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13 Other Endocrine Tissues and Organs, Eicosanoids, and Growth Factors

13 Other Endocrine Tissues and Organs, Eicosanoids, and Growth Factors

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CHAPTER 18







THE ENDOCRINE SYSTEM



Appearing in the blood in minute quantities, eicosanoids are present only briefly due to rapid inactivation.

To exert their effects, eicosanoids bind to receptors on targetcell plasma membranes and stimulate or inhibit the synthesis of

second messengers such as cyclic AMP. Leukotrienes stimulate

chemotaxis (attraction to a chemical stimulus) of white blood

cells and mediate inflammation. The prostaglandins alter smooth

muscle contraction, glandular secretions, blood flow, reproductive processes, platelet function, respiration, nerve impulse

transmission, lipid metabolism, and immune responses. They

also have roles in promoting inflammation and fever, and in intensifying pain.



C L I NI C AL C ON N E C T ION |



Nonsteroidal Antiinflammatory Drugs



In 1971, scientists solved the long-standing puzzle of how

aspirin works. Aspirin and related nonsteroidal antiinflammatory drugs (NSAIDs), such as ibuprofen (Motrin®), inhibit

cyclooxygenase (COX), a key enzyme involved in prostaglandin synthesis. NSAIDs are used to treat a wide variety of inflammatory disorders, from rheumatoid arthritis to tennis elbow. The success of

NSAIDs in reducing fever, pain, and inflammation shows how prostaglandins contribute to these woes. •



Growth Factors

Several of the hormones we have described—insulinlike growth

factor, thymosin, insulin, thyroid hormones, human growth hormone, and prolactin—stimulate cell growth and division. In addition, several more recently discovered hormones called growth

factors play important roles in tissue development, growth, and

repair. Growth factors are mitogenic substances—they cause

growth by stimulating cell division. Many growth factors act

locally, as autocrines or paracrines.

A summary of sources and actions of six important growth

factors is presented in Table 18.12.

CHECKPOINT



26. What hormones are secreted by the gastrointestinal

tract, placenta, kidneys, skin, adipose tissue, and heart?

27. What are some functions of prostaglandins,

leukotrienes, and growth factors?



18.14 The Stress Response

OBJECTIVE



• Describe how the body responds to stress.



It is impossible to remove all of the stress from our everyday

lives. Some stress, called eustress, prepares us to meet certain

challenges and thus is helpful. Other stress, called distress, is

harmful. Any stimulus that produces a stress response is called a

stressor. A stressor may be almost any disturbance of the human

body—heat or cold, environmental poisons, toxins given off by



TABLE

TABLE 18.12

1.3



Summary of Selected Growth Factors

GROWTH FACTOR



COMMENT



Epidermal growth

factor (EGF)



Produced in submaxillary (salivary) glands;

stimulates proliferation of epithelial cells,

fibroblasts, neurons, and astrocytes; suppresses

some cancer cells and secretion of gastric juice

by stomach.



Platelet-derived

growth factor (PDGF)



Produced in blood platelets; stimulates

proliferation of neuroglia, smooth muscle

fibers, and fibroblasts; appears to have

role in wound healing; may contribute to

atherosclerosis development.



Fibroblast growth

factor (FGF)



Found in pituitary gland and brain; stimulates

proliferation of many cells derived

from embryonic mesoderm (fibroblasts,

adrenocortical cells, smooth muscle fibers,

chondrocytes, and endothelial cells); stimulates

formation of new blood vessels (angiogenesis).



Nerve growth

factor (NGF)



Produced in submandibular (salivary) glands

and hippocampus of brain; stimulates growth

of ganglia in embryo; maintains sympathetic

nervous system; stimulates hypertrophy and

differentiation of neurons.



Tumor angiogenesis

factors (TAFs)



Produced by normal and tumor cells; stimulate

growth of new capillaries, organ regeneration,

and wound healing.



Transforming growth

factors (TGFs)



Produced by various cells as separate

molecules: TGF-alpha has activities similar

to epidermal growth factor; TGF-beta inhibits

proliferation of many cell types.



bacteria, heavy bleeding from a wound or surgery, or a strong

emotional reaction. The responses to stressors may be pleasant or

unpleasant, and they vary among people and even within the same

person at different times.

