The endocrine system

• Circulating hormones


  Hormones secreted into interstitial fluid and then absorbed into the bloodstream to be carried systemically to any cell that displays the appropriate type of receptor
• Local hormones

  Hormones that are secreted and act on nearby cells
• Chemical classes of hormones

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


  Steroid hormones, thyroid hormones, gases
• Lipid-soluble hormone action


  Bind to receptors in the cytoplasm or nucleus of a cell
• Water-soluble hormones


  Amine hormones, peptide/protein hormones, eicosanoid hormones
• Water-soluble hormone action


  Bind to receptors on the plasma membrane of a cell
• Autocrine hormones


  Local hormones that are secreted and bind to the same cell
• Paracrine hormones


  Local hormones that are secreted into interstitial fluid and act on nearby cells
• Endocrine glands/tissues

  • Hypothalamus and pituitary gland
  • Anterior pituitary
  • Posterior pituitary
  • Pineal gland
  • Thyroid gland
  • Parathyroid glands
  • Adrenal glands
  • Pancreas
  • Thymus
  • Ovaries
  • Testes
  • Other tissues with endocrine cells
• Hormones produced and basic actions

  • Antidiuretic hormone (ADH)
  • Oxytocin
  • Thyroid hormones (T3, T4)
  • Parathyroid hormone (PTH)
  • Calcitonin
  • Catecholamines (noradrenaline)
  • Cortisol
  • Insulin
  • Glucagon
  • Thymosin
  • Testosterone
  • Atrial natriuretic peptide (ANP)
  • Gastrin
  • Erythropoietin (EPO)
  • Secretin
  • Angiotensinogen
  • Vitamin D
• Negative feedback control

  Blood calcium level is controlled by parathyroid hormone (PTH) and calcitonin
• Positive feedback control

  Oxytocin stimulates uterine contractions which in turn stimulate more oxytocin release
• The endocrine system and nervous system coordinate all body systems
• The endocrine system uses hormones produced by endocrine structures to produce their effects
• General functions of hormones

  • Regulate extracellular fluid homeostasis
  • Regulate metabolism
  • Regulate muscle contraction
  • Regulate glandular secretions
  • Regulate some immune functions
  • Regulate growth and development
  • Regulate reproduction
  • Establish circadian rhythms
• Hormone responsiveness of target cells depends on hormone concentration, number of receptors, and influence of other hormones
• Permissive effect


  Requires recent or simultaneous exposure to a second hormone, e.g. increasing receptors or enzyme synthesis for the other hormone
• Synergistic effect

  Two hormones have greater impact together than individually
• Antagonistic effect


  Two hormones have opposing actions
• Endocrine glands are stimulated to secrete hormones in short bursts
• Hormone secretion is regulated by signals from the nervous system, chemical changes in the blood, and other hormones