Post on 13-Jan-2016
Chapter 9
The Endocrine System
• 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
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
• 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
• 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
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
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
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
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
Steroid Hormone Action
STEROID HORMONE ANIMATIONFigure 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
Nonsteroid Hormone Action
NONSTEROID HORMONE ANIMATIONFigure 9.1b
PRESS TO PLAY
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
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
• 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
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
• 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
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
Neural Stimuli of Endocrine Glands
• Nerve impulses stimulate hormone release
• Most are under control of the sympathetic nervous system
Figure 9.2c
• Sympathetic Nervous System stimulation of the adrenal gland releases norepinephrine and epinephrine during periods of stress.
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
• 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
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
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
Hormones of the Anterior Pituitary
Figure 9.4
• Characteristics of all anterior pituitary hormones– Proteins (or peptides)– Act through second-messenger systems– Regulated by hormonal stimuli, mostly
negative feedback
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
• 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
• Thyroid-stimulating hormone– TSH– Also called thyrotropic hormone - TH – Influences growth and activity of thyroid gland
• 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
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
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
• 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
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
Thyroid Hormones
• Thyroxine – T4
– 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
• 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
Parathyroid Glands
• Tiny masses of gland tissue found on posterior surface of thyroid
• Secretes parathyroid hormone which acts antagonistically to calcitonin
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
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
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
• 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
• Sex hormones including androgens and estrogens produced by innermost cortex layer– Both are produced regardless of gender
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
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
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
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
• 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
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
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
Gonads
• Male and female gonads produce same sex hormones as those produced in adrenal cortex
• Female gonads – ovaries
• Male gonads - testes
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
Ovarian Hormones
• Estrogen– 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
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
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