Your body’s homeostatic mechanisms attempt to counteract

stress. When they are successful, the internal environment remains within normal physiological limits. If stress is extreme,

unusual, or long lasting, the normal mechanisms may not be

enough. In 1936, Hans Selye, a pioneer in stress research, showed

that a variety of stressful conditions or noxious agents elicit a

similar sequence of bodily changes. These changes, called the

stress response or general adaptation syndrome (GAS), are

controlled mainly by the hypothalamus. The stress response occurs in three stages: (1) an initial fight-or-flight response, (2) a

slower resistance reaction, and eventually (3) exhaustion.



The Fight-or-Flight Response

The fight-or-flight response, initiated by nerve impulses from

the hypothalamus to the sympathetic division of the autonomic

nervous system (ANS), including the adrenal medulla, quickly

mobilizes the body’s resources for immediate physical activity



18.14 THE STRESS RESPONSE



(Figure 18.20a). It brings huge amounts of glucose and oxygen to

the organs that are most active in warding off danger: the brain,

which must become highly alert; the skeletal muscles, which may



649



have to fight off an attacker or flee; and the heart, which must

work vigorously to pump enough blood to the brain and muscles.

During the fight-or-flight response, nonessential body functions



Figure 18.20 Responses to stressors during the stress response. Red arrows (hormonal responses) and green arrows (neural

responses) in (a) indicate immediate fight-or-flight reactions; black arrows in (b) indicate long-term resistance reactions.

Stressors stimulate the hypothalamus to initiate the stress response through the fight-or-flight response and the resistance

reaction.

STRESSORS

stimulate



CRH

GHRH



Nerve

impulses



Key:

CRH = Corticotropin-releasing hormone

ACTH = Adrenocorticotropic hormone

GHRH = Growth hormone–releasing hormone

hGH = Human growth hormone

TRH = Thyrotropin-releasing hormone

TSH = Thyroid-stimulating hormone



Hypothalamus



TRH

Sympathetic centers

in spinal cord

Anterior

pituitary

TSH

hGH



ACTH



Adrenal

medulla



hGH



Thyroid

gland



Liver



Adrenal

cortex



TSH



Visceral effectors

Cortisol

Epinephrine

and

norepinephrine

Supplement

and prolong

“fight-or-flight”

responses



STRESS RESPONSES

1. Increased heart rate

and force of beat

2. Constriction of blood

vessels of most

viscera and skin

3. Dilation of blood

vessels of heart,

lungs, brain, and

skeletal muscles

4. Contraction of spleen

5. Conversion of glycogen

into glucose in liver

6. Sweating

7. Dilation of airways

8. Decrease in digestive

activities

9. Water retention and

elevated blood pressure

(a) Fight-or-flight responses



IGFs



STRESS RESPONSES

Lipolysis

Gluconeogenesis

Protein catabolism

Sensitized blood vessels

Reduced inflammation



STRESS

RESPONSES

Lipolysis

Glycogenolysis



(b) Resistance reaction



What is the basic difference between the stress response and homeostasis?



Thyroid hormones

(T3 and T4)



STRESS

RESPONSES

Increased use

of glucose to

produce ATP



C H A P T E R



18



ACTH

Sympathetic nerves



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CHAPTER 18







THE ENDOCRINE SYSTEM



such as digestive, urinary, and reproductive activities are inhibited. Reduction of blood flow to the kidneys promotes release of

renin, which sets into motion the renin–angiotensin–aldosterone

pathway (see Figure 18.16). Aldosterone causes the kidneys to

retain Naϩ, which leads to water retention and elevated blood

pressure. Water retention also helps preserve body fluid volume

in the case of severe bleeding.



The Resistance Reaction

The second stage in the stress response is the resistance reaction

(Figure 18.20b). Unlike the short-lived fight-or-flight response,

which is initiated by nerve impulses from the hypothalamus, the

resistance reaction is initiated in large part by hypothalamic releasing hormones and is a longer-lasting response. The hormones

involved are corticotropin-releasing hormone (CRH), growth hormone–releasing hormone (GHRH), and thyrotropin-releasing

hormone (TRH).

