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Crafton Hills - Anatomy 151 - Chapter 16
Endocrine System
A. Endocrine System: Overview
1. Endocrine system: Body's 2nd great control system; influences cellular metabolism via hormones
2. Endocrine glands: Pituitary, thyroid, parathyroid, adrenal, pineal, and thymus
3. Pancreas & Gonads produce hormones and exocrine products
4. Hypothalamus: Neural functions & releases hormones
5. Other tissues/organs produce hormones: Adipose cells, pockets of cells in the walls of the small
intestine, stomach, kidneys, and heart
6. Types of Chemical Messengers
a. Autocrines - chemicals acting on the very cells that secrete them
b. Paracrines - local effects, affect nearby cells
c. Fail to meet definition of Hormones (Long-distance chemical signals)
B. Hormones
1. Definition: Chemical substances secreted by cells into extracellular fluids affecting cells "at long
distance"
2. Characteristics
a. Alter target cell activity (e.g. Regulate cell metabolic fx)
b. Have lag times ranging from seconds to hours
c. Tend to have prolonged effects
d. Are classified as amino acid-based hormones, or steroids
3. Types of hormones
a. Amino acid based: Includes most hormones
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Amines, thyroxine, peptide, & protein hormones
b. Steroids: Gonadal & adrenocortical hormones
c. Eicosanoids: Leukotrienes & prostaglandins
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Biologically active lipids with local hormone-like activity
4. Mechanisms of Hormone action
a. Direct gene activation (e.g. steroid hormones)
b. 2nd messengers (Indirect) involving:
i. Regulatory G proteins & Amino acid-based hormones
ii. Examples: cAMP, PIP-Calcium (3rd messenger), Steroid Hormones (intracellular
receptors-mRNA-proteins-cellular effect)
1) 2nd Messenger (indirect) EXAMPLE: cAMP
a) Hormone (1st messenger) binds receptor, which binds G protein
b) G protein is activated, binds GTP
c) Activated G protein activates effector enzyme (adenylate cyclase)
d) Adenylate cyclase generates cAMP (2nd messenger) from ATP
e) cAMP activates protein kinases, which then cause cellular effects
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5. Target Cell Specificity
a. Hormones circulate to all tissues but only activate target cells
b. Target cells have specific receptors to bind hormone
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Receptors: Intracellular or on Plasma Membrane
6. Target Cell Activation
a. Critical Factors
i. Blood levels of the hormone
ii. Relative number of receptors on target cell
iii. Affinity of receptors for the hormone
b. Up-regulation: Target cells form more receptors in response to the hormone
c. Down-regulation: Target cells lose receptors in response to the hormone
7. Concentrations [ ] of hormone in Blood
a. Reflect release rate & rate of inactivation & removal
b. Removed from blood by:
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Degrading enzymes, Kidneys, Liver enzymes
8. Types of Hormone Interaction
a. Permissiveness: One hormone cannot exert its effects without another hormone present
b. Synergism: More than one hormone produces the same effects on a target cell
c. Antagonism: One or more hormones opposes the action of another hormone
9. Control of Hormone Release
a. Blood levels of hormones:
i. Controlled by negative feedback systems [narrow range]
b. Synthesized & released in response to stimuli (Humoral, Neural, and Hormonal stimuli)
i. Humoral stimuli: Secretion in direct response to changed blood levels of ions & nutrients
1) Example: Concentration of Calcium ions in blood
a) Low concentration blood Ca2+
b) Parathyroid glands secretes PTH (parathyroid hormone)
c) Ca2+ concentration rises
d) Negative feedback (PTH secretion stops)
ii. Neural stimuli: Nerve fibers stimulate hormone release
