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Bonewit-West: Today’s Medical Assistant
Chapter 11: Endocrine System
Content Outlines
Introduction to the Endocrine System
1. Endocrine glands: organs that make up the endocrine system
a. Secrete hormones into blood
b. Scattered throughout body
2. Endocrinology: study of endocrine glands and hormones
Introduction to the Endocrine System
Comparison of the Endocrine and Nervous Systems
1. Endocrine system and nervous system: function in regulation of body activities
a. Nervous system: acts through electrical impulses and neurotransmitters
• To cause muscle contraction and glandular secretion
• Effect: short duration, measured in seconds, localized
b. Endocrine system: acts through chemical messengers (hormones)
• Hormones influence growth, development, and metabolic activities
• Action is measured in minutes, hours, or weeks and is more generalized
Comparison of Exocrine and Endocrine Glands
1. Exocrine glands: have ducts that carry their secretory product to a surface
a. Examples: sweat, sebaceous, and mammary glands and glands that secrete
digestive enzymes
2. Endocrine glands: do not have ducts to carry their product to a surface (ductless
glands)
a. Secretory products: hormones
• Secreted directly into blood: carried throughout body
- Influence only those cells that have receptor sites for that hormone
Characteristics of Hormones
Chemical Nature of Hormones
1. Chemical classification:
a. Proteins: make up most of the hormones in the body
• Difficult to administer orally
- Inactivated by acid and pepsin in stomach
- Must be given by injection (example: insulin)
b. Steroids: make up sex hormones and hormones secreted by the adrenal
cortex
• May be given orally
Mechanism of Hormone Action
1. Hormones are very potent substances
a. Small amounts: profound effects on metabolic processes
2. Hormones are carried by the blood throughout the body
a. Affect only certain cells
3. The specific cells that respond to a given hormone have receptor sites for that
hormone
a. “Lock and key” mechanism
b. If a hormone fits the receptor site: there will be an effect
c. If a hormone and a receptor site do not match: there is no reaction
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4. Target tissue: tissues (cells) that respond to a particular hormone because they
have receptor sites for that hormone
a. May be localized in a single gland or organ
b. May be diffuse and scattered throughout body
• Many areas are affected
Endocrine Glands and Their Hormones
Pituitary Gland
1. Small gland: size of a pea
2. Connected to hypothalamus by a slender stalk: infundibulum
3. Two distinct regions:
a. Adenohypophysis: anterior portion
b. Neurohypophysis: posterior portion
Hormones of the Anterior Lobe (Adenohypophysis)
1. Growth hormone (GH)
a. Stimulates growth of bones, muscles, and other organs
• By promoting protein synthesis
b. Influences height
• Too little of hormone in a child: pituitary dwarf of normal proportions
but small stature
• Excess of hormone in a child: individual becomes exceptionally tall
• After ossification is complete (bone length no longer possible)
- Excess GH causes enlargement in the diameter of the bones:
acromegaly
1) Bones of the hands and face become abnormally large
2. Thyroid-stimulating hormone (TSH)
a. Causes glandular cells of thyroid to secrete thyroid hormone
b. Hypersecretion of TSH: thyroid gland enlarges and secretes too much
thyroid hormone
c. Hyposecretion of TSH: results in atrophy of thyroid gland and too little
hormone
3. Adrenocorticotropic hormone (ACTH)
a. Reacts with receptor sites in the cortex of adrenal gland
• To stimulate secretion of cortical hormones (cortisol)
b. Affects melanocytes in the skin: increases pigmentation
4. Gonadotropic hormones
a. React with receptor sites in the gonads (ovaries and testes)
• To regulate development, growth, and function of these organs
b. Follicle-stimulating hormone (FSH)
• Stimulates development of eggs in the ovaries and sperm in testes
• Stimulates estrogen production in female
c. Luteinizing hormone (LH)
• Causes ovulation
• Causes production and secretion of female sex hormones
- Progesterone
- Estrogen
• In the male: sometimes called interstitial cell-stimulating hormone
(ICSH)
- Stimulates interstitial cells of testes
1) To produce and secrete testosterone (male sex hormone)