CRH stimulates the anterior pituitary to secrete ACTH, which

in turn stimulates the adrenal cortex to increase release of cortisol. Cortisol then stimulates gluconeogenesis by liver cells,

breakdown of triglycerides into fatty acids (lipolysis), and catabolism of proteins into amino acids. Tissues throughout the

body can use the resulting glucose, fatty acids, and amino acids

to produce ATP or to repair damaged cells. Cortisol also reduces

inflammation.

A second hypothalamic releasing hormone, GHRH, causes

the anterior pituitary to secrete human growth hormone

(hGH). Acting via insulinlike growth factors, hGH stimulates

lipolysis and glycogenolysis, the breakdown of glycogen to

glucose, in the liver. A third hypothalamic releasing hormone,

TRH, stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH). TSH promotes secretion of thyroid

hormones, which stimulate the increased use of glucose for

ATP production. The combined actions of hGH and TSH supply additional ATP for metabolically active cells throughout

the body.

The resistance stage helps the body continue fighting a stressor

long after the fight-or-flight response dissipates. This is why your

heart continues to pound for several minutes even after the stressor

is removed. Generally, it is successful in seeing us through a

stressful episode, and our bodies then return to normal. Occasionally, however, the resistance stage fails to combat the stressor, and

the body moves into the state of exhaustion.



Exhaustion

The resources of the body may eventually become so depleted

that they cannot sustain the resistance stage, and exhaustion

ensues. Prolonged exposure to high levels of cortisol and other

hormones involved in the resistance reaction causes wasting of

muscle, suppression of the immune system, ulceration of the

gastrointestinal tract, and failure of pancreatic beta cells. In addition, pathological changes may occur because resistance reactions

persist after the stressor has been removed.



Stress and Disease

Although the exact role of stress in human diseases is not known, it

is clear that stress can lead to particular diseases by temporarily

inhibiting certain components of the immune system. Stress-related

disorders include gastritis, ulcerative colitis, irritable bowel syndrome, hypertension, asthma, rheumatoid arthritis (RA), migraine

headaches, anxiety, and depression. People under stress are at a

greater risk of developing chronic disease or dying prematurely.

Interleukin-1, a substance secreted by macrophages of the immune system (see the discussion of ACTH in Section 18.6), is an

important link between stress and immunity. One action of interleukin-1 is to stimulate secretion of ACTH, which in turn stimulates the production of cortisol. Not only does cortisol provide

resistance to stress and inflammation, but it also suppresses further production of interleukin-1. Thus, the immune system turns

on the stress response, and the resulting cortisol then turns off one

immune system mediator. This negative feedback system keeps

the immune response in check once it has accomplished its goal.

Because of this activity, cortisol and other glucocorticoids are

used as immunosuppressive drugs for organ transplant recipients.



CLIN ICA L CON N ECTION |



Posttraumatic Stress

Disorder



Posttraumatic stress disorder (PTSD) is an anxiety disorder that may develop in an individual who has experienced,

witnessed, or learned about a physically or psychologically distressing

event. The immediate cause of PTSD appears to be the specific stressors associated with the events. Among the stressors are terrorism,

hostage taking, imprisonment, military duty, serious accidents, torture, sexual or physical abuse, violent crimes, school shootings, massacres, and natural disasters. In the United States, PTSD affects 10%

of females and 5% of males. Symptoms of PTSD include reexperiencing the event through nightmares or flashbacks; avoidance of any

activity, person, place, or event associated with the stressors; loss of

interest and lack of motivation; poor concentration; irritability; and

insomnia. Treatment may include the use of antidepressants, mood

stabilizers, and antianxiety and antipsychotic agents. •



CHECKPOINT



28. What is the central role of the hypothalamus during stress?

29. What body reactions occur during the fight-or-flight

response, the resistance reaction, and exhaustion?

30. What is the relationship between stress and immunity?



18.15 Development of the

Endocrine System

OBJECTIVE



• Describe the development of endocrine glands.



The development of the endocrine system is not as localized as

the development of other systems because, as you have already

learned, endocrine organs are distributed throughout the body.



