Chapter 16 Endocrine System

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Chapter 16Endocrine System
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Introduction

• The endocrine and nervous systems function to
  achieve and maintain homeostasis
• In the endocrine system, secreting cells send
  hormone molecules via the blood to specific
  target cells contained in target tissues or
  target organs

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Introduction

• Hormones
• carried to almost every point in the body
• can regulate most cells
• effects work more slowly and last longer than
  those of neurotransmitters

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Classification of hormones

• Classification of hormones
• Classification by general function
• Tropic hormones target other endocrine glands
  and stimulate their growth and secretion
• Sex hormones target reproductive tissues
• Anabolic hormones stimulate anabolism in target
  cells
• Classification by chemical structure
• Steroid hormones
• Non-steroid hormones

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Steroid hormones

• Synthesized from cholesterol
• Lipid-soluble and can easily pass through the
  phospholipid plasma membrane of target cells
• Examples cortisol, aldosterone, estrogen,
  progesterone, and testosterone

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Nonsteroid hormones

• Synthesized primarily from amino acids
• Protein hormones
• long, folded chains of amino acids
• Ex insulin and parathyroid hormone
• Glycoprotein hormones protein hormones with
  carbohydrate groups attached to the amino acid
  chain
• Peptide hormones
• smaller than protein hormones
• short chain of amino acids
• Ex oxytocin and antidiuretic hormone (ADH)

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Nonsteroid hormones

• Amino acid derivative hormones each is derived
  from a single amino acid molecule
• Amine hormones
• synthesized by modifying a single molecule of
  tyrosine
• produced by neurosecretory cells and by neurons
• Ex epinephrine and norepinephrine
• Amino acid derivatives
• produced by the thyroid gland
• synthesized by adding iodine to tyrosine

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How do hormones work?

• Hormones bind to the target cells receptors in a
  lock-and-key mechanism
• Different hormone-receptor interactions produce
  different regulatory changes within the target
  cell through chemical reactions
• Combined hormone actions
• Synergism combinations of hormones acting
  together have a greater effect on cell than the
  sum of the effects that each would have if acting
  alone
• Permissiveness when a small amount
• of one hormone permits a second one
• to have its full effects on a target cell
• Antagonism one hormone produces
• the opposite effects of another
• hormone used to fine tune the activity
• of target cells with great accuracy

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How do hormones work?

• Most hormones have primary effects that directly
  regulate target cells and many secondary effects
  that influence or modulate other regulatory
  mechanisms in target cells
• Endocrine glands produce more hormone molecules
  than are actually needed
• the unused hormones are quickly excreted by the
  kidneys or broken down by metabolic processes

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How do steroid hormones work?
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Mechanism of steroid hormone action

• Steroid hormone ? cell
• Forms a hormone-receptor complex
• complex ? nucleus ? binds to DNA
• activates a certain gene sequence ? mRNA
• mRNA ? cytosol ? protein molecules that produce
  the effects of the hormone
• Steroid hormones regulate cells by regulating
  production of certain critical proteins
• The amount of steroid hormone present determines
  magnitude of a target cells response
• Because transcription and protein synthesis take
  time, responses to steroid hormones are often slow

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What about nonsteroid hormones?
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Mechanisms of nonsteroid hormone action

• The second messenger mechanism
• A nonsteroid hormone molecule acts as a first
  messenger and delivers its chemical message to
  receptors in the target cells plasma membrane
• message ? in cell, where a second messenger
  triggers cell changes
• Second messenger mechanism much different from
  steroid hormone effects
• Effects are amplified by the cascade of reactions
• variety of second messenger mechanisms
• much more quickly than the steroid mechanism

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One exampleYou do not need to know the specifics
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Regulation of hormone secretion

• usually part of a negative feedback loop and is
  called endocrine reflexes
• Simplest mechanism an endocrine gland is
  sensitive to the changes produced by its target
  cells
• secretion may also be regulated by a hormone
  produced by another gland
• secretions may be influenced by nervous system
  input

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• When lactation in a woman consumes Ca and thus
  lowers blood Ca concentration, the parathyroids
  sense the change and respond by increasing their
  secretion of parathyroid hormone (PTH). PTH
  stimulates osteoclasts in bone to release more
  Ca from storage in bone tissue (among other
  effects), which increases maternal blood Ca
  concentration to the set point level.

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Regulation of target cell sensitivity

• Sensitivity of target cell depends in part on
  number of receptors
• Up-regulation increased number of hormone
  receptors increases sensitivity
• Down-regulation decreased number of hormone
  receptors decreases sensitivity
• Sensitivity of target cell may also be regulated
  by factors that affect transcription

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Prostaglandins (PGs)

• Unique group of lipid hormones that do not meet
  the usual definition of a hormone
• important and widespread functions
• Called tissue hormones
• Secreted from a tissue
• Diffuses only a short distance to other cells
  within the same tissue
• PGs tend to integrate activities of neighboring
  cells

