
release and GHIF-somatostatin-inhibits release of GH. Blood glucose levels
regulate both GHRH and GHIF. Hypoglycemia stimulates production of GHRH
and hyperglycemia stimulates production of GHIH.
TSH, thyroid stimulating hormone stimulates secretory activity of the thyroid
gland. When thyroid hormone is low in the blood TRH-thyrotropin RH from the
hypothalamus causes thyrotrope cells of the anterior pituitary to produce TSH.
TSH stimulates the thyroid to make thyroid hormones-T3 and T4.Thyroid hormone
feedsbacks on TRH and TSH to inhibit secretion.
ACTH or adrenocorticotropic hormone is secreted by corticotrophs of the anterior
pituitary. Hypothalamuscorticotropin releasing hormone (CRH)anterior
pituitary to make ACTHadrenal cortexglucocoricoids, especially cortisol
which help resist stress. Glucocorticoids feedback to block secretion of CRH and
ACTH.
Gonadotropins include FSH and LH which regulate the function of the gonadsovaries & testes. Follicle stimulating hormone (FSH) targets the ovaries in females
causing the follicular cells to produce estrogen. In males FSH stimulates sperm
production. LH (leutinizing hormone) in females triggers ovulation and with FSH
stimulates estrogen secretion. In males LH stimulates interstitial cells of testes to
secrete testosterone. GnRH (gonadotrophin releasing hormone) from the
hypothalamus stimulates the anterior pituitary to release both FSH and LH.
Gonadal hormones feedback to suppress FSH and LH release.
Prolactin or PRL is released from the anterior pituitary in response to PRH from
the hypothalamus. In females prolactin initiates and maintains milk production by
the mammary glands. Ejection of milk depends on oxytocin from the posterior
pituitary gland. The hypothalamus secretes PIH (prolactin inhibiting hormone)
which is believed to be dopamine which inhibits prolactin secretion. PRH,
believed to be serotonin causes lactotropes to stimulate milk production.
MSH, melanocyte stimulating hormone stimulates melanocytes in amphibians and
reptiles to make melanin, increasing pigmentation. The role of MSH in humans is
unknown. There are MSH receptors in the brain suggesting it is important in brain
activity. There is little circulating MSH.
Posterior Pituitary
The posterior pituitary or neurohypophysis does not make hormones. It stores and
releases two hormones made in the hypothalamus- ADH or antidiuretic hormone
and oxytocin. Cell bodies in the paraventricular (oxytocin) and supraoptic nucleus
(ADH or vasopressin) of the hypothalamus make the hormones and they travel to
the posterior pituitary through the axons of the nerve cells. The axons make up the
hypothalamic hypophyseal tract.
ADH functions to control water balance. It inhibits or decreases urine formation
which helps avoid dehydration or water overload. Hypothalamic neurons called
osmoreceptors monitor solute or water concentration of blood. Solutes too
concentratedosmoreceptorsexcite supraoptic nucleisynthesis and release of
ADHtarget-kidney tubules reabsorb water from urineless urineblood
volume increases. Solute concentration decreasesosmoreceptors end ADH
release. Alcohol inhibits ADH secretioncopious urine. Excessive water drinking
 inhibits ADH release. At high concentrations ADH causes vasoconstriction
primarily visceral blood vessels-referred to as vasopressin.
The target organs for oxytocin are the uterus and the breasts. It is released in high
amounts during childbirth and in nursing mothers. During delivery, the cervix
stretches which stimulates release of oxytocin which enhances smooth muscle
contraction. After delivery oxytocin stimulates milk ejection. Uterus stretches
impulse to hypothalamusoxytoxin madereleased from posterior
pituitaryoxytocin in bloodcontractions increase. This is a form of positive feed
back control. Oxytocin is called the cuddle hormone; it is believed to
promotesnurturing and affectionate behavior.
Thyroid Gland.
The thyroid gland is located in the anterior neck on the trachea just inferior to the
larynx. It is make up of 2 lateral lobes connected by an isthmus and is the largest
pure endocrine gland. Thyroid follicles make up most of the gland. The walls of
each follicle consists of cuboidal, follicular cells. These cells make the thyroid
hormones T4, thyroxin or tetraiodothyronine and T3 or triiodotyronine. Both are
iodinated derivatives of tyrosines; thyroxine has 4 bound iodine atoms and T3 has
3.
Synthesis & Secretion of Thyroid Hormones
The first step in the synthesis and secretion of thyroid hormone is Iodide trapping.
The follicular cells trap I- by actively transporting the ions from the blood into the
cytosol of the cells. In step two thryroglobulin (TGB) is made. While trapping
iodide follicle cells make TGB. Thyroglobulin is made in the ribosome and
transproted to the Golgi body where sugar residues are attached and it is packaged
into vesicles. These undergo exocytosis and release TGB into the lumen of the
follicle. The third step is the oxidation to iodide. Iodides, I- are negatively charged
and cannot bind with tyrosines to make thyroid hormones until theylose the
negative charge. This requires them to be oxidated or to lose an electron.