18.15 DEVELOPMENT OF THE ENDOCRINE SYSTEM



About 3 weeks after fertilization, the pituitary gland (hypophysis) begins to develop from two different regions of the ectoderm.

The posterior pituitary (neurohypophysis) is derived from an outgrowth of ectoderm called the neurohypophyseal bud (nooЈ-roˉ-hı¯poˉ-FIZ-e¯-al), located on the floor of the hypothalamus (Figure 18.21).

The infundibulum, also an outgrowth of the neurohypophyseal bud,

connects the posterior pituitary to the hypothalamus. The anterior

pituitary (adenohypophysis) is derived from an outgrowth of ectoderm from the roof of the mouth called the hypophyseal pouch or

Rathke’s pouch. The pouch grows toward the neurohypophyseal

bud and eventually loses its connection with the roof of the mouth.

The thyroid gland develops during the fourth week as a midventral outgrowth of endoderm, called the thyroid diverticulum

(dı¯Ј-ver-TIK-uˉ-lum), from the floor of the pharynx at the level of

the second pair of pharyngeal pouches (Figure 18.21a). The out-



651



growth projects inferiorly and differentiates into the right and left

lateral lobes and the isthmus of the gland.

The parathyroid glands develop during the fourth week from

endoderm as outgrowths from the third and fourth pharyngeal

pouches (fa-RIN-je¯-al), which help to form structures of the head

and neck.

The adrenal cortex and adrenal medulla develop during the

fifth week and have completely different embryological origins.

The adrenal cortex is derived from the same region of mesoderm

that produces the gonads. Endocrine tissues that secrete steroid

hormones all are derived from mesoderm. The adrenal medulla is

derived from ectoderm from neural crest cells that migrate to

the superior pole of the kidney. Recall that neural crest cells also

give rise to sympathetic ganglia and other structures of the nervous system (see Figure 14.27b).



Figure 18.21 Development of the endocrine system.

Glands of the endocrine system develop from all three primary germ layers: ectoderm, mesoderm, and endoderm.



18



PHARYNGEAL

POUCHES



4



3



2



C H A P T E R



Hypothalamus

NEUROHYPOPHYSEAL

BUD



1



HYPOPHYSEAL

(RATHKE’S) POUCH



Stomodeum

Esophagus

Respiratory diverticulum



THYROID DIVERTICULUM



(a) Location of neurohypophyseal bud, hypophyseal (Rathke’s) pouch,

thyroid diverticulum, and pharyngeal pouches in 28-day embryo

Hypothalamus



NEUROHYPOPHYSEAL BUD

Infundibulum



Infundibulum

Pars intermedia

Infundibulum



HYPOPHYSEAL

(RATHKE’S)

POUCH



Anterior

pituitary



Mouth cavity



Posterior pituitary



Mesenchyme

(b) Development of pituitary gland between 5 and 16 weeks



Which endocrine gland develops from tissues with two different embryological origins?



652



CHAPTER 18







THE ENDOCRINE SYSTEM



The pancreas develops during the fifth through seventh weeks

from two outgrowths of endoderm from the part of the foregut

that later becomes the duodenum (see Figure 29.12c). The two

outgrowths eventually fuse to form the pancreas. The origin of the

ovaries and testes is discussed in Section 28.5.

The pineal gland arises during the seventh week as an outgrowth between the thalamus and colliculi of the midbrain from

ectoderm associated with the diencephalon (see Figure 14.28).

The thymus arises during the fifth week from endoderm of the

third pharyngeal pouches.

CHECKPOINT



31. Compare the origins of the adrenal cortex and adrenal

medulla.



18.16 Aging and the

Endocrine System

OBJECTIVE



• Describe the effects of aging on the endocrine system.



Although some endocrine glands shrink as we get older, their

performance may or may not be compromised. Production of human growth hormone by the anterior pituitary decreases, which

is one cause of muscle atrophy as aging proceeds. The thyroid

gland often decreases its output of thyroid hormones with age,

causing a decrease in metabolic rate, an increase in body fat, and

hypothyroidism, which is seen more often in older people. Because

there is less negative feedback (lower levels of thyroid hormones),

the level of thyroid-stimulating hormone increases with age (see

Figure 18.12).

