Endocrine System.


Properties of the Endocrine System.

  • Regulates longer term metabolic processes.
  • Shares some features with nervous system.
  • Releases hormones from endocrine cells.
  • Hormones are chemicals.
  • Hormones alter metabolism of many cells.
  • Release regulated by negative feedback.
  • Hormones act on target cells.
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Overview of the Endocrine System.

  • Hypothalamus- Production of ADH, oxytocin and regulatory hormones.
  • Pituitary Gland- Anterior pituitary: ACTH, TSH, GH, PRL, FSH, LH and MSH. Posterior pituitary: Release of oxytocin and ADH.
  • Pineal Gland- Melatonin.
  • Thyroid Gland- Thyroxin (T₄),Triiodothyronin (T₃) and Calcitonin (CT)
  • Parathyroid- Parathyroid hormone (PTH).
  • Thymus- Thymosins.
  • Heart- Atrial Natriuretic Peptide (ANP).
  • Adrenal Glands - Subdivided into Adrenal Medulla: Epinephrine (E), Norepinephrine (NE) amd the Adrenal Cortex: Cortisol, Corticosterone, Aldosterone and Androgens.
  • Kidney- Renin, Erythropoietin (EPO) and Calcitriol.
  • Adipose tissue- Leptin, Resistin.
  • Digestive Tract- Numerous hormones.
  • Pancreatic Islets- Insulin and Glucagon.
  • Gonads- Testes (male): Androgens (especially testosterone) and Inhibin. Overies (female): Estrogens, Progestins and Inhibin.
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The structure of hormones.

  • Three chemical families:
    - Amino acid derivatives e.g. epinephrine, thyroid homones.
    - Peptides and proteins - chains of amino acids e.g. insulin and ADH.
    -Lipid derivatives- Steroids e.g. testosterone. Eicosanoids e.g. prostaglandins.
  • Target cells- Peripheral cells that can respond to a particular hormone.
  • Hormone receptor- A protein in the cell membrane , or within the cytoplasm or nucleus, to which a hormone specifically binds to trigger its actions on a target cell.
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Classes of Hormone Receptor.

  • Extracellular Receptors
    -Located in the cell membrane.
    -Targeted by: Amino acid derivatives, peptidess and eicosanoids.
    -Operates through a second messenger such as cyclic-AMP.
  • Intracellular Receptors
    -Located in the cytoplasm or nucleus.
    -Targeted by steroid and thyroid hormones.
    -Operates through changes in gene expression.
    -Thyroid hormones also act on mitochondria.
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Hormone Secretion and Distribution.

  • Hormones directly enter capillaries.
  • Distributed widely in the circulation.
    -Most hormones travel free in solution.
    -Steroid, thyroid hormones bind to blood protiens for transport.
  • Hormones are inactivated by:
    -Binding to cell receptors.
    -Removal by liver, kidney cells.
    -Breakdown by extracellular enzymes.
  • Hormones coordinate cells on a sustained basis. They circulate in the blood and bind to specific receptors on or in target cells. They alter membrane permeability, activate or inactivate key enzymes or change genetic activity.
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Hypothalamus and Endocrine Control.

  • Three mechanisms of action:
    -Hypothalamus secretes hormones as an endocrine organ.
    - Hypothalamus secretes regulatory hormones to control pituitary gland endocrine cells.
    -Autonomic centers excert direct neural control of adrenal meduliae.
  • Three mechanisms of hypothalamic control over Endocrine organs:
    - Secretion of regulatory hormones to control activity of anterior pituitary gland.
    -Production of ADH and oxytocin.
    -Control of sympathetic output to adrenal medulia.
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Pituitary Gland.

Pituitary Gland:

  • Also called the hypophysis.
  • Releases nine important hormones:
    -All are peptide hormones.
    -All bind to membrane (extracellular) receptors.
    -Most use cyclic-AMP as second messenger.
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Anterior Pituitary Hormones.

Anterior Pituitary Hormones:

  • Thyroid-Stimulating Hormone (TSH).
    -Triggers thyroid hormone release.
  • Adrenoacorticotropic Hormone (ACTH).
    -Stimulates glucocorticoid release from adrenal gland.
  • Follicle-Stimulating Hormone (FSH).
    -Stimulates estrogen secretion, egg production (females) and sperm production (males).
  • Luteinizing Hormone (LH).
    -Triggers ovulation, progestin production (females) and androgen production (males).
  • Prolactin (PRL).
    -Stimulates mammary gland development and milk secretion.
  • Growth hormone (hGH).
    - Stimulates cell growth via somatomedins released from liver.
  • Melanocyte Stimulating Hormone (MSH).
    -Chemical that determins skin pigmentation.
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Posterior Pituitary Gland.

  • Releases hormones from hypothalamic axons.
  • Antidiuretic Hormone (ADH).
    -Reduces water loss in the urine.
    -Increases thirst.
  • Oxytocin.
    -Stimulates uterine contraction, milk delivery.
    -Stimulates prostate gland smooth muscle.
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Posterior Pituitary Gland and Key Note.

  • Releases hormones from hypothalamic axons.
  • Antidiuretic Hormone (ADH).
    -Reduces water loss in the urine.
    -Increases thirst.
  • Oxytocin.
    -Stimulates uterine contraction, milk delivery.
    -Stimulates prostate gland smooth muscle.

Key Note:

Hypothalamic regulatory factors control the anterior pituitary (source of of seven hormones). Most of these control other glans (thyroid, adrenal, gonads). It also produces growth hormone. The posterior pituitary releases two hormones produced in the hypothalamus. ADH (restricts water loss) and oxytocin (stimulates contractions in the mammary glands and uterus and the prostate gland).

