Endocrinology

• Endocrine system


  A network of ductless glands that secrete hormones directly into the blood
• Endocrinology


  The study of the endocrine system
• The endocrine system is considered to be the regulatory system of the body
• The endocrine system is regulated by means of control of hormone synthesis rather than by degradation
• Hormones


  Chemical signals produced by specialized cells, secreted into the blood stream and carried to a target tissue
• Hormones


  • They play an important role in the growth and development of an individual
  • They are regulated by the metabolic activity either positive or negative feedback mechanism
  • Their major function is to maintain the constancy of chemical composition of extracellular and intracellular fluids, and control metabolism, growth, fertility, and responses to stress
• There are hormones influenced by physiologic factors such as age - the elderly secrete less triiodothyronine, parathyroid hormone, aldosterone, and cortisol
• Positive feedback system


  A system in which an increased in the product results to elevation of the activity of the system and the production rate (example: gonadal, thyroidal and adrenocortical hormones)
• Negative feedback system


  A system in which an increased in the product results to decreased activity of the system and the production rate (example: leutenizing hormone)
• Types of Hormone Actions


  • Endocrine
  • Paracrine
  • Autocrine
  • Juxtacrine
  • Intracrine
  • Exocrine
  • Neurocrine
  • Neuroendocrine
• Endocrine


  Secreted in one location and released into blood circulation; binds to specific receptor to elicit physiological response
• Paracrine


  Secreted in endocrine cells and released into interstitial space; binds to specific receptor in adjacent cell and affects its function
• Autocrine


  Secreted in endocrine cells and sometimes released into interstitial space; binds to specific receptor on cell of origin resulting to self-regulation of its function
• Juxtacrine


  Secreted in endocrine cells and remains in relation to plasma membrane; acts on immediately adjacent cell by direct cell-to-cell contact
• Intracrine


  Secreted in endocrine cells and remained as well as function inside the synthesis of origin
• Exocrine


  Secreted in endocrine cells and released into lumen of gut; it affects their function
• Neurocrine


  Secreted in neurons and released into extracellular space; binds to receptor in nearby cell and affects its function
• Neuroendocrine


  Secreted in neurons and released from nerve endings; interacts with receptors of cells at distant site
• Endocrine functions


  Majority are regulated through the pituitary gland, which in turn is controlled by secretions from the hypothalamus
• Classification of hormones according to composition or structure


  • Peptides and proteins
  • Steroids
  • Amines
• Peptides and proteins


  • Synthesized and stored within the cell in the form of secretory granules and are cleaved as needed
  • Cannot cross the cell membrane due to their large molecular size and thus produce their effects on the outer surface of the cell
  • Water soluble and not bound to carrier protein
• Peptide and protein hormones


  • Glycoproteins: FSH, HCG, TSH, erythropoietin
  • Polypeptides: ACTH, ADH, GH, angiotensin, calcitonin, cholecystokinin, gastrin/glucagon, insulin, melanocyte-stimulating hormone (MSH), oxytocin, PTH, prolactin, somatostatin
• Steroids


  • Lipid molecules that have cholesterol as a common precursor
  • Produced by adrenal glands, ovaries, testes and placenta
  • Water insoluble (hydrophobic) and circulate bound to a carrier protein
• Steroid hormones


  • Aldosterone, cortisol, estradiol, progesterone, testosterone, activated vitamin D
• Amines


  • Derived from an amino acid and are intermediary between steroid and protein hormones
• Amine hormones


  • Epinephrine, norepinephrine, triiodothyronine, thyroxine
• Hypothalamus


  • Portion of the brain located in the walls and floor of the third ventricle
  • Above the pituitary gland, and is connected to the posterior pituitary by the infundibulum (pituitary stalk)
  • Link between the nervous system and the endocrine system
• Hypothalamus


  • Supraoptic and paraventricular nuclei produce vasopressin and oxytocin
  • Anterior portion releases the following hormones: thyrotropin-releasing hormone (TRH); gonadotropin-releasing hormone(GnRH); somatostatin also known as growth hormone inhibiting hormone (GH-IH); growth hormone releasing hormone (GH-RH); and prolactin-inhibiting factor (PIF)