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Chapter 9 The Endocrine System
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Along with Nervous System, the Endocrine System coordinates and directs the activities of body cells
Nervous System uses impulses to make muscles and glands to respond quickly to changes inside and outside the body Endocrine System uses chemical messengers released into the blood to respond slowly to changes
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Hormones Chemical messengers are called hormones
Hormones control several major processes Reproduction Growth and development Mobilization of body defenses Maintenance of much of homeostasis Regulation of metabolism
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Hormones are produced by specialized cells
Cells secrete hormones into extracellular fluids Blood transfers hormones to target sites These hormones regulate the metabolic activity of other cells
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Hormones regulate processes that go on for long periods
Hormones can be classified as Amino acid-based molecules – proteins, peptides, amines Steroids – made from cholesterol Prostaglandins – made from highly active lipids
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Mechanisms of Hormone Action
Hormones work by binding to target cell membrane Hormones affect only certain tissues or organs (target cells or organs) Target cells must have specific protein receptors Hormone binding influences the working of the cells
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Effects Caused by Hormones
Changes that follow hormone binding depend on the specific hormone and the target cell type Typical changes include: Changes in plasma membrane permeability or electrical state Synthesis of proteins, such as enzymes Activation or inactivation of enzymes Stimulation of mitosis
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Mechanisms of Hormone Action
Two hormone mechanisms trigger changes in the cell Figure 9.1 Direct gene activation – the steroid hormone mechanism A second messenger system – nonsteroid (amino acid-based) mechanism
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Steroid Hormone Action
Diffuse through the plasma membrane of target cells – lipid-soluble Enter the nucleus Bind to a specific protein within the nucleus Bind to specific sites on the cell’s DNA Activate genes that result in synthesis of new proteins
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Steroid Hormone Action
PRESS TO PLAY STEROID HORMONE ANIMATION Figure 9.1a
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Nonsteroid Hormone Action
Hormone binds to a membrane receptor Hormone does not enter the cell Sets off a series of reactions that activates an enzyme Catalyzes a reaction that produces a second messenger molecule – cAMP or various other possiblities Oversees additional intracellular changes to promote a specific response
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Nonsteroid Hormone Action
PRESS TO PLAY NONSTEROID HORMONE ANIMATION Figure 9.1b
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Prostaglandins Group of biochemicals that also regulate cells like hormones Produced by many types of cells Act more locally than hormones Effect organs where they are produced Synthesized just before release, not stored in cells
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Control of Hormone release
Negative Feedback mechanisms Regulate blood levels of hormones Secretion is triggered (internal or external stimulus), rising hormone levels inhibit further hormone release Blood levels of hormones vary within a narrow range
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3 major categories of stimuli to activate endocrine organs
Hormonal stimulation – organs stimulated by other hormones Humoral stimulation – changing blood levels stimulate release of hormones Neural stimulation – nerve fibers stimulate release of hormones
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Hormonal Stimuli of Endocrine Glands
Endocrine glands are activated by other hormones Hypothalamus hormones stimulate pituitary gland secretion Pituitary hormones stimulate other endocrine organs Figure 9.2a
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Hormones secreted by thyroid, adrenal cortex, testis (final target glands) “feed back” to pituitary gland and inhibit more pituitary gland secretion When target gland secretion is too low, hypothalamus again stimulates the pituitary gland
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Humoral Stimuli of Endocrine Glands
Changing blood levels of certain ions stimulate hormone release Low blood Ca levels stimulate release of PTH When Ca levels rise, PTH release is inhibited Figure 9.2b
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Neural Stimuli of Endocrine Glands
Nerve impulses stimulate hormone release Most are under control of the sympathetic nervous system Figure 9.2c
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Sympathetic Nervous System stimulation of the adrenal gland releases norepinephrine and epinephrine during periods of stress.
