The Endocrine System. Part B. Oxytocin is a strong stimulant of uterine contraction

PowerPoint® Lecture Slides prepared by Vince Austin, University of Kentucky The Endocrine System Part B 16 Oxytocin ƒ Oxytocin is a strong stimula...
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PowerPoint® Lecture Slides prepared by Vince Austin, University of Kentucky

The Endocrine System Part B

16

Oxytocin

ƒ Oxytocin is a strong stimulant of uterine contraction ƒ Regulated by a positive feedback mechanism to oxytocin in the blood ƒ This leads to increased intensity of uterine contractions, ending in birth

Human Anatomy & Physiology, Sixth Edition Elaine N. Marieb Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Oxytocin

ƒ Oxytocin triggers milk ejection (“letdown” reflex) in women producing milk Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Antidiuretic Hormone (ADH)

ƒ ADH helps to avoid dehydration or water overload

ƒ Synthetic and natural oxytocic drugs are used to induce or hasten labor ƒ Plays a role in sexual arousal and satisfaction in males and nonlactating females

ƒ Prevents urine formation

ƒ Osmoreceptors monitor the solute concentration of the blood ƒ With high solutes, ADH is synthesized and released, thus preserving water ƒ With low solutes, ADH is not released, thus causing water loss from the body ƒ Alcohol inhibits ADH release and causes copious urine output

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Thyroid Gland

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Thyroid Gland

ƒ The largest endocrine gland, located in the anterior neck, consists of two lateral lobes connected by a median tissue mass called the isthmus ƒ Composed of follicles that produce the glycoprotein thyroglobulin ƒ Colloid (thyroglobulin + iodine) fills the lumen of the follicles and is the precursor of thyroid hormone ƒ Other endocrine cells, the parafollicular cells, produce the hormone calcitonin Figure 16.7 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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Thyroid Hormone

Effects of Thyroid Hormone ƒ TH is concerned with:

ƒ Thyroid hormone – the body’s major metabolic hormone

ƒ Glucose oxidation

ƒ Consists of two closely related iodine-containing compounds

ƒ Heat production

ƒ Increasing metabolic rate

ƒ TH plays a role in:

ƒ T4 – thyroxine; has two tyrosine molecules plus four bound iodine atoms

ƒ Maintaining blood pressure

ƒ T3 – triiodothyronine; has two tyrosines with three bound iodine atoms

ƒ Developing skeletal and nervous systems

ƒ Regulating tissue growth ƒ Maturation and reproductive capabilities

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Synthesis of Thyroid Hormone ƒ Thyroglobulin is synthesized and discharged into the lumen

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Synthesis of Thyroid Hormone

ƒ Iodides (I–) are actively taken into the cell, oxidized to iodine (I2), and released into the lumen ƒ Iodine attaches to tyrosine, mediated by peroxidase enzymes, forming T1 (monoiodotyrosine, or MIT), and T2 (diiodotyrosine, or DIT) ƒ Iodinated tyrosines link together to form T3 and T4 ƒ Colloid is then endocytosed and combined with a lysosome, where T3 and T4 are cleaved and diffuse into the bloodstream Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Transport and Regulation of TH

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Figure 16.8

Calcitonin

ƒ T4 and T3 bind to thyroxine-binding globulins (TBGs) produced by the liver ƒ Both bind to target receptors, but T3 is ten times more active than T4 ƒ Peripheral tissues convert T4 to T3 ƒ Mechanisms of activity are similar to steroids ƒ Regulation is by negative feedback

ƒ A peptide hormone produced by the parafollicular, or C, cells ƒ Lowers blood calcium levels in children ƒ Antagonist to parathyroid hormone (PTH)

ƒ Hypothalamic thyrotropin-releasing hormone (TRH) can overcome the negative feedback Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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Calcitonin

Parathyroid Glands

ƒ Calcitonin targets the skeleton, where it:

ƒ Tiny glands embedded in the posterior aspect of the thyroid

ƒ Inhibits osteoclast activity (and thus bone resorption) and release of calcium from the bone matrix ƒ Stimulates calcium uptake and incorporation into the bone matrix

ƒ Regulated by a humoral (calcium ion concentration in the blood) negative feedback mechanism Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

ƒ Cells are arranged in cords containing oxyphil and chief cells ƒ Chief (principal) cells secrete PTH ƒ PTH (parathormone) regulates calcium balance in the blood Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Parathyroid Glands

