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The Endocrine System

This chapter provides an overview of the endocrine system, its glands, and the hormones they produce. It discusses the differences between endocrine and exocrine glands, explaining how hormones are secreted into circulation and their slow response in target cells. The text covers various glands including the pituitary, thyroid, and adrenal glands, detailing the roles of hormones in regulating growth, metabolism, and homeostasis. Additionally, it explains hormone chemistry, mechanisms of action, and regulation of secretion, emphasizing the critical connection between the nervous and endocrine systems.

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The Endocrine System

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  1. The Endocrine System Chapter 13

  2. Endocrine • to interstitial fluid circulation • exocrine- secreted to ducts  lumen or outside the body • Endocrine glands: • Pituitary, thyroid, parathyroid, adrenal & pineal • Hormone secretion + other functions: • Hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, adipose tissue & placenta

  3. Figure 13.1

  4. Hormone Operation • General chemical signal in circulation • Slower than nerve responses • Target cells must have a specific receptor • Response determined by responding cell, i.e. different cells may respond differently to the same hormone • Cell may respond to more than one hormone, • i.e. has more than one type receptor

  5. Hormone Chemistry • Soluble in lipids = Hydrophobic • steroids, e.g. testosterone, estrogens, etc. • thyroid hormones, e.g. T3, T4 • Nitric oxide (NO) • Water soluble= Hydrophillic • Amino acid derivatives, e.g. epinephrine, norepinephrine • Peptides, e.g. antidiuretic Hormone (ADH), oxytocin • Proteins, e.g. insulin & growth hormone • General Action depends on chemistry

  6. Lipid Soluble Action • Hormone detaches from carrier in blood stream • Diffusion through interstitial fluid & cell membrane into cell • Binds to & activates receptor • Receptor-hormone complex alters gene expression • If new mRNA protein synthesis • New proteins alter cell activity

  7. Figure 13.2

  8. Water-Soluble Action • Diffuses from blood and binds to receptor in plasma membrane • Starts reaction inside cell forming second messenger • Cyclic AMP is a common one • Second messenger causes activation of several proteins (enzymes) • Activated proteins produce physiological responses • Second messenger is inactivated

  9. Figure 13.3

  10. Control of Secretions • Release occurs in short bursts • Regulated by: • Signals from nervous system, e.g. adrenal medulla release of epinephrine • Chemical changes in blood, e.g Blood Ca2+ affects parathyroid hormone • Other hormones, e.g. ACTH from pituitary stimulates cortisol release from adrenal cortex

  11. Hypothalamus & Pituitary • Major link between nervous & endocrine systems • Hypothalamic Cells synthesize at least 9 hormones • Pituitary synthesizes 7 • Regulate growth, development, metabolism & homeostasis

  12. Pituitary • Two lobes; anterior & posterior • Hypophyseal portal veins • Connect capillaries in hypothalamus to capillaries in anterior pituitary

  13. Hypothalamus  Pituitary • Axons of hypothalamic neurons (neurosecretory cells) end near capillaries of hypothalamus • Secrete Releasing hormones or Inhibiting hormones portal veins • Regulate release of anterior pituitary hormones

  14. Figure 13.4

  15. Human Growth Hormone (hGH) • Promotes synthesis of IGFs = somatomedins • in liver, muscle, cartilage & bone • Released in bursts (~2 hour intervals) • Hypothalamus Growth Hormone Releasing Hormone (GHRH) & Growth Hormone Inhibiting Hormone (GHIH ) • Regulated by blood glucose levels

  16. Thyroid Stimulating Hormone • Stimulates the formation & secretion of Thyroid hormones from thyroid gland • Hypothalamus Thyrotropin Releasing Hormone (TRH)- no TIH • Regulated by circulating thyroid hormone levels

  17. Follicle Stimulating Hormone (FSH) & Luteinizing Hormone (LH) • In females: • FSH starts follicle development • LH stimulates formation of corpus luteum & secretion of progesterone • In males: • FSH stimulates sperm production in testes • LH stimulates release of testosterone • Gonadotrophin releasing Hormone (GnRH) from hypothalamus is suppressed by high levels of estrogen in females and testosterone in males

  18. Prolactin (PRL) • Initiates & maintains milk production by mammary glands • Ejection of milk depends on oxytocin • Prolactin inhibiting hormone (PIH) suppresses prolactin release • High levels of Estrogens  PRH prolactin release • Unknown function in males • Hypersecretion  impotence

  19. Adrenocortcotrophic Hormone (ACTH) • Controls production & secretion of glucocorticoids from adrenal cortex • Corticotrophin Releasing Hormone (CRH) from hypothalamus stimulates secretion of ACTH • Stress related stimuli can also stimulate ACTH release • Glucocorticoids inhibit CRH & ACTH release

