Section 3, chapter 13

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  • 1. Section 3, Chapter 13 The Peripheral Endocrine Glands
  • 2. Thyroid Gland Location: The thyroid gland is located just inferior to the larynx. Structure: • It consists of two lateral lobes connected by an isthmus • Contains several follicles.
  • 3. Thyroid Gland Follicles Follicles consists of simple cuboidal epithelium & a colloid center Follicular Cells: produce T3 & T4 Coloid: contains Thyroglobulin, which is a storage form of thyroid hormones. Extrafollicular Cells: produce Calcitonin Follicular Cells take up thyroglobulin by endocytosis, then release the thyroid hormones into the bloodstream.
  • 4. Thyroid Hormones Target Cells: T3 & T4 affect many cells throughout the body. Actions of T3 & T4: Raise Metabolic Rate • Increase rate of carbohydrate catabolism • Enhance protein synthesis • Promotes the breakdown and use of lipids T3 & T4 are major factors in determining the basal metabolic rate (BMR) BMR = calories required to sustain life
  • 5. Thyroid Hormones Follicular cells require iodine salts (iodide) to produce T3 and T4. T3 & T4 are hydrophobic molecules (insoluble in water) • Nearly 75% of thyroid hormones are attached to thyroid binding globulins. • Only the small amounts of the unbound hormones act on target cells.
  • 6. Transport of Thyroid Hormones T4 accounts for 95% of circulating Thyroid Hormone, But… T3 is physiologically more active. • T3 is 5 times as potent as T4 • T3 also has a 50-fold higher “free” concentration in the plasma (see figure below).
  • 7. Thyroid Disorders Hypothyroidism – insufficient T3 & T4 • During infancy – results in intellectual disability, stunted growth, abnormal bone formation (cretinism) • During adulthood – low metabolic weight, sluggishness, poor appetite, and sensitivity to cold Infantile hypothyroidism Hyperthyroidism – excess T3 & T4 • Results in high metabolic rate, hyperactivity, weight loss, sensitivity to heat, and exophthamia (protruding eyes) • Grave’s Disease • Autoimmune Disorder: Antibodies target the thyroid gland and mimic TSH. Thyroid antibodies Grave’s disease may overstimulate thyroid gland, resulting in cause exophthalmia
  • 8. Calcitonin Extrafollicular cells (C-cells) secrete Calcitonin Calcitonin lowers blood calcium concentrations. Actions of Calcitonin • Stimulates Osteoblast activity – increases bone deposition • Inhibits osteoclast activity – reduces bone resorption • Promotes the excreting of calcium from the kidneys Major Source of Control: elevated blood calcium ion concentrat
  • 9. Parathyroid Glands • Location: 4 small parathyroid glands are located on the posterior aspect of the thyroid gland •Hormone: PTH (parathyroid hormone) One parathyroid gland surrounded by thyroid
  • 10. Parathyroid Hormone (PTH) Parathyroid Hormone elevates blood calcium levels. Actions of PTH: • Stimulates Osteoclast activity – increases bone resorption • Inhibits osteoblast activity – reduces bone deposition • Promotes calcium reabsorption from the kidneys. • PTH also promotes the activation of Vitamin D, which enhances calcium absorption from the small intestine. Major Source of Control: Inadequate blood calcium ion concent
  • 11. Figure 13.27 Parathyroid Hormone (PTH) stimulates bone to release Calcium (Ca2+) and the kidneys to conserve calcium. It indirectly stimulates the intestine to absorb calcium. The resulting increase in blood calcium concentration inhibits secretion of PTH by negative feedback.
  • 12. Calcitonin and PTH have opposing effects on the levels of calcium ions in circulation. Both work together to maintain calcium
  • 13. Adrenal Glands Location: The adrenal glands are located on the superior aspect of the kidneys. Structure: • Adrenal glands are pyramid shaped organs that consist of two parts • Adrenal Medulla = secretions controlled by sympathetic nerve fibers Adrenal Cortex = Under hormonal control
  • 14. Hormones of the Adrenal Medulla Nerve fibers control secretions: Hormones of the adrenal medulla are under control by the sympathetic division (fight or flight) of the ANS. Hormones: Norepinephrine (noradrenalin) & Epinephrine (adrenalin) • Both are classified as catecholamines. Actions: Effects are similar to sympathetic nerve fibers, but longer lasting. • Increases heart rate and force of contraction • Increases blood pressure • Increases metabolic rate • Increases blood glucose levels (primarily epinephrine) • Decreases digestion
  • 15. Beta Blockers • Epinephrine & Norepinephrine exert their effects by binding to Beta (ß) adrenergic receptors in heart and walls of the blood vessels. • Beta blockers bind to ß-receptors, thus obstructing the binding of catecholamines. • Hence beta blockers reduce sympathetic influences of the heart and blood vessels. • Therefore, beta blockers decrease heart rate, contractility, and reduce blood pressure.