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Thyroid Follicles, Thyroid Hormones and C Cells of

  • Follicles produce and store colloid.
  • Production requires adequate iodine in the diet.
  • Occurs in two forms: Thyroxine and Triiodothyronine.
  • Increases metabolism and heat production (calorigenic effect).
  • Required for normal development
  • C Cells scattered outside of follicle epithelium.
  • Also called parafollicular  cells.
  • Secrete calcitonin.
    -Lowers blood calcium levels.
    -Inhibits osteoclasts of bone.
    -Increases urinary calcium loss.
  • Triggered by high blood calcium level.
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Parathyroid Glands and Key Note.

  • Four glands embedded on thyroid posterior.
  • Chief cells produce parathyroid hormone.
  • Low blood calcium triggers secretion.
  • PTH speeds bone breakdown by osteoclasts, increases dietary absorption, slows loss in kidney.
  • PYH raisess blood calcium.

Key Note:

The thyroid gland produces (1) hormones that adjust tissue metabolic rates and (2) a hormone that usually plays a minor role in calcium ion homeostasis by opposing the action of parathyroid hormone.

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Adrenal Gland Anatomy and Key Note.

  • Lie along superior border of each kidney.
  • Surrounded by fibrous capsule.
  • Made of two parts:
    -Adrenal Cortex (outer).
    -Adrenal Medulla.

Key Note:

The adrenal glans produce hormones that adjust metabolic activities at specific sites, affecting either the pattern of nutrient utilization, mineral ion balance or the rate energy consumption by active tissues.

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Adrenal Cortex and Adrenal Medulla.


  • Makes steroid hormones (Coticosteroids).
    -Glucocorticoids (e.g. cortisol).
      -Stimulated by ACTH.
      -Affect glucose metabolism.
    -Mineralocorticoids (e.g. aldosterone)
      -Stimulated by angiotensin II
      -Restricts loss of water. Sodium in urine, sweat, digestive tract, saliva.
  • Androgens (male hormone).


  • Produces two related hormones:
    -Epinephrine (adrenaline).
    -Norepinephrine (noradrenaline).
  • Innervated by preganglionic nerve fibres.
  • Axons stimulate hormone secretion.
  • Increases hear rate and force, releases glucose, fatty acids into blood, opens airways.
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The Pineal Gland and overview of the Pancreas.

Pineal gland:

  • Synthesizes melatonin.
    -Inhibits reproductive function.
    -Protects neural tissue from free radicals.
    -Establishes daily wake-sleep cycle.

Pancreas overview:

  • Lies behind stomach and beneath liver.
  • Has both exocrine and endocrine cells.
  • Endocrine cells organised into islets of Langerhans.
  • Islet cells secrete insulin and glucagon.
    -Insulin produced by beta cells.
    -Glucagon produced by alpha cells.
  • Exocrine cells secrete enzyme-rich digestive fluid.
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The Pancreas.

Actions of Insulin and Glucagon:

  • Insulin:
    -Lowers blood glucose concentration.
    -Increase glucose uptake, storage and use by target cells.
    -Targets liver, muscle and fat cells.
  • Glucagon:
    -Raises blood glucose concentration.
    -Increases glycogen breakdown and glucose synthesis.
    -Targets liver cells.
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Endocrine tissues of other organs.

  • Intestines.
    -Secretes hormones to control digestion.
  • Kidneys.
    -Secretes three hormones. 
      -Calcitriol- stiumulates calcium and phosphate absorption in intestine.
      -Erythropoietin (EPO)- stimulates red blood cell production by bone marrow.
  • Heart.
    -Specialised muscle cells secrete atrial natriuretic peptide (ANP) to lower blood volume/pressure.
  • Thymus.
    -Secretes thymosins that control immune system defenses.
  • Adipose Tissue (Fat cells).
    -Secrtes leptin to control appetite.
    -Secretes resistin to reduce insulin response.
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Endocrine tissues of other organs cont.

  • Testies (male gonad).
    -Interstitial cells secrete androgens (testosterone).
    -Sustentacular cells secrete inhibin.
  • Ovary (female gonad).
    -Follicle cells secrete estrogens and inhibin.
    -Corpus luteum cells secrete estrogens and progesterone.
  • Placenta.
    -Secretes several hormones during pregancy.
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Patterns of Hormone Regulation/ Hormones and behav

  • Sex hormones.
    -Testosterone fosters aggressiveness.
    -Estrogen fosters sexual receptivity.
  • Thryoid hormones.
    -Excess leads to nervousness, restlessness.
    -Deficiency leads to sluggishness.
  • Antidiuretic hormone.
    -Leads to feeling of thirst and water intake.
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  • Homeostasis- Normal Na+ concentration in ECP.
  • Homeostasis disturbed- Decreased Na+ levels in ECP.
  • Osmorecpetors in hypothalamus inhibited.
  • ADH secretion decreases- As soon as the osmotic concentration of the ECP drops by 2% or more ADH secretion decreases, so thirst is suppressed and water losses at the kidneys increase.
  • Water loss reduces ECP volume, concentrates ions.
  • Homeostasis restored- Increased Na+ levels in ECP.
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Fluid balance and Hormones and Aging.

Fluid balance.

If you drink too much fluid, osmoreceptors detect too much dilution of Na+ in body fluids, Anti Diuretic (against water loss) hormone secretion decreases and water loss at the kidneys increases. If you are dehydrated, ADH secretion increases and water loss at the kidneys decreases.

Hormones and aging.

  • Many hormones are unaffected by age.
  • Reduced or absent reproductive hormones.
  • hGH, insulin release reduced.
    -Leads to loss of bone density, muscle mass.
  • Tissue response to ADH , glucocorticoids declines.
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