I- I2 . As the iodide is oxidized it passes into the lumen of the follicle. Step four
is iodination of tyrosine. I2 binds with tyrosines. Coupling of one iodineT1
ormonoiodotyrosine. Attachment of 2diiodotyrosine. TGB with iodine atoms
attached is called colloid. Step five is coupling of T1 and T2 within the colloid.
Enzymes in colloid link T1 and T2 into T3 and T4. The next step is pinocytosis and
digestion of colloid. Droplets of colloid reenter the follicular cells by pinocytosis
(cell sipping) and merge with lysosomes. The enzymes in the lysosomes digest the
colloid breaking off molecules of T3 and T4. Secretion of the thyroid hormones is
the next step. Since thyroid hormones are lipid soluble they diffuse through the
membrane and into the blood. More thyroxin is secreted; T3 is more potent. The
last step is transport of the hormones in the blood. Binds to proteins in
bloodstream, thyroxine-binding globulins or TBGs for transport to target tissues.
Actions of Thyroid Hormone
Most cells have receptors for thyroid hormones. It affects all cells except adult
brain, spleen, testes, uterus and thyroid. It increases BMR (rate of oxygen
consumption under standard basal conditions). When BMR increases metabolism
of carbohydrates, proteins and lipids increase. Thyroid hormones also stimulate the
synthesis of Na/K ATPase pumps. They stimulate protein synthesis, enhance
actions of catecholamines and accelerate body growth.
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Thyroid Hormone-Mechanism of Action
Low blood levels of thyroid hormonehypothalamusTRHanterior pituitary
TSH (anterior pituitary)binds to follicle cell receptorsforms & stores
thyroid hormone. There is feedback inhibition by thyroid hormone. When levels
are high enough they suppress release of both TRH and TSH.
Calcitonin
Another cell type in the thyroid gland are parafollicular or C cells which lie in
follicular epithelium and protrude into the connective tissue separating and
surrounding follicles. These cells make calcitonin, a polypeptide hormone that acts
to lower blood Ca++. One target organ is the skeleton; the hormone inhibits
osteoclast activity or bone resorption and release of Ca++.
Parathyroid glands
The four parathryroid glands are found embedded in the posterior surface of the
thyroid gland. There are two types of cells, chief or principle cells which make
parathyroid hormone and oxyphilic cells whose function is unknown. PTH,
parathormone or parathyroid hormone is the single most important hormone
controlling Ca++ balance. It also functions in the balance of magnesium and
phosphate ions.
Lowered blood Ca++ or hypocalcemia stimulates secretion of PTH. PTH increases
Ca++ levels in blood by stimulating 3 target organs: skeleton, kidneys and the
intestines. The hormone stimulates osteoclasts, bone resorbing cells to digest the
bone matrix and release Ca++ and PO4. It enhances the reabsorption of Ca++ and
excretion of PO4 by the kidneys and increases absorption of Ca++ by intestional
mucosal cells. PTH increases production of the hormone calcitriol from the
kidneys. Calcitriol is the active form of vitamin D3 which enhances Ca++
absorption by the gut.
Adrenal Glands-Suprarenal Glands
The adrenal glands are two pyramid shaped organs on top of the kidneys. Each is
surrounded by a fibrous capsule and fat. The glands have two structurally and
functionally distinct regions: the inner, adrenal medulla and the outer, adrenal
cortex which produces the steroid hormones that are essential for life.
The adrenal cortex is divided into three zones, each which secrete a different
hormone. The outer zone, the zona glomerulosa secretes mineralocorticoids
which affect mineral homeostasis. The zona fasciculate is the middle zone. Here
the cells are formed in linear cords and secrete glucocorticoids or metabolic
hormones. The last area is the zona reticularis where the cells are found in a net
like arrangement. This zone synthesizes androgrens or gonadocorticoids.
Mineralocorticoids regulate electrolyte or mineral salt concentration in
extracellular fluids, particularly Na and K. Aldosterone is the major
mineralocorticoids. Its primary target is the distal part of the kidney tubules
where it stimulates reabsorption of Na and water from urine. It enhances K
elimination and enhances Na absorption from sweat, saliva and gastric juice.
Sodium ion regulation is critical to overall body homeostasis because where Na
goes water follows which can lead to blood volume and blood pressure changes.
Aldosterone is stimulated by rising K levels, low Na, decreasing blood volume and
blood pressure. Reverse conditions end secretion.
The renin-angiotensin-aldosterone system (RAA) is the major regulator of
aldosterone release. Blood pressure or volume decreases or plasma osmolaritysolute concentration decreasescells in the kidney’s juxtaglomerular
apparatusrelease renincleaves off part of angiotensinogenenzymatic
cascadeangiotensin II glomerulosa cells-adrenal cortexaldosterone release.
Glucocorticoids are produced in the zona fasciculata. The major one is cortisol or
hydrocortisone. Cortisone and corticosterone are two other glucocorticoids.
