Ch 18 - Endocrine System

• Master Control Systems: The nervous and endocrine systems work together to coordinate the functions of all body systems
• Functions of Hormones: Regulation of different processes
• Two kinds of glands in the body

  • Exocrine glands
  • Endocrine glands
• Endocrine Signaling
• Activation of cells by hormones: In order to activate a cell, a hormone needs to bind to its receptor non-covalently (reversibly; non-permanently)
• 3 factors determine the level of activation of a target cell: Blood levels of hormones, Number
• Types of cell signaling

  • Autocrine
  • Paracrine
  • Paracrine cell
  • Nearby target cell
• Activation of cells by hormones

  In order to activate a cell, a hormone needs to bind to its receptor non-covalently (reversibly; non-permanently)
• Factors determining the level of activation of a target cell

  • Blood levels of hormones
  • Number of receptors on target cell
  • Affinity of receptor for its hormone
• Hormone receptors are continually being synthesized and destroyed

  1. Down-regulation

  2. Cell responds to excess hormone by decreasing number of receptors

  3. Surface receptors undergo endocytosis and are degraded

  4. Decreases sensitivity of target cell to hormone

  5. Up-regulation

  6. Cell responds to deficiency of hormone by increasing number of receptors

  7. Target tissue more sensitive to the hormone
• General Mechanism of Action

  1. Hormone binds to receptor

  2. Binding of hormone to receptor triggers a response in the target cell
• Chemical classes of hormones

  • Lipid-soluble hormones
  • Water-soluble hormones
• Lipid-soluble hormones

  • Steroids (e.g. estrogens, testosterone, glucocorticoids)
  • Thyroid hormones (T3, T4)
  • Nitric oxide – NO (gas)
• Water-soluble hormones

  • Epinephrine, serotonin, histamine, oxytocin, ADH, insulin, prostaglandins
• Transport of lipid-soluble hormones in the blood

  1. Lipid-soluble hormones are generally bound to a transport protein (e.g. albumin)

  2. Transport protein is amphipathic

  3. Prevents clumping and increases molecule size, so less likely to be filtered out by kidneys

  4. Hormone dissociates from transport protein as it approaches tissues, and diffuses into cells
• Action of Lipid-soluble hormones

  1. Hormone binds to receptor in cytoplasm or nucleus

  2. Activated receptor-hormone complex alters gene expression

  3. Newly formed mRNA directs synthesis of specific proteins on ribosomes

  4. New proteins alter cell's activity
• Water-soluble hormones

  • Amine hormones, Peptide/protein hormones, Eicosanoid hormones
• Transport of water-soluble hormones in the blood

  Water-soluble hormones are hydrophilic – freely travel in blood
• Action of Water-soluble hormones

  1. Binding of hormone (first messenger) to its receptor activates G protein, which activates adenylate cyclase

  2. Activated adenylate cyclase converts ATP to cAMP, which serves as a second messenger to activate protein kinases
• Humoral stimuli trigger the release of hormones from glands
• Neural stimuli trigger the release of hormones from glands
• Adenylate cyclase


  Converts ATP to cAMP, which serves as a second messenger to activate protein kinases
• Triggers for the release of hormones from glands

  • Humoral stimuli
  • Neural stimuli
  • Hormonal stimuli
• Most hormonal regulatory systems work via negative feedback, but a few operate via positive feedback
• Positive feedback and negative feedback are different in hormonal regulation
• Release of most hormones occurs in short bursts, with little/no secretion between bursts
• When stimulated, an endocrine gland releases its hormone in more frequent bursts

  Increasing its concentration in the blood
• In the absence of stimulation

  The blood level of the hormone decreases
• Hypothalamus


  • Part of the brain
  • Regulates the autonomic nervous system, body temperature, thirst, hunger, sexual behavior, fear, anger
  • Links the nervous and endocrine systems
  • Controls the pituitary gland through the production of hormones
• Hypothalamus receives signals from different parts of the brain


  Then sends signals to the pituitary gland, which produces several hormones that regulate many body functions
• The pituitary gland attaches to the hypothalamus and has two anatomically and functionally separate lobes
• The infundibulum attaches the pituitary gland to the hypothalamus
• The anterior lobe of the pituitary gland secretes hormones in response to hypothalamic hormones
• Hormone delivery to anterior pituitary gland

  Hypothalamic hormones are delivered via the hypophyseal portal system; a portal system connects two capillary beds together
• Regulatory hormones released into the hypophyseal portal system

  • Corticotropin-releasing hormone (CRH)
  • Thyrotropin-releasing hormone (TRH)
  • Growth hormone-releasing hormone (GH-RH)
  • Growth hormone-inhibiting hormone (GH-IH)
  • Prolactin-releasing factor (PRF)
  • Prolactin-inhibiting hormone (PIH)
  • Gonadotropin-releasing hormone (GnRH)
• Endocrine glands

  • Testes
  • Mammary glands
  • Bone, muscle, other tissues
  • Thyroid gland
  • Adrenal glands
  • Adrenal cortex
  • Anterior lobe of pituitary gland
  • Liver
• Hormones


  • ACTH
  • TSH
  • GH
  • PRL
  • FSH
  • LH
  • MSH
  • OXT
  • ADH
  • Somatomedins
  • Glucocorticoids (steroid hormones)
  • Thyroid hormones
  • Inhibin
  • Testosterone
  • Estrogen
  • Progesterone
• Human Growth Hormone and Insulin-like Growth Factors


  1. hGH promotes skeleton and muscle growth

  2. IGFs promote cell growth and cell division, increase uptake of amino acids and protein synthesis, inhibit breakdown of proteins, stimulate breakdown of glycogen, make glucose available for ATP synthesis, stimulate lipolysis
• Abnormal secretion of hGH causes gigantism or acromegaly
• Hyposecretion of hGH results in pituitary dwarfism