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Endocrine System

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Title: Endocrine System


1
Endocrine System
  • Chapter 13

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  • Exocrine glands secrete their products through
    ducts into body cavities or onto body surfaces.
  • Endocrine glands secrete hormones into the
    interstitial fluid where it enters the blood.

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  • Hormones that pass into the blood and act on
    distant cells are called circulating hormones
  • Paracrine secretions are restricted to the
    interstitial fluid and affect only nearby cells.
    Autocrine secretions affect only the secreting
    cell itself.

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  • Paracrine and autocrine secretions are called
    local hormones and are inactivated quickly.
  • Circulating hormones have longer lasting effects,
    until they are inactivated by the liver and
    excreted by the kidneys.

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  • Nerves Hormones
  • Nerve impulses Hormones
  • Causes muscles to Affects virtually all
  • contract and glands body tissues
  • to secrete
  • Act in milliseconds May take hours
  • Effects are brief Effects long lasting

8
  • The nervous and endocrine systems are coordinated
    into the neuroendocrine system
  • Certain parts of the nervous system stimulate or
    inhibit the release of hormones
  • Hormones may promote or inhibit the generation of
    nerve impulses
  • Some neurotransmitters act as hormones

9
Hormones
  • Chemicals, produced by endocrine cells, that
    travel in blood and influence certain cells
    elsewhere in the body.
  • Hormones affect only specific target cells that
    have receptors which recognize that particular
    hormone.

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Chemistry of hormones
  • Classified as
  • Steroids
  • (biogenic) amines
  • Peptides or proteins
  • Eicosanoids
  • Nitric oxide, a gas, acts as a local hormone in
    several tissues

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Steroids
  • Lipids formed from cholesterol
  • Examples cortisol, aldosterone, estrogen,
    testosterone

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Biogenic amines
  • The simplest hormones, are formed by modifying
    amino acids
  • T3, epinephrine, norepinephrine, and histamine
    are examples

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Peptides and Proteins
  • Chains of 3-200 amino acids
  • Some may also have attached carbohydrate groups
    and be glycoproteins, such as thyroid stimulating
    hormone

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Peptide Hormones
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Eicosanoids
  • Most recently discovered
  • Derived from a 20-carbon fatty acid called
    arachidonic acid
  • Two major types
  • Prostaglandins affect a wide variety of
    tissues
  • Leukotrines affect white blood cells

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Administration of Hormones
  • Steroid hormones can be given orally because they
    are not broken down and are fat-soluble and
    easily take up.
  • Peptide and protein hormones are digested and
    must be given by another route.

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Mechanisms of Hormone Action
  • Lipid-soluble steroid hormones and thyroid
    hormones affect cell function by entering the
    cell and activating receptors inside the cell.
  • The activated receptor then turns genes on or
    off. If it activates a gene, a new protein is
    produced which alters the activity of the cell.

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Water-soluble Hormone Action
  • Alter cell function by using a second-messenger.
  • The hormone itself is the first messenger. When
    it binds with a receptor on the cell surface the
    binding activates the receptors active site.
    This site activates a second-messenger, such as
    cyclic AMP.
  • The second-messenger then activates other
    enzymes, called protein kinases, in the cell.

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  • Second messengers act to amplify the effect of
    hormones because they initiate a cascade of
    reactions within the cell.

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Prostaglandins
  • Paracrine substances
  • Produced as needed
  • Rapidly inactivated
  • Relax smooth muscle in blood vessels
  • Contract smooth muscle in uterus
  • Also have a role in inflammation and influnecence
    the action of other hormones.

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Control of Hormonal Secretions
  • Continually excreted in urine and broken down by
    enzymes, esp. in liver.
  • Changing blood level, therefore, depends on
    increased or decreased secretion of that hormone.

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Hormone secretion is controlled in three ways
  • The anterior pituitary gland is called the
    master gland of the body because it releases
    tropic hormones , which then stimulate other
    endocrine glands to secrete their hormones. The
    anterior pituitary gland is regulated by signals
    from the hypothalamus, which constantly receives
    information from sensory receptors and by
    monitoring the cerebrospinal fluid.

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  • 2. Some glands respond directly to changes in the
    internal environment.(e.g. insulin is produced
    when blood glucose is high.)
  • 3. The nervous system stimulates some glands
    directly.
  • (adrenal medulla)Most often, negative-feedback
    systems regulate hormonal secretions.

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Suggested note card format
  • Hormone
  • Produced by
  • Target
  • Effect
  • Regulation

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Hypothalamus and the Pituitary Gland (Hypophysis)
  • The hypothalamus is the integrating link between
    the nervous system and the endocrine system.
  • The hypothalamus and pituitary regulate virtually
    all aspects of growth, development, metabolism
    and homeostasis.