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Prostaglandins

• Many prostaglandins
• Prostaglandin A (PGA)
• intraarterial ? immediate fall in blood pressure
  and increase in regional blood flow to several
  areas
• Prostaglandin E (PGE)
• vascular effects clotting, inflammation (which
  can be blocked with drugs- aspirin)
• gastrointestinal effects regulates hydrochloric
  acid secretion
• Prostaglandin F (PGF)
• especially important in reproductive system
  causes uterine contractions
• also affects intestinal motility and is required
  for normal peristalsis
• Many tissues are known to secrete PGs
• PGs have diverse physiological effects

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Pituitary Gland

• Structure of the pituitary gland
• Size 1.2 to 1.5 cm (about ½ inch) across
• Weight 0.5 g (1/60 ounce)
• Located on the ventral surface of the brain
  within the skull
• connects to the hypothalamus
• Made up of two separate glands
• anterior pituitary gland
• posterior pituitary gland

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Pituitary Gland

• Anterior pituitary
• irregular clumps of secretory cells
• rich vascular network
• Three types of cells identified by how they
  stain
• Chromophobes do not stain
• Acidophils stain with acid stains
• Basophils stain with basic stains

chromophobes (arrowheads)
a (acidophil) and b (basophil)
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Pituitary Gland

• Adenohypophysis (anterior pituitary) (cont.)
• Five functional types of secretory cells exist
• Somatotrophssecrete GH
• Corticotrophssecrete ACTH
• Thyrotrophssecrete TSH
• Lactotrophssecrete prolactin (PRL)
• Gonadotrophssecrete LH and FSH

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Anterior Pituitary Gland

• Growth hormone (GH)
• Promotes growth of bone and muscle by ? amino
  acid transport into the cells
• Stimulates fat metabolism by mobilizing lipids
  from storage in adipose cells and speeding up
  catabolism of the lipids after they have entered
  another cell
• GH shifts cell away from glucose catabolism and
  toward lipid catabolism as an energy source ? ?
  blood glucose levels

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• GH in 23 year old female

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Anterior Pituitary Gland

• Prolactin (PRL)
• Produced by acidophils
• Also known as lactogenic hormone
• During pregnancy, PRL promotes development of the
  breasts, anticipating milk secretion
• After baby born, PRL stimulates the mothers
  mammary glands to produce milk

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Anterior Pituitary Gland

• Tropic hormones
• stimulates other endocrine glands
• Secreted by basophils
• Four principal tropic hormones
• Thyroid-stimulating hormone (TSH)
• promotes development of thyroid
• causes thyroid to secrete its hormones
• Adrenocorticotropic hormone (ACTH)
• promotes development of cortex of adrenal gland
• stimulates adrenal cortex to secrete some of its
  hormones

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Anterior Pituitary Gland

• Follicle-stimulating hormone (FSH)
• Female
• stimulates primary follicles to grow
• stimulates follicle cells to secrete estrogens
• Male
• stimulates development of seminiferous tubules of
  testes and maintains spermatogenesis
• Luteinizing hormone (LH)
• FSH and LH are called gonadotropins because they
  stimulate growth and maintenance of gonads
• Female
• stimulates formation corpus luteum of ovary
• corpus luteum secretes progesterone and estrogens
  when stimulated by LH
• supports FSH in stimulating maturation of
  follicles
• Male
• stimulates secretion of testosterone

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Anterior Pituitary Gland

• Control of secretion
• Hypothalamus secretes releasing hormones
• Releasing hormones influence secretion of
  hormones by acidophils and basophils
• Through negative feedback, hypothalamus adjusts
  secretions, which then adjusts secretions of
  target glands that, in turn, adjust activity of
  their target tissues
• In stress, hypothalamus translates nerve impulses
  into hormone secretions by endocrine glands,
  basically creating a mind-body link

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Anterior Pituitary
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Anterior Pituitary
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Adrenal Glands

• Adrenal cortex
• Mineralocorticoids
• regulate sodium in the body
• Aldosterone
• Only important mineralocorticoid in the human
• maintains sodium homeostasis in the blood by
  increasing sodium reabsorption in the kidneys
• also ? water retention and ? K and H ions
• controlled by the renin-angiotensin-aldosterone
  system (RAAS) and by blood potassium
  concentration

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Adrenal Glands

• Adrenal cortex (cont.)
• Glucocorticoids
• Main glucocorticoids cortisol, cortisone, and
  corticosterone, with cortisol the only one
  secreted in significant quantities
• Affect every cell in the body!
• protein-mobilizing, gluconeogenic, and
  hyperglycemic
• Cells shift from carbohydrate catabolism to lipid
  catabolism as an energy source
• Essential for maintaining normal blood pressure
  by aiding norepinephrine and epinephrine to have
  their full effect, causing vasoconstriction

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Adrenal Glands

• Glucocorticoids (cont.)
• High blood concentration causes atrophy of
  lymphatic tissues
• Act with epinephrine to bring about normal
  recovery from injury produced by inflammatory
  agents
• Secretion increases in response to stress
• Except during stress, secretion is mainly
  controlled by a negative feedback mechanism
  involving ACTH from the anterior pituitary
• the large pulse occurs just before waking
• Gonadocorticoids sex hormones (androgens) that
  are released from the adrenal cortex