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Major Endocrine Organs
Pituitary, thyroid, parathyroid, adrenal, pineal, thymus, pancreas, gonads (ovaries and testis) Hypothalamus is part of the CNS, but is also considered an endocrine organ since it produces several hormones
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Some glands are purely endocrine, others are mixed – endocrine and exocrine
Exocrine glands – release substances onto body substances or into body cavities through ducts Endocrine glands – release substances into blood
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Pituitary Gland Size of a grape Hangs by a stalk from the hypothalamus
Protected by the sphenoid bone Has two functional lobes Anterior pituitary – glandular tissue Posterior pituitary – nervous tissue
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Hormones of the Anterior Pituitary
Six anterior pituitary hormones Two affect non-endocrine targets Growth hormone Prolactin Four stimulate other endocrine glands (tropic hormones) Thyrotropic hormone Adrenocorticotropic hormone 2 Gonadotropic hormones
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Hormones of the Anterior Pituitary
Figure 9.4
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Characteristics of all anterior pituitary hormones
Proteins (or peptides) Act through second-messenger systems Regulated by hormonal stimuli, mostly negative feedback
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Hormones of the Anterior Pituitary
Growth hormone - GH Effects growth of skeletal muscle and long bones Builds up proteins Cells grow in size and divide Breaks down fats to produce energy
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Adrenocorticotropic hormone - ACTH
Prolactin – PRL Stimulates and maintains milk production in mothers after birth Function in males is not known Adrenocorticotropic hormone - ACTH Regulates endocrine activity of adrenal cortex
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Thyroid-stimulating hormone– TSH
Also called thyrotropic hormone - TH Influences growth and activity of thyroid gland
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Gonadotropic hormones Follicle-stimulating hormone – FSH
Matures follicles of ovaries, produces estrogen In males, stimulates sperm development Luteinizing hormone – LH Triggers ovulation of egg from the ovary Produces progesterone and estrogen In males, called interstitial cell-stimulating hormone, stimulates testosterone production
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Pituitary-Hypothalamus
Anterior pituitary controls the activity of other glands : adrenals, thyroid, ovaries, testes Anterior pituitary is controlled by hypothalamus Hypothalamus produces 2 other hormones that are transported along neurosecretory axons to posterior pituitary – figure 9.5
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Hormones of Posterior Pituitary
Hormones are made by hypothalamus and transported to and stored by posterior pituitary Nerve impulse from hypothalamus triggers release of posterior hormones Oxytocin Released during childbirth and nursing Stimulates contraction of uterine muscles
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Antidiuretic hormone – ADH
Diuresis – urine production ADH causes kidneys to reabsorb water from urine, urine volume decreases, blood volume increases Alcoholic drinks inhibit ADH secretion resulting in increase in urine output, dehydration Diuretic drugs inhibit ADH
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Thyroid gland Located at the base of the throat
Consists of 2 lobes joined by a central isthmus Composed of hollow structures called follicles which are filled with a colloid which produces thyroid hormones Produces 2 iodine-containing hormones and calcitonin
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Thyroid Hormones Thyroxine – T4 Triiodothyronine - T3
Major hormone of the thyroid follicles 4 iodine atoms Controls metabolism Triiodothyronine - T3 Formed by conversion of thyroxine at the target cell 3 iodine atoms
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Calcitonin Decreases blood calcium levels by helping calcium to be deposited in the bones Made by C cells, parafollicular cells, in the connective tissue around the follicles Released into blood in response to high blood levels of calcium
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Parathyroid Glands Tiny masses of gland tissue found on posterior surface of thyroid Secretes parathyroid hormone which acts antagonistically to calcitonin
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Parathyroid Hormone Parathyroid hormone – PTH
Increases calcium levels in the blood by stimulating osteoclasts to break down bone tissue to increase calcium levels in blood
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Adrenal Glands Bean-shaped gland on top of kidneys