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Pituitary gland
  • Located in the sella turcica of the sphenoid bone
  • Connected to hypothalamus by a stalk called the
    infundibulum.
  • Divided into anterior (75) and posterior (25)
    pituitary glands.

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Posterior Pituitary (Neurohypophysis)
  • Contains axons whose cell bodies are in the
    hypothalamus. The neurons are called
    neurosecretory cells because their secretions
    function as hormones instead of
    neurotransmitters.
  • Hormones made in the hypothalamus are stored and
    released by the posterior pituitary gland.

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Hormones of the Post. Pit.
  • Oxytocin stimulates contraction of the uterus
    during labor and milk let down or ejection from
    alveoli into ducts.
  • Secretion is controlled by positive-feedback.
  • No known effect in males, but may stimulate
    movement of fluids in reproductive tract during
    sexual activity.

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  • Antidiuretic Hormone controlled by the osmotic
    pressure of the blood and blood volume.
  • Acts on distal convoluted tubule and collecting
    ducts to increase their permeability to water, so
    that more water is reabsorbed into the body and
    less urine is formed.
  • At high concentrations causes constriction of
    arterioles, which raises blood pressure
    (vasopressin)

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  • A lack of ADH or receptor causes diabetes
    insipidus urine output can 20 L/day.
  • ADH can be administered by injection or nasal
    spray
  • Alcohol, caffeine, nicotine, pain, stress and
    trauma also affect secretion of ADH.

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Anterior Pituitary Gland (Adenohypophysis)
  • Hormones of Ant. Pit. are controlled by
    inhibiting and releasing hormones produced by the
    hypothalamus.
  • No direct connection between the two, so hormones
    are carried the short distance through the
    hypophyseal portal veins to the capillary bed
    around the anterior pit.

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  • Five types of cells secrete seven major hormones
  • Somatotropes human Growth Hormone (hGH) or
    somatotropin
  • Mammotropes Prolactin (PRL)
  • Thyrotropes Thyroid Stimulating Hormone (TSH)
  • Corticotropes Adrenocorticotropic Hormone
    (ACTH) and Melanocyte-stimulating hormone (MSH)
  • Gonadotropes follicle stimulating hormone (FSH)
    and Luteinizing hormone (LH)

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Human Growth Hormone
  • Protein hormone that stimulates cells to increase
    in size and reproduce.
  • Enhances movement of amino acids into cells and
    stimulates protein synthesis.
  • Stimulates break down of lipids
  • Slows breakdown of glucose, so less leaves the
    blood, and blood levels ?
  • Stimulates bone growth directly, but needs an
    intermediary (somatomedin) to stimulate cartilage.

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  • Release of hGH has a diurnal rhythm very little
    secreted during the day one or more prolonged
    bursts of secretion after falling asleep.
  • Hyposecretion causes pituitary dwarfism
  • Hypersecretion causes gigantism before epiphyseal
    plates close, and acromegaly after.

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Prolactin
  • Protein hormone responsible for breast
    development during pregnancy, and together with
    other hormones, initiates and maintains milk
    secretion.
  • In males, PRL decreases the secretion of LH,
    which is necessary for the production of male sex
    hormones. Excess PRL can cause impotence and
    infertility.
  • Hypersecretion can cause absence of menstrual
    cycles in women.

58
Thyroid Stimulating Hormone
  • Or thyrotropin, is a glycoprotein that
    stimulates the thyroid gland to produce its own
    hormones T3 and T4 which control basal
    metabolic rate.
  • External factors can also influence the secretion
    of TSH exposure to extreme cold and emotional
    stress.

59
Adrenocorticotropic Hormone
  • A peptide that acts on the adrenal cortex,
    causing it to produce larger amounts of its
    hormones, especially cortisol (hydrocortisone).
  • Produced during physiological stress injury,
    hypoglycemia, or exercise.

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Follicle Stimulating Hormone and Luteinizing
Hormone
  • Both are glycoproteins called gonadotropins,
    meaning they act on the gonads or primary
    reproductive organs

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Follicle Stimulating Hormone (FSH)
  • In the female stimulates the growth and
    development of a follicle and ovum, and estrogen
    secretion by the follicle.
  • In a male it stimulates the seminiferous tubules
    to grow and produce sperm.

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Luteinizing Hormone (LH)
  • Acts with FSH to stimulate the growth and
    development of a follicle and ovum. A sharp rise
    in the level of LH causes rupture of the follicle
    and the release of the ovum. Sometimes called the
    ovulating hormone.
  • Also stimulates the formation of the corpus
    luteum .
  • In males, LH was called interstitial cell
    stimulating hormone (ICSH) because it causes the
    interstitial cells of the testes to secrete
    testosterone.