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Secretion of cortisol
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Adrenal Glands

• Adrenal medulla
• Neurosecretory tissue composed of neurons
  specialized to secrete their products into the
  blood
• Adrenal medulla secretes two important
  hormonesepinephrine and norepinephrine
• Both hormones bind to the receptors to prolong
  and enhance the effects of sympathetic
  stimulation by the ANS

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Pancreas

• Composed of endocrine and exocrine tissues
• Pancreatic islets (islets of Langerhans)
  endocrine portion
• Acini exocrine portion secretes digestive
  enzymes into the small intestine

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Pancreatic Islets

• Structure of the pancreatic islets
• Each islet contains four primary types
• Alpha cells (A cells) secrete glucagon- ?? blood
  glucose
• Beta cells (B cells) secrete insulin- ? blood
  concentration of glucose, amino acids, and fatty
  acids
• 75 of all pancreatic islet cells

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Pancreatic Islets

• Delta cells (D cells) secrete somatostatin-
  regulates the other cells
• Pancreatic polypeptide cells (F cells)secrete
  pancreatic polypeptides- influences digestion

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Too much and too little

• Is VERY Bad!
• Many diseases of endocrine system
• Diabetes
• Type I insulin hyposecretion (too little)
• Type II insulin insensitivity (possibly due to
  down regulation because too much was released)

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Anabolic Steroids

• Stimulate building of large molecules, especially
  in muscle and bone
• Athletes may take synthetic derivatives of
  testosterone to increase their performance
• BUT prolonged use ? negative feedback
• Gonadotropin levels will drop in response ? ? of
  testes and possibly permanent sterility

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Gonads

• Testes
• Paired organs within the scrotum in the male
• Composed of seminiferous tubules
• Testosterone is produced and is responsible for
  male sexual characteristics
• Testosterone secretion is mainly regulated by
  gonadotropin levels in the blood

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Prostate Cancer

• one of the most commonly diagnosed cancer among
  men in the US, representing about 25 of all
  tumors
• Harvard review
• 19/23 studies showed a positive association
  between dairy and prostate cancer
• This is one of the most consistent dietary
  predictors for prostate cancer in the published
  literature.
• Men with the highest dairy intakes had
  approximately double the risk of total prostate
  cancer, and up to fourfold increase in risk of
  metastatic spreading or fatal cancer relative
  to low consumers.

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Gonads

• Ovaries
• Paired glands produce several types of sex
  hormones
• Estrogens
• steroid hormones secreted by follicles
• promote female sexual characteristics
• Progesterone
• secreted by corpus luteum
• maintains the lining of the uterus necessary for
  successful pregnancy
• Ovarian hormone secretion depends on the changing
  levels of FSH and LH from anterior pituitary

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Placenta

• Tissues that form on the lining of the uterus
  connecting the circulatory systems of the mother
  and the developing fetus
• Serves as a temporary endocrine gland that
  produces human chorionic gonadotropin (hCG),
  estrogens, and progesterone

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Thymus

• Gland located in the mediastinum just beneath the
  sternum
• Thymus is large in children, atrophies at
  puberty, by old age, the gland is a vestige of
  fat and fibrous tissue
• Primarily a lymphatic organ, but the hormone
  thymosin has been isolated from thymus tissue
• Thymosin stimulates development of T cells

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Gastric and Intestinal Mucosa

• GI tract cells produce both endocrine and
  exocrine secretions
• GI hormones gastrin, secretin, and
  cholecystokinin (CCK) all play regulatory roles
  in coordinating the secretory and motor
  activities involved in the digestive process
• Ghrelin
• hormone secreted by endocrine cells in gastric
  mucosa
• stimulates hypothalamus to boost appetite
• slows metabolism and fat burning
• may be a contributor to obesity

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Heart

• The heart has a secondary endocrine role
• Hormone-producing cells produce atrial
  natriuretic hormone (ANH)
• ANH opposes increases in blood volume or blood
  pressure
• an antagonist to ADH and aldosterone

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Other Endocrine Glands and Organs

• Endocrine glands produce many more hormones that
  are outlined in this book!
• Many tissues (perhaps all tissues) produce
  hormones, most of which are beyond the scope of
  this book
• Ex leptin and resistin secreted by adipose tissue

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Cycle of Life Endocrine System

• Endocrine regulation begins in the womb
• Many hormones are active from gestational period
• Evidence that a hormonal signal from fetus to
  mother signals the onset of labor
• Hormones related to reproduction begin at puberty
• Secretion of male reproductive hormonescontinuous
  production from puberty, slight decline in late
  adulthood
• Secretion of female reproductive hormones
  declines suddenly and completely in middle
  adulthood (menopause)

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The Big Picture The Endocrine System and the
Whole Body

• Nearly every process in the body is kept in
  balance by the intricate interaction of different
  nervous and endocrine regulatory chemicals
• The endocrine system operates with the nervous
  system to finely adjust the many processes they
  regulate
• Neuroendocrine system adjusts nutrient supply
• The nervous system and hormones regulate
  reproduction