Contains glandular cortex and neural tissue medulla region Adrenal cortex produces 3 groups of steroid hormones called corticosteroids: mineralocorticoids, glucocorticoids, and sex hormones Adrenal medulla produces catecholamines: epinephrine and norepinephrine
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Adrenal cortex Hormones
Mineralcorticoids, mainly aldosterone, produced by outermost cortex cell layer Regulates concentration of sodium and potassium ions by causing kidney to absorb sodium ions and water, and excrete potassium ions Increases blood volume and blood pressure
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Glucocorticoids including cortisone and cortisol are produced by the middle cortical layer
Promote normal cell metabolism, especially breakdown of fats and proteins, to help the body resist long-term stresses (Figure 9.12) Increase blood glucose levels Control inflammation Regulated by blood levels of ACTH
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Sex hormones including androgens and estrogens produced by innermost cortex layer
Both are produced regardless of gender
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Adrenal Medulla Hormones
Stimulated by sympathetic nervous system, releases catecholamines epinephrine and norepinephrine Epinephrine - also called adrenaline Coping with short-term stressful situations Increase heart rate, blood pressure, blood glucose level, dilate air passageways in lungs Increase oxygen to the brain
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Controlling stress Glucocorticoids ( ) vs. Catecholamines ( )
Figure 9.12 Cortisone & cortisol – adrenal cortex Cope with prolonged or continuing stressors, resistance stage Epinephrine – adrenal medulla Prepare body for alarm stage of stress
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Pancreas Located close to the stomach in abdominal cavity
A mixed gland – Figure 9.13 Exocrine cells – acinar tissue – work with the digestive system, produce enzymes Endocrine gland cells – pancreatic islets – scattered throughout acinar tissue, produce 2 hormones: insulin & glucagon Insulin produced in beta cells, glucagon produced in alpha cells
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Pancreatic Hormones Insulin – beta cells
Increases cells ability to transport glucose from the blood into cell through the cell membrane Speeds up cell’s ability to convert glucose into energy or glycogen to be stored in the liver Insulin secretion is stimulated by high glucose levels in blood. As glucose levels decrease, insulin release ends. Negative feedback Without insulin, no glucose gets into cells
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Figure 9.14 – antagonistic effects of insulin and glucagon
Glucagon – alpha cells Antagonist to insulin Stimulates liver to break down glycogen to glucose and release glucose into blood Stimulated by low blood glucose levels Figure 9.14 – antagonistic effects of insulin and glucagon
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Pineal Gland Small, cone-shaped Roof of third ventricle of brain
Only one hormone secreted, melatonin “sleep trigger” Establishes the body’s day-night cycle Peak levels at night cause drowsiness, low levels at daylight
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Thymus Gland Upper thorax, posterior to sternum
Large in infants and children Size decreases in adulthood Produces thymosin Incubator to help white blood cells mature Important role in immune response
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Gonads Male and female gonads produce same sex hormones as those produced in adrenal cortex Female gonads – ovaries Male gonads - testes
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Ovaries Almond-shaped, located in pelvic cavity
Produce sex cells, eggs Produce 2 steroid hormones: estrogen and progesterone Do not function until gonadotropic hormones of anterior pituitary gland stimulate them at puberty
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Ovarian Hormones Estrogen Progesterone
stimulates maturation of reproductive organs Prepares uterus to receive fertilized egg by causing cyclic changes in uterine lining – menstrual cycle Progesterone Acts with estrogen to bring about menstrual cycle Assures that uterine muscle will not abort implanted embryo Produced by corpus luteum, glandular structure of ovary
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Testes Suspended in a sac outside pelvic cavity
Produce sex cells, sperm Produce testosterone Stimulates maturation of reproductive organs Necessary for production of sperm Stimulated by gonadotropic hormone of anterior pituitary hormone at puberty
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Development of Endocrine System
Embryonic development varies Pituitary – epithelium of oral cavity, neural tissue of hypothalamus Pineal – neural tissue Thyroid, thymus, pancreas – mucosa of digestive tract Late middle age – estrogen production declines menopause Most endocrine organs function well until old age
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