Anatomy

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Chapter 17

• Anatomy Physiology
• Fifth Edition
• Seeley/Stephens/Tate
• (c) The McGraw-Hill Companies, Inc.

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FUNCTIONAL ORGANIZATION OF THE ENDOCRINE SYSTEM

• There are two major means of communication in the
  body the neuronal system and the endocrine
  system.
• The neuronal system has clear pathways which
  are connected with neurons. Thus it is reasonably
  clear for each neuron where it starts and where
  it ends. (reflex --- cerebral cortex ----
  targets)
• At the end knob, neurotransmitters are released.
• Its complexity makes it possible to stimulate
  more than one tissue and organ simultaneously.
• The effect is relatively short lived.

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• The endocrine system the active substances,
  hormones, are made in specific organs.
• The glands may be stimulated with neurons.
• Hormones have target tissue and organs.
• Long distance transport of its active substance
  is done by the circulatory system.
• Local effect is also possible.
• Again, one hormone may effect more than one
  target organs.
• The effects are usually slow and relatively long
  lasting.

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• An overview
• The word endocrine is in contrast with exocrine.
  Thus we are concerned with internal secretion.
• Endocrine glands secrete specific hormones.
• The strength of a hormone is determined by its
  concentration, while the strength for neurons is
  often the frequency of the action potential.
• Also recall the neurohormones may be released by
  neurons into the circulatory system.
• Functional classification of chemical signals is
  shown in Fig. 17.1

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• Review Table 17.1
• Autocrine chemical signals released by cells
  and have a local effect of the same cell type.
  Example are prostaglandins and platelets.
• Paracrine chemical signals released by cells
  and effect local other cell types. Somatostatin
  from pancreas.
• Pheromones hormones secreted into environment.

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• Structure of hormone there are type major
  types.
• Amino acid derivates epinephrine,
  norepinephrine, the thyroid hormones, pineal
  hormone (melatonin)
• Peptide/protein hormone antidiuretic hormone,
  glucagon, oxytocin, growth hormone, prolactin,
  insulin.
• Lipid derivatives
• Steroid hormones estrogens, testosterone
• Prostaglandins derived from arachidonic acid.

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• Control of secretion rate
• In essence the control is in a form of negative
  feedback.
• Three major patterns
• By other substance such as sugar controls the
  regulation of insulin release.
• Increased blood sugar
• Stimulates insulin release from the pancreas
• Insulin stimulates glucose uptake by tissues
• Decrease in blood sugar

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• Neuronal control of endocrine glands
• Stress or exercise stimulates sympathetic
  division of ANS.
• Release of epinephrine or smaller amounts of
  norepinephrine from the adrenal medulla.
  (Fig.17.5)
• Regulation of hormone secretion by another
  hormone
• Thyroid-releasing hormone (TRH) is released from
  neurons in the hypothalamus.
• TRH stimulates the release of thyroid stimulating
  hormone (TSH) from the anterior pituitary gland.
• TSH stimulates the secretion of thyroid hormones
  from the thyroid gland.
• Thyroid hormone stimulates tissues
• That includes the hypothalamus and the anterior
  pituitary (negative-feedback) to inhibit both TRH
  and TSH.(fig.17.6)
• Regulation is usually complex and more than one
  factor could affect organs. For example, the
  level of insulin may be regulated by blood
  glucose and ANS., Ex of positive feedback. (fig
  17.7)

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• The level of hormone in the blood steam may
• Maybe relatively short
• May respond in a matter of hours
• May respond in a matter of days

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• Transport
• Hormones are transported through the circulatory
  system dissolved in plasma.
• At the target sites they diffuse out from the
  capillaries.
• Some hormones such as, steroids and thyroid
  hormones, are bound to specific plasma proteins
  reversibly.
• hormone plasma protein hormone-protein
• While this equilibrium is held in plasma, there
  are free hormones and proteins bound hormones.
• Free hormones may diffuse out through the walls
  of capillary pores at the target sites, while
  proteins bound hormones remain within the
  capillaries. But eventually they will establish a
  new equilibrium (carry more hormone) and make
  more free hormones available (sustained release).

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• Metabolism and Excretion
• Once hormones are poured into the blood steam, it
  is equally important to excrete them from the
  circulation to avoid excessively long effects of
  hormones.
• The lengths of time it takes for elimination by
  half is called half-life time.
• The half-life time of water soluble hormones,
  such as proteins, glycoproteins, epinephrine and
  norepinephrine, is relatively short.
• The half-life time of lipid soluble hormones,
  such as steroid and thyroid hormones, is longer.

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• As shown in Table 17.3 hormones are eliminated
  from the circulatory system in four ways
• Excretion
• Kidneys-urine the liver-bile
• Metabolism
• Enzymatic digestion
• Active transport
• Recycling (neurotransmitters)
• Conjugation
• Sulfate and glucuronic acid groups bind to
  hormones and they are released into urine or bile.

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• Interaction of hormones with their target tissues
• At their designated sites hormones interact with
  the target cells and active their intrinsic
  function, usually stimulating specific enzymes.
• Hormone receptors protein or glycoprotein.
• Each target cell has a specific binding site for
  the specific hormone.
• However, a hormone may bind to the/ other target
  receptor which are similar.
• After prolonged exposure to a hormone, the tagged
  cells may reduce the number of hormone receptors
  called, down regulation.
• On the other hand, exposure to stimulation may
  increase the number of hormone receptors called,
  up-regulation. Example is LH receptors are up
  regulated by FSH.

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The End.