  20. Melanocyte Stimulating Hormone (MSH) • Small circulating amounts • Excess causes skin darkening

  21. Posterior Pituitary • axon terminals from hypothalamus- • Release hormones • Oxytocin- enhance smooth muscle contraction during birth & milk ejection • may play role in emotional bonding • Antidiuretic Hormone (ADH) = vasopressin • Causes kidney to retain more water • Vasoconstriction increase in blood pressure • high blood osmotic pressure increase secretion

  22. Figure 13.5

  23. Figure 13.6

  24. Thyroid Gland • Below larynx- two lobes • follicular cells surround follicles •  thyroxin (T4) & triiodothyronine (T3) • Stored in follicle • Parafollicular cells (C-cells) • calcitonin

  25. Figure 13.7a

  26. Figure 13.7b

  27. Thyroid Hormones • T4 & T3 increase basal metabolic rate, protein synthesis & growth • Blood level is controlled via feedback through hypothalamus • Increased body ATP demand can also raise blood levels • Calcitonin inhibits osteoclasts  decrease in blood Ca2+ • Feedback control on blood levels

  28. Figure 13.8

  29. Parathyroid Glands • Small round masses in posterior of thyroid gland • Chief cells release parathyroid hormone (PTH) • Regulator of Ca2+, Mg2+ & HPO42- • Increases number & activity of osteoblasts • Slows loss of Ca2+ & Mg2+ in urine • Promotes production of calcitriol  increases rate of Ca2+, Mg2+ & HPO42- absorption in GI tract

  30. Figure 13.9

  31. Figure 13.10

  32. Pancreas • Fattened organ in curve of duodenum • Mostly an exocrine organ for digestion • Endocrine cells in pancreatic islets • Several cell types: • alpha cells glucagon • beta cells insulin

  33. Figure 13.11a

  34. Figure 13.11b

  35. Figure 13.11c

  36. Actions of Insulin & Glucagon • Low blood glucose stimulates glucagon release • Glucagon stimulates liver glucose release  increased blood glucose • High glucose levels stimulate insulin release • Insulin increase glucose transport into skeletal muscle and adipose cells  decreased blood glucose • Insulin promotes Amino Acid uptake, protein synthesis & lipid storage • ANS also modulates hormone release

  37. Figure 13.12

  38. Adrenal Gland • Near kidneys • Two separate gland structures- • Adrenal cortex and adrenal medulla • 3 zones in Cortex-3 steroid hormones • Outer zone  mineralocorticoids • Middle zone  glucocorticoids • Inner Zone  androgens

  39. Figure 13.13a

  40. Figure 13.13b

  41. Mineralocorticoids • Aldosterone is the major form • Stimulates Na+ reabsorption from urine to blood • Stimulates excretion of K + into urine • Part of renin-angiotensin-aldosterone pathway • Decreased BP  release of renin from kidney • Renin causes angiotensinogen angiotensin I • In lungs Angiotensin converting enzyme (ACE) causes Angiotensin I  angiotensin II • Angiotensin II causes Aldosterone release

  42. Figure 13.14

  43. Glucocorticoid action • Increase rate of protein breakdown • Stimulate liver formation of glucose • Breakdown of triglycerides in adipose • Anti-inflammatory effects- • Inhibit white blood cells • Depresses immune system • Regulated by negative feedback through hypothalamus

  44. Androgens • Small amount secreted from adrenal cortex • Contribute to libido in females • Converted to estrogens by other body tissues • Stimulate axillary hair growth in both boys & girls • Contribute to adolescent growth spurt

  45. Adrenal Medulla • Consists of sympathetic post ganglionic cells • stimulated by preganglionic sympathetic neurons • Releases Epinephrine and norepinephrine • gives systemic sympathetic effects • occurs during strong physiological stress

  46. Gonads • Produce gametes • Release sex steroids (testosterone or estrogen & progesterone) • Also hormone inhibin • Inhibits FSH release • hormones from pituitary (FSH & LH) • Ovaries also produce a hormone relaxin during pregnancy • details later in course

  47. Pineal • Small gland attached to roof of third ventricle of brain • Produces melatonin • Sets bodies biological clock • More released in darkness

  48. Other hormones • Prostaglandins (PG) & leukotrienes (LT) • Derived from fatty acids • Act locally in most tissues & released from most body cells • LTs stimulate white blood cells & mediate inflammation • PGs affect many visceral functions & also modulate inflammation, promote fever & intensify pain

  49. Stress Responses • Part of homeostatic responses • When successful leads to extra physiological capacity and long term adaptation • Initial “fight-or-flight” response • Nerve mediated response-sympathetic

  50. Stress- Resistance Reaction Slower & longer Than initial response • Hypothalamus  Increased CRH, GHRH, TRH • CRHACTHCortisol  mobilize metabolites (amino acids, glucose & fat) • GHRHhGH mobilize fats & glucose for energy and promote tissue growth & repair • TRHTSHthyroid hormones increased Metabolic capacity

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