Severe stress including hemorrhage, infection and emotional trauma produce high
output of cortisol. Release is regulated by negative feedback. Low levels of
glucocorticoidsCRH from the hypothalamusACTH from the anterior pituitary
adrenal cortexcortisol release.
The primary metabolic effect of glucocorticoids is gluconeogenesis, the formation
of glucose from non-carbohydrate sources. It also increases the rate of protein
breakdown and stimulates lipolysis. These hormones help resist stressors and are
absolutely essential to life. Additional glucose provides tissues with a source of
ATP which help to reduce stress. Glucocoricoids have anti-inflammatory effects by
inhibiting white blood cells. In so doing they also depress the immune response.
The zona reticularis makes androgens. The major one is DHEA-inhi. In females
this helps promote libido and is converted into estrogen in other body tissues. The
main hormone that stimulates its secretion is ACTH.
Adrenal Medulla
The adrenal medulla secretes catecholamines, epinephrine, norepinephrine and
dopamine. The medullas is a modified, sympathetic ganglion of the autonomic
nervous system. Chromaffin cells produce epinephrine and norepinephrine. Stress
and exercise cause the hypothalamus to send impulses to the medulla to release the
catecholamines. This begins the fight or flight response. Epinephrine is a potent
stimulator of heart and metabolic activities; norepinephrine has a greater effect on
peripheral vasoconstriction and blood pressure. Stresssympathetic nervous
systemblood sugar increases, blood vessels constrict, heart rate increases, blood
pressure increases.
Pancreas
The pancreas is located partially behind the stomach in the abdomen. It is a soft,
triangular, mixed gland containing both exocrine and endocrine parts. 99% of the
exocrine cells are arranged in clusters called acini. These cells produce digestive
enzymes. Scattered among the acini are pancreatic islets or Islets of Langerhans.
These contain four different cells types: alpha, beta, delta and F.
Alpha or A cells produce glucagon. These account for 17% of the islet cells.
Glucagon is hyperglycemic; it raises blood glucose levels. It stimulus for release is
low blood glucose levels. Beta or B cells account for 70% of the islets cells and
make insulin. Insulin is hypoglycemic and is released in response to high blood
glucose levels. Delta or D cells account for 7% of islet cells and make
somatostatin. This hormone inhibits glucagon and insulin release and inhibits
growth hormone secretion. F cells make pancreatic polypeptide which inhibits
somatostatin, inhibits gall bladder secretion and inhibits secretion of digestive
enzymes.
Gonads
The gonads, ovaries and testes produce steroid hormones and produce gametes.
Ovaries, paired, small, oval organs in the abdominopelvic cavity make estrogen,
progesterone and inhibin. Estrogen is needed for the maturation of reproductive
organs and is responsible for secondary sex characteristics with progesterone, these
hormones regulate the menstrual cycle. Testes found in the extra abdominal skin
pouch-scrotum make testosterone which is responsible for the maturation of
reproductive organs and maintenance of the secondary sex characteristics. The
testes also make inhibin. GnRH from the hypothalamusanterior pituitaryFSH
and LH ( gonadotropins) control release. Inhibin feeds back and inhibits FSH
release.
Pineal Gland
The pineal gland is a tiny, pine coned shaped body hanging from the roof of the
third ventricle. It secretes melatonin from pinealocytes arranged in cords and
clusters. Melatonin rises and falls in a diurnal cycle. Itpeaks during the night and
promotes sleep. Pineal gland receives information regarding intensity and duration
of day light through the retina of the eye. This information is sent to the
suprachiasmatic nucleus (biological clock) in the hypothalamussuperior
cervical ganglion pineal glandmelatonin.
Thymus
The thymus gland is found deep in the sternum in the thorax. It is lobulated and is
large and conspicuous in infants and children. It diminishes in size with age. This
gland makes thymopoietins , thymic humoral factor (THF), thymic factor (TF) and
thymosins. These promote the development of T lymphocytes; a type of white
blood cell needed for the immune response. The gland populates other lymphatic
organs before it involutes.
Other Hormone Producing Structures
There are other tissues that make hormones that are not endocrine glands. The
heart’s atrial cells make ANP, atrial natriuretic peptide. This hormone reduces
blood volume, blood pressure and blood Na. The gastrointestional tract contains
enteroendocrine cells which make gastrin, secretin and GIP. Gastrin promotes
gastric juice secretion, secretin promotes secretion of pancreatic enzymes and GIP
(glucose dependent insulinotropic peptide promotes release of insulin. The
placenta produces hormones that influence pregnancy such as estrogens and
progesterones; it also makes hCG-human chorionic gonadotrophin and hCs, human
chorionic somatommotrophin which stimulates development of the mammary
glands. The kidneys make renin, erythropoietin and calcitriol.
Erythropoietinincreases RBC production. Renin is needed for blood pressure and
salt balance and calcitriol is used to absorb calcium from the digestive system. The
skin makes cholecalciferol, an inactive vitamin D form. Adipose tissue produces
leptin which binds to CNS neurons for appettite control; it provides the sensation
of satiety.