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Melanocyte Stimulating Hormone(MSH)
  • In amphibians, causes production of melanin
  • In humans exact role is not known, but does
    cause darkening of the skin. May play a role in
    learning.
  • From the intermediate lobe of the pituitary which
    atrophies during fetal development and some cells
    migrate to the anterior pituitary.
  • Secretion stimulated by corticotropin releasing
    hormone and inhibited by dopamine.

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Thyroid gland
  • Located below the larynx
  • Made of two lobes connected by the isthmus
  • Gland is made of follicles that secrete and store
    the thyroid hormones tetraiodothyronine (T4) and
    triiodothyronine (T3) and extrafollicular cells
    or C cells which secrete calcitonin (CT)
  • Follicles are spheres made of a single layer of
    cuboidal cells and filled with colloid.

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  • T3 and T4 can be released into the capillaries
    that surround the follicle or they can be stored
    within the colloid.
  • T3 and T4 are synthesized from iodine and
    tyrosine ( an amino acid) and are transported in
    the blood bound to special plasma proteins.
  • This is the only endocrine gland that stores
    large quantities of its product 100 day supply

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  • Virtually all the iodine in the body is located
    in the thyroid gland, and hyperthyroidism can be
    treated with injections of radioactive iodine.
  • Thyroid hormones (T3 and T4 ) control the basal
    metabolic rate, growth and development,
    thermoregulation, and the reactivity of the
    nervous system.
  • Secretion is controlled by release of TSH.

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  • Hypersectretion
  • Graves Disease exophthalmos and goiter
  • Hyposecretion
  • Hashimotos Disease (autoimmune)
  • Cretinism in childhood stunted growth, mental
    retardation
  • Myxedema as adult edema of facial tissues,
    sensitive to cold, dulled mental function

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Calcitonin
  • Also produced by the (extrafollicular cells)
    thyroid gland.
  • Lowers the blood level of calcium and phosphorus
    by inhibiting osteoclasts (which break bone down)
    and accelerates the uptake of these minerals into
    bones.
  • Also causes increases loss of Calcium and
    Phosphorus in urine.
  • Secretion controlled by blood Ca levels.
  • No clinical problems as long as parathyroid
    glands are functional.

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Parathyroid Hormone
  • Produced by the parathyroid glands four small
    glands on the posterior surface of the thyroid.

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  • Parathyroid hormone (PTH) or parathormone
    regulates the levels of calcium and phosphate in
    the blood.
  • PTH increases osteoclast activity and decreases
    the activity of osteoblasts, so that calcium and
    phosphorus are released into the blood.
  • It also stimulates the kidneys to reabsorb more
    Ca and to excrete more phosphates, so just
    blood calcium is increased.

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  • PTH also indirectly stimulates absorption of
    calcium from the intestines by influencing the
    metabolism of vitamin D.
  • Stimulates the kidneys to convert
    hydroxycholecalciferol from the liver to
    dihydroxycholecalciferol, which is the active
    form.

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  • Hyposecretion leads to muscle cramps and
    seizures
  • Hypersecretion osteitis fibrosa cystica
    brittle bones

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Adrenal Glands
  • Located superior to kidneys, and are enclosed in
    the mass of adipose tissue that surrounds the
    kidneys.
  • Made up of an outer cortex and inner medulla.

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Adrenal Cortex
  • Divided into three zones which produce different
    steroid hormones
  • Zona glomerulosa (outer layer) produces
    mineralocorticoids, mostly aldosterone.
  • Aldosterone causes kidneys to reabsorb (keep)
    sodium and water, and lose potassium and H.
  • Control is by K levels in blood (only mildly by
    Na) and by renin-angiotensin- aldosterone
    pathway.

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  • Zona Fasciculata (middle layer) produces
    glucocorticoids, mostly cortisol
    (hydrocortisone)
  • Regulate metabolism, esp. of glucose, and
    resistance to stress.
  • Increase protein breakdown and decrease
    synthesis, raising blood amino acid levels.
  • Lipolysis promotes breakdown of adipose tissue,
    increasing use of fatty acids for energy and
    decreasing use of glucose
  • Gluconeogenesis production of new glucose from
    other substances, increasing blood glucose

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  • Resistance to stress increased glucose is
    available to tissues, and makes blood vessels
    more sensitive to agents that cause
    vasoconstriction
  • Anti-inflammatory effects inhibits cells that
    participate in inflammatory response slower
    wound healing
  • Depression of the immune response prevents
    rejection of transplanted organs, but decreases
    resistance to infection.