Effects of Parathyroid Hormone

ƒ PTH release increases Ca2+ in the blood as it: ƒ Stimulates osteoclasts to digest bone matrix ƒ Enhances the reabsorption of Ca2+ and the secretion of phosphate by the kidneys ƒ Increases absorption of Ca2+ by intestinal mucosal cells

ƒ Rising Ca2+ in the blood inhibits PTH release Figure 16.10a Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Adrenal (Suprarenal) Glands

Effects of Parathyroid Hormone

ƒ Adrenal glands – paired, pyramid-shaped organs atop the kidneys ƒ Structurally and functionally, they are two glands in one ƒ Adrenal medulla – nervous tissue that acts as part of the SNS ƒ Adrenal cortex – glandular tissue derived from embryonic mesoderm Figure 16.11 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

3

Adrenal Cortex

Adrenal Cortex

ƒ Synthesizes and releases steroid hormones called corticosteroids ƒ Different corticosteroids are produced in each of the three layers ƒ Zona glomerulosa – mineralocorticoids (chiefly aldosterone) ƒ Zona fasciculata – glucocorticoids (chiefly cortisol) ƒ Zona reticularis – gonadocorticoids (chiefly androgens) Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Mineralocorticoids

Mineralocorticoids

ƒ Regulate the electrolyte concentrations of extracellular fluids

ƒ Aldosterone secretion is stimulated by:

ƒ Aldosterone – most important mineralocorticoid ƒ Maintains Na+ balance by reducing excretion of sodium from the body

Figure 16.12a

ƒ Rising blood levels of K+ ƒ Low blood Na+ ƒ Decreasing blood volume or pressure

ƒ Stimulates reabsorption of Na+ by the kidneys

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

The Four Mechanisms of Aldosterone Secretion

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The Four Mechanisms of Aldosterone Secretion

ƒ Renin-angiotensin mechanism – kidneys release renin, which is converted into angiotensin II that in turn stimulates aldosterone release ƒ Plasma concentration of sodium and potassium – directly influences the zona glomerulosa cells ƒ ACTH – causes small increases of aldosterone during stress ƒ Atrial natriuretic peptide (ANP) – inhibits activity of the zona glomerulosa Figure 16.13 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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Glucocorticoids (Cortisol)

Excessive Levels of Glucocorticoids

ƒ Help the body resist stress by: ƒ Keeping blood sugar levels relatively constant ƒ Maintaining blood volume and preventing water shift into tissue

ƒ Cortisol provokes: ƒ Gluconeogenesis (formation of glucose from noncarbohydrates)

ƒ Excessive levels of glucocorticoids: ƒ Depress cartilage and bone formation ƒ Inhibit inflammation ƒ Depress the immune system ƒ Promote changes in cardiovascular, neural, and gastrointestinal function

ƒ Rises in blood glucose, fatty acids, and amino acids Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Gonadocorticoids (Sex Hormones)

Adrenal Medulla

ƒ Most gonadocorticoids secreted are androgens (male sex hormones), and the most important one is testosterone

ƒ Made up of chromaffin cells that secrete epinephrine and norepinephrine

ƒ Androgens contribute to: ƒ The onset of puberty ƒ The appearance of secondary sex characteristics ƒ Sex drive in females

ƒ Androgens can be converted into estrogens after menopause Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Adrenal Medulla

ƒ Secretion of these hormones causes: ƒ Blood glucose levels to rise ƒ Blood vessels to constrict ƒ The heart to beat faster ƒ Blood to be diverted to the brain, heart, and skeletal muscle Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Stress and the Adrenal Gland

ƒ Epinephrine is the more potent stimulator of the heart and metabolic activities ƒ Norepinephrine is more influential on peripheral vasoconstriction and blood pressure

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Figure 16.15

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Glucagon

Pancreas

ƒ A triangular gland, which has both exocrine and endocrine cells, located behind the stomach ƒ Acinar cells produce an enzyme-rich juice used for digestion (exocrine product)

ƒ A 29-amino-acid polypeptide hormone that is a potent hyperglycemic agent ƒ Its major target is the liver, where it promotes:

ƒ Pancreatic islets (islets of Langerhans) produce hormones (endocrine products)

ƒ Glycogenolysis – the breakdown of glycogen to glucose

ƒ The islets contain two major cell types:

ƒ Gluconeogenesis – synthesis of glucose from lactic acid and noncarbohydrates

ƒ Alpha (α) cells that produce glucagon

ƒ Release of glucose to the blood from liver cells

ƒ Beta (β) cells that produce insulin Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Insulin