• Without gonadotropins:
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- Ovaries and testes decrease in size
- Ova and sperm are not produced
- Sex hormones are not secreted
5. Prolactin (PRL)
a. Promotes development of glandular tissue in female breast during pregnancy
b. Stimulates milk production after birth of infant
Hormones of the Posterior Lobe (Neurohypophysis)
1. Antidiuretic hormone (ADH)
a. Promotes reabsorption of water by the kidney tubules
• Less water is lost as urine
• Conserves water for the body
b. Insufficient amounts of ADH: excessive water loss in urine
• Large amounts of very dilute urine: known as diabetes insipidus
c. Large amounts of ADH: cause blood vessels to constrict
• Increases blood pressure
• ADH sometimes called vasopressin
2. Oxytocin
a. Causes contraction of smooth muscle in wall of uterus
b. Stimulates ejection of milk from lactating breast
c. Pitocin: commercial preparation of this hormone
• Used to induce labor
Thyroid Gland
1. Very vascular organ: located in neck
2. Consists of two lobes: one on each side of trachea
a. Connected by narrow band of tissue: isthmus
Thyroxine and Triiodothyronine
1. Thyroid hormone consists of:
a. Thyroxine: 95%
b. Triiodothyronine: 5%
2. Requires iodine for synthesis
3. Iodine deficiency: thyroid cannot make sufficient hormone
a. Stimulates thyroid gland to increase in size
• In an attempt to produce more thyroid hormone
- Cannot produce more hormone: does not have iodine
• Known as simple goiter or iodine deficiency goiter
4. Help to regulate the metabolism of carbohydrates, proteins, and lipids
a. Increases rate at which cells release energy from carbohydrates
b. Enhances protein synthesis
c. Necessary for normal growth and development
d. Stimulates nervous system
5. Do not have a single target organ
a. Affect most of the cells in the body
6. Hypothyroidism: deficiency of thyroid hormone
a. Cretinism: in a child
• Mentally retarded dwarf with abnormal skeletal features
• Treatment: thyroid hormone therapy
b. Myxedema: in an adult
• Lethargy, weight gain, loss of hair, low metabolic rate
• Thyroid hormone therapy: usually alleviates symptoms
7. Hyperthyroidism: enlarged thyroid gland that produces too much hormone
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a. Symptoms:
• High metabolic rate, hyperactivity, insomnia, nervousness, irritability,
chronic fatigue
• Exophthalmos: protruding eyes
- Due to swelling in tissues behind the eyes
b. Treatment: removal or destruction of a portion of the thyroid gland
Calcitonin
1. Secreted by thyroid gland
2. Reduces the calcium level in the blood
a. Opposes the action of parathyroid glands
3. Works by:
a. Reducing rate at which calcium is released from bone
b. Increasing rate of calcium excretion by kidneys
c. Reducing calcium absorption in intestines
Parathyroid Glands
1. Four small masses of epithelial tissue
a. Embedded in a connective tissue capsule
b. Located on the posterior surface of the thyroid glands
2. Secrete parathyroid hormone (PTH)
3. Secreted in response to low blood calcium levels
a. Increase blood calcium levels
4. Works by:
a. Increasing osteoclast activity in bones: calcium is released from bones into
blood
b. Increasing calcium reabsorption from kidney tubules into blood: decreases
amount lost in urine
c. Increasing absorption of dietary calcium in intestines
5. Vitamin D: necessary for dietary calcium to be absorbed in intestines
6. PTH has opposite effect of calcitonin from the thyroid gland
7. Hypoparathyroidism: insufficient secretion of PTH
a. Increased nerve excitability due to low blood calcium levels
8. Hyperparathyroidism: excessive secretion of PTH
a. Excess calcium in the blood:
• May precipitate in abnormal locations
• Cause kidney stones
Adrenal (Suprarenal) Glands
1. Paired glands: one located near upper portion of each kidney
2. Divided into regions:
a. Adrenal cortex: outer region
b. Adrenal medulla: inner region
Hormones of the Adrenal Cortex
1. Mineralocorticoids
a. Function:
• Regulate blood volume
• Regulate the concentration of mineral electrolytes in the blood
b. Types:
• Aldosterone
- Primarily affects the kidneys
- Conserves sodium ions and water in the body and eliminates
potassium ions
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1) Important in:
a) Maintaining blood pressure