1) Sympathetic nervous system (SNS) fibers stimulate adrenal medulla to secrete
catecholamines
iii. Hormonal stimuli: Release of hormones in response to hormones produced by other
endocrine organs
1) Hypothalamic hormones stimulate anterior pituitary
2) Pituitary hormones, in turn, stimulate targets to secrete still more hormones
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C. Pituitary
1. Definition: Two-lobed organ that secretes 9 major hormones
a. Neurohypophysis - posterior lobe (neural tissue) & infundibulum
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Receives, stores & releases hormones from hypothalamus (e.g. ADH)
b. Adenohypophysis - anterior lobe (glandular tissue): Synthesizes & secretes several hormones
2. Hormones Released (Adenophypophysis): GH, TSH, ACTH, FSH, LH, and PRL
a. Characteristics
i. Regulate activity of other endocrine glands
ii. Hypothalamus sends a chemical stimulus to anterior pituitary
1) Releasing hormones: Stimulate synthesis & release of hormones at anterior pituitary
2) Inhibiting hormones: Inhibit synthesis & release of hormones at anterior pituitary
b. Growth Hormone (GH)
i. Source: Somatotropic cells of anterior lobe
1) Stimulate most cells, target bone & skeletal muscle
2) Promote protein synthesis & to use of fats for fuel
ii. Metabolic action
1) Stimulates liver, skel. muscle, bone, & cartilage to make insulin-like growth factors
2) Directly promote lipolysis & inhibits glucose uptake
c. Thyroid Stimulating Hormone (Thyrotropin/TSH)
i. Stimulates develop. & secretory activity of thyroid
ii. Negative Feedback: High blood levels of TSH act on pituitary & hypothalamus to block
release of TSH
d. Adrenocorticotropic Hormone (ACTH)
i. Stimulates adrenal cortex to release corticosteroids
ii. Triggered by hormones assoc. w/ daily rhythm, which, in turn, can be triggered by
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Internal & external factors: Fever, hypoglycemia, & stressors
e. Gonadotropins: FSH & LH (Follicle-stimulating hormone & Luteinizing Hormone)
i. Regulate function of ovaries & testes
1) FSH stimulates gamete (egg or sperm) production
2) Absent from blood in prepubertal boys & girls
ii. Females
1) LH with FSH causing maturation of ovarian follicle
2) LH alone triggers ovulation (expelling egg from follicle)
3) LH promote synthesis & release of estrogens & progesterone
iii. Males
1) LH stimulates interstitial cells of testes to produce testosterone
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f. Prolactin (PRL)
i. Stimulates milk production by breasts ( _S )
ii. Blood levels rise toward end of pregnancy
iii. Suckling stimulates Prolactin Releasing Hormone (PRH) release & promotes continued
milk production
3. Hormones Released (Posterior Pituitary / Hypothalamus)
a. Stores antidiuretic hormone (ADH) & Oxytocin
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Synthesized in hypothalamus
b. Oxytocin
i. Stimulates smooth muscle contraction (positive feedback) in breasts & uterus (important
in delivery! & milk ejection)
ii. Used in drugs to induce or hasten labor
iii. Sexual arousal/satisfaction in males & nonlactating females
c. ADH (Antidiuretic Hormone)
i. Influences water balance by helping to avoid dehydration by preventing urine formation
1) Osmoreceptors monitor solute [ ] of blood
2) High solutes, ADH released; Low solutes, ADH not released
D. Thyroid
1. Characteristics
a. Largest endocrine gland, in anterior neck, consists of two lateral lobes connected by a median
tissue mass called the isthmus
b. Composed of follicles whose product eventually becomes thyroid hormone
c. Other endocrine cells, the parafollicular cells, produce the hormone calcitonin
2. Hormones Released
a. Thyroid hormone: Major metabolic hormone
i. Made of 2 closely related iodine-based compounds
