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The Endocrine System

Learn about the functions and mechanisms of the endocrine system, including hormone classifications, intercellular communication, and regulatory processes. Explore the diverse roles of hormones in maintaining homeostatic balance and influencing metabolic activities. Discover the impact of endocrine reflexes on various bodily functions.

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The Endocrine System

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  1. The Endocrine System Chapter 18

  2. Function of the Endocrine System • Maintain homeostatic balance of the body • Endocrine system – the body’s second great controlling system which influences metabolic activities of cells by means of hormones • The endocrine system is a collection of “ductless” glands and tissues • The products of these glands are hormones

  3. Endocrine glands

  4. Hormones • Functions • Regulation: • Water balance and body fluid chemistry • Metabolic rate and energy balance • Cardiac and smooth muscle activity • Immune system activity • Control growth and development • Reproductive organ function and cycles • Circadian rythms

  5. Mechanisms of Intercellular Communication Table 18–1

  6. Hormones • Can be divided into 3 groups: • amino acid derivatives • peptide hormones • lipid derivatives

  7. Amino Acid Derivatives • Small molecules structurally related to amino acids • Synthesized from the amino acids tyrosine and tryptophan

  8. Tyrosine Derivatives • Thyroid hormones • Compounds: • epinephrine (E) • norepinephrine (NE) • dopamine, also called catecholamines

  9. Tryptophan Derivative • Melatonin: • produced by pineal gland

  10. Peptide Hormones • Chains of amino acids • Synthesized as prohormones: • inactive molecules converted to active hormones before or after secretion

  11. 2 Groups of Peptide Hormones • Group 1: • glycoproteins: • more than 200 amino acids long, with carbohydrate side chains: • thyroid-stimulating hormone (TSH) • luteinizing hormone (LH) • follicle-stimulating hormone (FSH)

  12. 2 Groups of Peptide Hormones • Group 2: • all hormones secreted by: • hypothalamus • heart • thymus • digestive tract • pancreas • posterior lobe of pituitary gland • anterior lobe of pituitary gland

  13. 2 Classes of Lipid Derivatives • Eicosanoids: • derived from arachidonic acid • Steroid hormones: • derived from cholesterol

  14. Eicosanoids • Are small molecules with five-carbon ring at one end • Are important paracrine factors • Coordinate cellular activities • Affect enzymatic processes in extracellular fluids

  15. Leukotrienes • Are eicosanoids released by activated white blood cells, or leukocytes • Important in coordinating tissue responses to injury or disease

  16. Prostaglandins • A second group of eicosanoids produced in most tissues of body • Are involved in coordinating local cellular activities • Sometimes converted to thromboxanes and prostacyclins

  17. Steroid Hormones • Are lipids structurally similar to cholesterol • Released by: • reproductive organs (androgens by testes, estrogens, and progestins by ovaries) • adrenal glands (corticosteroids) • kidneys (calcitriol)

  18. Steroid Hormones • Remain in circulation longer than peptide hormones • Are absorbed gradually by liver • Are converted to soluble form • Are excreted in bile or urine

  19. Chemical Classifications of Hormones: Lipid & Water soluble • Lipid soluble – • This characteristic allows hormones to pass directly through the plasma membrane of the target cell • Lipid soluble hormones include: • Steroid hormones • Thyroid hormones • Nitric oxide (a gas) • Require a carrier protein to travel in the plasma

  20. Fig. 18.03

  21. Water soluble Hormones • These are insoluble in lipid but travel freely in the blood but cannot enter the cell • They have their activity by binding to receptors on the plasma membrane and include: • Amines • Catecholamines – epinephrine & norepinephrine • Melatonin • Histamine • Serotonin

  22. More water soluble hormones • Peptides & Proteins • Hypothalamic hormones - hypothalamus • ADH & oxytocin – posterior pituitary • hGH, TSH, ACTH, FSH, LH, PRL, MSH – anterior pituitary • Insulin, glucagon, somatostatin, pancreatic polypeptide – pancreas • PTH – parathyroid • Calcitonin – thyroid • Gastrin, secretin, CCK, GIP – GI tract • EPO – Kidneys • Leptin – adipose tissue

  23. Second messenger activity of water soluble hormones

  24. Cyclic AMP

  25. PIP2 – IP3

  26. Steroid Hormones Figure 18–4a

  27. Thyroid Hormones Figure 18–4b

  28. Endocrine Reflexes • Functional counterparts of neural reflexes • In most cases, controlled by negative feedback mechanisms

  29. Endocrine Reflex Triggers • Humoral stimuli: • changes in composition of extracellular fluid • Hormonal stimuli: • arrival or removal of specific hormone • Neural stimuli: • arrival of neurotransmitters at neuroglandular junctions

  30. Simple Endocrine Reflex • Involves only 1 hormone • Controls hormone secretion by: • heart • pancreas • parathyroid gland • digestive tract

  31. Complex Endocrine Reflex • Involves: • 1 or more intermediary steps • 2 or more hormones

  32. Hormone activity • Down-regulation – • Excessively high concentrations of hormones reduces the number of receptors on the target tissues. This makes the tissue less sensitive to that hormone • Up-regulation – • Low concentrations cause cells to produce more receptors resulting in increased sensitivity of the tissue

  33. The Hormones and their glands

  34. Hypothalamus Figure 18–5

  35. the Pituitary

  36. Anterior Lobe • Also called adenohypophysis: • pars distalis • pars intermedia • pars tuberalis

  37. Anterior Lobe Figure 18–6

  38. HypothalamicRegulatory Hormones • Rate of secretion is controlled by negative feedback Figure 18–8a

  39. Hypothalamic Regulatory Hormones Figure 18–8b

  40. Thyroid-Stimulating Hormone (TSH) • Also called thyrotropin • Triggers release of thyroid hormones

  41. Adrenocorticotropic Hormone (ACTH) • Also called corticotropin • Stimulates release of steroid hormones by adrenal cortex • Targets cells that produce glucocorticoids

  42. Gonadotropins • Regulate activities of gonads (testes, ovaries) • Follicle-stimulating hormone • Luteinizing hormone

  43. Follicle-Stimulating Hormone (FSH) • Also called follitropin • Stimulates follicle development and estrogen secretion in females • Stimulates sustentacular cells in males: • promotes physical maturation of sperm • Production inhibited by inhibin: • peptide hormone released by testes and ovaries

  44. Luteinizing Hormone (LH) • Also called lutropin • Causes ovulation and progestin production in females • Causes androgen production in males

  45. FSH and LH Production • Stimulated by gonadotropin-releasing hormone (GnRH) from hypothalamus: • GnRH production inhibited by estrogens, progestins, and androgens

  46. Prolactin (PRL) • Also called mammotropin • Stimulates development of mammary glands and milk production • Production inhibited by prolactin-inhibiting hormone (PIH)

  47. Prolactin (PRL) • Stimulates PIH release • Inhibits secretion of prolactin-releasing factors (PRF)

  48. Prolactin (PRL) Figure 18–8b

  49. Growth Hormone (GH) • Also called somatotropin • Stimulates cell growth and replication • Production regulated by: • growth hormone–releasing hormone (GH–RH) • growth hormone–inhibiting hormone (GH–IH)

  50. Melanocyte-Stimulating Hormone (MSH) • Also called melanotropin • Stimulates melanocytes to produce melanin • Inhibited by dopamine

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