  • 16. Hormones of the Adrenal Cortex 3 Layers of the adrenal cortex secrete over 30 types of steroid hormones. Hormones Aldosterone – produced in zona glomerulosa Cortisol – produced in zona fasciculata Androgens – produced in zona reticularis
  • 17. Hormones of the Adrenal Cortex 1. Aldosterone (mineralocorticoid) • regulates Na+ and K+ concentrations • regulates blood pressure Actions • Aldosterone causes the kidneys to reabsorb Na+ and to excrete K+ • Aldosterone indirectly raises blood pressure: Increased Na+ reabsorption increases water reabsorption by osmosis. Controls of Aldosterone Secretion • Low blood pressure stimulates aldosterone secretion (renin-angiotensin-aldosterone pathway) • Elevated blood K+ concentration promotes aldosterone secretion • Low Na+ has only a slight effect on aldosterone secretion.
  • 18. Renin-Angiontensin-Aldosterone System ACE Inhibitors block the actions of ACE, and thus lower blood pressure.
  • 19. Hormones of the Adrenal Cortex 2. Cortisol (glucocorticoid) • Its primary effect is to build up and conserve blood glucose supplies • Its actions keep blood glucose levels constant between meals. Actions • Promotes gluconeogenesis in the liver gluconeogenesis = glucose synthesis from non-carbohydrates • Promotes the release and used of fatty acids from adipose for energy. Using fatty acids for energy allows glucose to be conserved. • Inhibits protein synthesis: amino acids used in gluconeogenesis
  • 20. Hormones of the Adrenal Cortex 3. Androgens • Supplement the sex hormones secreted from the gonads. • Androgens may be converted into testosterone and estrogens.
  • 21. The Pancreas Structure & Location: The pancreas is located posterior to the stomach, attached to the duodenum. The pancreas has both digestive and endocrine functions. • Pancreatic Islets (Islets of Langerhans) = endocrine cells • Digestion cells (we’ll discuss these with the digestive system)
  • 22. Cells of the Pancreatic Islets 3 distinct type of cells secrete 3 hormones: • Alpha Cells – secrete glucagon • Beta Cells – secrete insulin • Delta Cells – secrete somatostatin Pancreatic hormones regulate the storage, use, and release of fuels (glucose).
  • 23. Pancreatic Hormones 1. Glucagon Overall Effect: During fasting, when blood glucose levels drop, glucagon elevates blood glucose levels Actions of Glucagon: • Stimulates glycogenolysis in the liver (breakdown of glycogen into glucose) • Glucagon also promotes gluconeogenesis • Glucagon also stimulates the breakdown of fats into glycerol and fatty acids. • Glycerol is used in gluconeogenesis • Fatty Acids are metabolized for energy
  • 24. Liver Amino acids glycerol Gluconeogenesis glycogen Glycogenolysis glucose glucose Glucagon secretions elevates blood glucose concentrations. • Gluconeogenesis converts noncarbohydrates, such as amino acids and glycerol, into glucose. • Glycogenolysis breaks down large glycogen molecules into glucose.
  • 25. Pancreatic Hormones 2. Insulin Overall Effect: Following a meal, when blood carbohydrate levels are high, insulin removes excess glucose from the blood. Actions of Insulin: • Stimulates glycogenesis in the liver (formation of glycogen from glucose). • It inhibits gluconeogenesis. • Insulin promotes glucose uptake in adipose tissue, skeletal muscles, and cardiac muscle. 3. Somatostatin Overall Effect: Helps regulate glucose metabolism by inhibiting the secretion of glucagon and insulin.
  • 26. Hormonal Control of Glucose Insulin and glucagon function together to stabilize blood glucose concentration. Negative feedback responding to blood glucose concentration controls the levels of both hormones.
  • 27. Diabetes Mellitus Type I Diabetes Mellitus (juvenile) • Autoimmune disease – immune system destroys beta cells, resulting in the loss of insulin production. • Without insulin, blood glucose cannot be taken up and used for energy. • Glucose accumulates in the blood and urine = hyperglycemia. Type II Diabetes Mellitus (adult onset) • Receptors on target cells wear down and become insensitive to insulin. • Target cells resist glucose uptake, even in the presence of insulin. • Insulin levels must be higher than normal just to maintain normal glucose concentrations.
  • 28. Other Endocrine Glands Pineal Gland • Located posterior to thalamus. • The pineal gland secretes melatonin, which regulates circadian rhythms (sleep/wake cycle) • Melatonin secretions are greatest in dark. Light inhibits secretions. Thymus Gland • Secretes thymosins • Promotes development of certain lymphocytes • Important in role of immunity
  • 29. Other Endocrine Glands Reproductive Organs • Ovaries produce estrogens and progesterone • Testes produce testosterone • Placenta produces estrogens, progesterone, and gonadotropin Other organs: digestive glands, heart, and kidney End of Section 3, Chapter 13