Effects of Insulin Binding

ƒ A 51-amino-acid protein consisting of two amino acid chains linked by disulfide bonds

ƒ The insulin receptor is a tyrosine kinase enzyme

ƒ Synthesized as part of proinsulin and then excised by enzymes, releasing functional insulin

ƒ After glucose enters a cell, insulin binding triggers enzymatic activity that: ƒ Catalyzes the oxidation of glucose for ATP production

ƒ Insulin: ƒ Lowers blood glucose levels

ƒ Polymerizes glucose to form glycogen

ƒ Enhances transport of glucose into body cells ƒ Counters metabolic activity that would enhance blood glucose levels Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

ƒ Converts glucose to fat (particularly in adipose tissue) Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Regulation of Blood Glucose Levels

Diabetes Mellitus (DM)

ƒ Results from hyposecretion or hypoactivity of insulin ƒ The hyperglycemic effects of glucagon and the hypoglycemic effects of insulin

ƒ The three cardinal signs of DM are: ƒ Polyuria – huge urine output ƒ Polydipsia – excessive thirst ƒ Polyphagia – excessive hunger and food consumption

ƒ Hyperinsulinism – excessive insulin secretion, resulting in hypoglycemia Figure 16.17 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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Diabetes Mellitus (DM)

Gonads: Female

ƒ Paired ovaries in the abdominopelvic cavity produce estrogens and progesterone ƒ They are responsible for: ƒ Maturation of the reproductive organs ƒ Appearance of secondary sexual characteristics ƒ Breast development and cyclic changes in the uterine mucosa Figure 16.18 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Gonads: Male

ƒ Testes located in an extra-abdominal sac (scrotum) produce testosterone ƒ Testosterone: ƒ Initiates maturation of male reproductive organs ƒ Causes appearance of secondary sexual characteristics and sex drive ƒ Is necessary for sperm production ƒ Maintains sex organs in their functional state Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Pineal Gland

ƒ Small gland hanging from the roof of the third ventricle of the brain ƒ Secretory product is melatonin ƒ Melatonin is involved with: ƒ Day/night cycles ƒ Physiological processes that show rhythmic variations (body temperature, sleep, appetite) Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Thymus

Other Hormone-Producing Structures

ƒ Lobulated gland located deep to the sternum in the thorax

ƒ Heart – produces atrial natriuretic peptide (ANP), which reduces blood pressure, blood volume, and blood sodium concentration

ƒ Major hormonal products are thymopoietins and thymosins ƒ These hormones are essential for the development of the T lymphocytes (T cells) of the immune system

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

ƒ Gastrointestinal tract – enteroendocrine cells release local-acting digestive hormones ƒ Placenta – releases hormones that influence the course of pregnancy

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Other Hormone-Producing Structures

ƒ Kidneys – secrete erythropoietin, which signals the production of red blood cells ƒ Skin – produces cholecalciferol, the precursor of vitamin D ƒ Adipose tissue – releases leptin, which is involved in the sensation of satiety, and stimulates increased energy expenditure

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Developmental Aspects

ƒ Hormone-producing glands arise from all three germ layers ƒ Endocrine glands derived from mesoderm produce steroid hormones ƒ Endocrine organs operate smoothly throughout life ƒ Most endocrine glands show structural changes with age, but hormone production may or may not be affected Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Developmental Aspects

Developmental Aspects

ƒ Exposure to pesticides, industrial chemicals, arsenic, dioxin, and soil and water pollutants disrupts hormone function

ƒ Ovaries undergo significant changes with age and become unresponsive to gonadotropins

ƒ Sex hormones, thyroid hormone, and glucocorticoids are vulnerable to the effects of pollutants

ƒ Female hormone production declines, the ability to bear children ends, and problems associated with estrogen deficiency (e.g., osteoporosis) begin to occur

ƒ Interference with glucocorticoids may help explain high cancer rates in certain areas

ƒ Testosterone also diminishes with age, but effect is not usually seen until very old age

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Developmental Aspects

ƒ GH levels decline with age and this accounts for muscle atrophy with age ƒ Supplemental GH may spur muscle growth, reduce body fat, and help physique ƒ TH declines with age, causing lower basal metabolic rates ƒ PTH levels remain fairly constant with age, and lack of estrogen in women makes them more vulnerable to bone-demineralizing effects of PTH Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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