b) Nerve impulse conduction
c) Muscle contraction
2. Glucocorticoids
a. Cortisol: principal glucocorticoid
b. Function:
• Increases blood glucose levels
- Helps to maintain blood glucose levels between meals
• Counteracts inflammatory response
- Used clinically to reduce inflammation (allergic reactions, bursitis,
arthritis, infections)
3. Gonadocorticoids
a. Sex hormones
• Androgens: male hormones
• Estrogens: female hormones
b. Secreted in minimal amounts by both sexes
• Effect is masked by hormones from the testes and ovaries
4. Addison’s disease: hyposecretion of hormones from adrenal cortex
5. Cushing’s syndrome: hypersecretion of hormones from adrenal cortex
Hormones of the Adrenal Medulla
1. Epinephrine and norepinephrine
2. Prepares body for strenuous activity
a. Sometimes called fight-or-flight hormones
3. Effect on body: increased heart rate, increased respiratory rate, increased blood
supply to skeletal muscles
Pancreas—Islets of Langerhans
1. Long, soft organ
2. Located posterior to stomach
3. Exocrine portion: secretes digestive enzymes
a. Carried through a duct to duodenum
4. Endocrine portion: secretes hormones into blood
a. Consists of pancreatic islets (islets of Langerhans)
• Contain alpha cells: secrete glucagon
• Also contain beta cells: secrete insulin
b. Glucagon and insulin: hormones that regulate blood glucose levels
c. Glucose: primary source of energy for nervous system
• Blood glucose levels too low: NS does not function properly
• Blood glucose levels too high: kidneys produce large quantities of urine
- May lead to dehydration
Glucagon
1. Secreted by alpha cells in pancreatic islets
a. In response to low concentration of glucose in blood
2. Action: raises blood glucose levels
a. Mobilizes glucose and fatty acids from storage forms
b. Stimulates liver to break down glycogen into glucose
3. Prevents hypoglycemia from occurring
a. Between meals
b. When glucose is being used rapidly
Insulin
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1. Secreted by beta cells in pancreatic islets
a. In response to a high concentration of glucose in blood
2. Action of insulin: decreases blood glucose level (opposite or antagonistic to
glucagon)
a. Promotes cellular uptake and use of glucose for energy
b. Stimulates liver and muscle to remove glucose from blood
• Store it as glycogen
3. When the liver has stored all the glycogen possible
a. Glucose is converted to fat
4. Hypoactivity of insulin
a. Insufficient insulin secretion
b. Insufficient receptor sites on target cell membranes
c. Defective receptor sites that do not recognize insulin
• Leads to diabetes mellitus (abnormally high blood glucose)
Gonads (Testes and Ovaries)
1. Primary reproductive organs
a. Testes: male
b. Ovaries: female
Testes
1. Androgens: male sex hormones
a. Testosterone: secreted by testes
• Small amount also produced by adrenal cortex
• Production of testosterone
- Begins during fetal development
- Continues for a short time after birth
- Ceases during childhood
- Resumes at puberty
• Function (at onset of puberty):
- Growth and development of the male reproductive structures
- Increased skeletal and muscular growth
- Enlargement of the larynx accompanied by voice changes
- Growth and distribution of body hair
- Increased male sexual drive
Ovaries
1. Produce female sex hormones:
a. Estrogens
• Function (at onset of puberty)
- Development of breasts
- Distribution of fat evidenced in the hips, legs, and breasts
- Maturation of reproductive organs (uterus and vagina)
b. Progesterone
• Causes uterine lining to thicken in preparation for pregnancy
Pineal Gland
1. Small cone-shaped structure
2. Extends from the third ventricle of brain
3. Contain specialized secretory cells: pinealocytes
a. Synthesize melatonin
• Secrete it directly into cerebrospinal fluid: takes it into blood
• High levels: secreted at night
• Low levels: secreted during day
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4. Function of melatonin:
a. Acts on hypothalamus: inhibits gonadotropin-releasing hormone (GnRH)
• GnRH inhibits gonad development
b. Regulation of circadian rhythms
• Example: sleepiness/wakefulness cycle
- Increased plasma melatonin levels (night): associated with sleepiness
c. Plays a role in hunger/satiety cycles, mood changes, and jet lag
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