ii. Plays a role in:
1) Glucose oxidation
2) Increasing metabolic rate & Heat production
3) Maintaining blood pressure
4) Regulating tissue growth
5) Maturation & reproductive capabilities
b. Calcitonin
i. Lowers blood calcium levels in children
ii. Antagonist to parathyroid hormone (PTH)
iii. Stimulates calcium uptake into bone matrix
iv. Regulated by a humoral (Ca+ [ ] in blood) negative feedback mechanism
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E. Parathyroid
1. Characteristics
a. Tiny glands embedded in posterior aspect of thyroid
b. Chief (principal) cells secrete PTH
c. PTH (parathormone) regulates calcium balance in the blood
2. Hormones Released
a. PTH (Parathyroid hormone) release raises blood Ca2+ level and
i. Stimulates osteoclasts
ii. Enhances reabsorption of Ca2+ & secretion of phosphate by kidneys
iii. Increases absorption of Ca2+
b. Rising Ca2+ in the blood inhibits PTH release
c. Antagonist to Calcitonin
F. Adrenal (Suprarenal) Glands
1. Paired, pyramid-shaped organs atop the kidneys
2. Structurally & functionally: Two glands in one (Adrenal Medulla & Adrenal Cortex)
3. Adrenal Cortex: Glandular tissue
a. Synthesizes and releases steroid hormones called corticosteroids
b. Different corticosteroids are produced in each of the three layers (outer to inner)
i. Zona glomerulosa - mineralocorticoids (Aldosterone: (K+/(Na+ keeps [Na+] by (
Na+secretion)
ii. Zona fasciculata - glucocorticoids (Cortisol: [bloodsugar] & blood volume constant )
iii. Zona reticularis - gonadocorticoids (Androgens: Testosterone
- Help w/
o
2 sex characteristics, female sex drive)
4. Adrenal Medulla: Nervous tissue that acts as part of SNS
a. Made of cells secreting Epinephrine & Norepinephrine which cause:
i. Rise in Blood glucose levels
ii. Blood vessels to constrict
iii. Rise in Heart rate
iv. Diversion of blood to Brain, Heart, & Skeletal muscle
b. Adrenal Glands: Effect of Stress: Be familiar with Fig. 16.15
G. Pancreas
1. Description
a. Triangular gland, with exocrine & endocrine cells, behind stomach
b. Acinar cells: Make enzyme-rich solution for digestion (exocrine)
2. Pancreatic islets (islets of Langerhans)
a. Alpha (") cells: Produce Glucagon
i. Targets liver: (raises blood sugar)
1) Glycogenolysis: Glycogen into glucose
2) Gluconeogenesis: Glucose from lactic acid
3) Release glucose: From liver cells to blood
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b. Beta ($) cells: Produce Insulin
i. Lowers "blood sugar" blood glucose levels (into cells)
ii. Helps oxidize glucose for ATP production
3. Diabetes Mellitus (DM)
a. Results from low secretion/activity of insulin
b. Three major symptoms
i. Polyuria: Huge urine output
ii. Polydipsia: Excessive thirst
iii. Polyphagia: Excessive hunger & eating
c. Hyperinsulinism: Excessive insulin secretion resulting in hypoglycemia
H. Sex Organs
1. Gonads: Female
a. Paired ovaries in abdominopelvic cavity produce hormones
i. Eestrogens & Progesterone
1) Maturation of reproductive organs
2) Development of secondary sexual characteristics
3) Breast development & cyclic changes in uterine mucosa
2. Gonads: Male
a. Testes located in an exterior sac (scrotum) produce testosterone
b. Testosterone
i. Initiates maturation of male reproductive organs
ii. Causes development of 2o sexual characteristics & sex drive
iii. Necessary for sperm production
iv. Maintains functionality of sex organs
I. Pineal Gland
1. Small, hangs from roof of Brain's 3rd ventricle
2. Secreted Product
a. Melatonin
i. Involved with day/night cycles: Variation in physiological processes
(body
temperature, sleep, appetite)
J. Thymus
1. Lobulated gland located behind sternum in the thorax
2. Hormonal products
a. Thymopoietins & Thymosins: Essential for development of the T lymphocytes (T cells of
immune system)
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