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  • Control through blood levels of cortisol and
    ACTH which is controlled by hypothalamus through
    corticotropin releasing hormone.
  • Hyposecretion leads to Addisons disease
  • Hypersecretion leads to Cushings syndrome

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  • Zona Reticularis (inner layer) produces
    androgens (male sex hormones).
  • These are overwhelmed by testosterone in males
    and so are insignificant.
  • In females, they contribute to the female sex
    drive, and may be converted into estrogen in
    other tissues, which is especially important
    after menopause.

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Adrenal Medulla
  • Made up of hormone producing cells called
    chromaffin cells, which surround large blood
    vessels.
  • Sympathetic postganglionic cells that are
    specialized to secrete hormones which are
    sympathomimetic.
  • Secretions are 80 epinephrine and 20
    norepinephrine

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  • Control through nervous impulses that originate
    in the hypothalamus
  • Not essential for life
  • Tumors are called pheochromocytomas, and produce
    extended Fight or Flight response. Treated by
    surgical removal of tumor.

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Pancreas
  • Inferior to stomach, in C of duodenum, and is
    retroperitoneal.
  • Histologically, see pancreatic islets of islets
    of Langerhans (endocrine cells) and clusters of
    enzyme producing cells called acini. (exocrine
    cells)

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Pancreatic Hormones
  • Alpha cells secrete glucagon (?blood glucose)
  • Beta cells secrete insulin (?blood glucose)
  • Delta cells - somatostatin

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Alpha Cells - Glucagon
  • Protein hormone, increases blood glucose by
    promoting glyogenolysis and gluconeogenesis in
    the liver.
  • Also stimulates the breakdown of lipids into
    fatty acids and glycerol
  • Secretion is stimulated by low blood levels of
    glucose.

106
Beta Cells - Insulin
  • Protein hormone, decreases blood glucose levels
    by
  • Accelerating facilitated diffusion into cells
  • Speeds conversion of glucose to glycogen
  • Slows glycogenolysis and gluconeogenesis
  • Increases the uptake of amino acids and protein
    and fatty acid synthesis by cells.
  • Secretion stimulated by high blood glucose levels.

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Diabetes mellitus
  • Type I insulin-dependent (IDDM)or juvenile
    onset diabetes due to underproduction of
    insulin, and so must inject insulin to control
    blood glucose levels. Long term uncontrolled
    diabetes affects blood vessels.
  • Type II Noninsulin-dependent (NIDDM) or adult
    onset diabetes due to lack of insulin receptors
    on cell membranes. Diet, exercise and oral
    medication.

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Delta Cells - Somatostatin
  • Growth hormone inhibiting hormone - similar to
    the hormone secreted by the hypothalamus.
  • Helps to regulate carbohydrate metabolism by
    inhibiting secretion of glucagon and insulin, and
    slows absorption of nutrients from the digestive
    tract.

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Ovaries and Testes
  • Will be covered shortly in the chapter on
    reproductive systems.

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Other Endocrine Glands
  • Pineal gland the third eye secretes
    melatonin.
  • Production controlled by light outside the body.
  • Part of the regulation of circadian rhythms and
    promotes sleepiness.

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  • There is no basis for these claims!!
  • Has been used for jet lag, but only resets your
    biological clock by about an hour.
  • It inhibits the secretion of gonadatropins from
    the anterior pituitary gland, and helps regulate
    the female reproductive cycle.
  • May also control the onset of puberty.

116
Thymus
  • Part of the lymphatic system, produces several
    hormones called thymosins, which promote the
    production and maturation of T lymphocytes.
  • (Review What type of immunity are Tc Cells
    responsible for? What do helper T cells do? )

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Kidneys
  • Release erythropoietin which stimulates red blood
    cell production.
  • Also calcitriol, the active form of vitamin D,
    and renin.

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Digestive Glands
  • Stomach and small intestines produce many
    hormones which affect digestion and the
    production of digestive enzymes (gastrin,
    cholecystokinin, secretin, etc.)

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The Heart
  • The right atrium produces Atrial Natriuretic
    Peptide ( ANP) when central venous pressure is
    increased. ANP is the sodium and water losing
    hormone, causing sodium (and therefore water) to
    be excreted into the urine and lost from the
    body, which decreases blood volume and therefore
    blood pressure.

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stress
  • Survival depends on homeostasis.
  • When stress upsets this balance, impulses are
    sent to the hypothalamus, which initiates the
    general stress (general adaptation) syndrome.
  • The hypothalamus resists change, usually by
    increased activity of the sympathetic nervous
    system and increased secretions from the adrenal
    medulla and adrenal cortex, and secretion of
    ADH, glucagon, and possibly renin.

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