With aging, the blood level of PTH rises, perhaps due to inadequate dietary intake of calcium. In a study of older women who

took 2400 mg/day of supplemental calcium, blood levels of PTH



were as low as those of younger women. Both calcitriol and calcitonin levels are lower in older persons. Together, the rise in PTH

and the fall in calcitonin level heighten the age-related decrease in

bone mass that leads to osteoporosis and increased risk of fractures (see Figure 18.14).

The adrenal glands contain increasingly more fibrous tissue

and produce less cortisol and aldosterone with advancing age.

However, production of epinephrine and norepinephrine remains

normal. The pancreas releases insulin more slowly with age, and

receptor sensitivity to glucose declines. As a result, blood glucose

levels in older people increase faster and return to normal more

slowly than in younger individuals.

The thymus is largest in infancy. After puberty, its size begins to

decrease, and thymic tissue is replaced by adipose and areolar connective tissue. In older adults, the thymus has atrophied significantly. However, it still produces new T cells for immune responses.

The ovaries decrease in size with age, and they no longer respond

to gonadotropins. The resultant decreased output of estrogens

leads to conditions such as osteoporosis, high blood cholesterol,

and atherosclerosis. FSH and LH levels are high due to less negative feedback inhibition of estrogens. Although testosterone

production by the testes decreases with age, the effects are not

usually apparent until very old age; and many elderly males can

still produce active sperm in normal numbers, even though there

are higher numbers of morphologically abnormal sperm and

decreased sperm motility.

CHECKPOINT



32. Which hormone is related to the muscle atrophy that

occurs with aging?



•••

To appreciate the many ways the endocrine system contributes to

homeostasis of other body systems, examine Focus on Homeostasis: Contributions of the Endocrine System. Next, in Chapter 19,

we will begin to explore the cardiovascular system, starting with

a description of the composition and functions of blood.



D I S O R D E R S : H O M E O S TAT I C I M B A L A N C E S

Disorders of the endocrine system often involve either hyposecretion (hypo- ϭ too little or under), inadequate release of a hormone,

or hypersecretion (hyper- ϭ too much or above), excessive release

of a hormone. In other cases, the problem is faulty hormone receptors, an inadequate number of receptors, or defects in secondmessenger systems. Because hormones are distributed in the blood to

target tissues throughout the body, problems associated with endocrine dysfunction may also be widespread.



Pituitary Gland Disorders

Pituitary Dwarfism, Giantism, and Acromegaly

Several disorders of the anterior pituitary involve human growth

hormone (hGH). Hyposecretion of hGH during the growth years

slows bone growth, and the epiphyseal plates close before normal



height is reached. This condition is called pituitary dwarfism

(see Clinical Connection: Hormonal Abnormalities That Affect

Height in Section 6.5). Other organs of the body also fail to grow,

and the body proportions are childlike. Treatment requires administration of hGH during childhood, before the epiphyseal plates

close.

Hypersecretion of hGH during childhood causes giantism, an

abnormal increase in the length of long bones. The person grows to

be very tall, but body proportions are about normal. Figure 18.22a

shows identical twins; one brother developed giantism due to a pituitary tumor. Hypersecretion of hGH during adulthood is called acromegaly (akЈ-roˉ-MEG-a-leˉ). Although hGH cannot produce further

lengthening of the long bones because the epiphyseal plates are

already closed, the bones of the hands, feet, cheeks, and jaws thicken

and other tissues enlarge. In addition, the eyelids, lips, tongue, and



FOCUS on HOMEOSTASIS

LYMPHATIC SYSTEM

and IMMUNITY



INTEGUMENTARY

SYSTEM



Glucocorticoids such as cortisol depress

inflammation and immune responses

Thymic hormones promote maturation of

T cells (a type of white blood cell)



Androgens stimulate growth of axillary

and pubic hair and activation of

sebaceous glands

Excess melanocyte-stimulating hormone

(MSH) causes darkening of skin



RESPIRATORY

SYSTEM

Epinephrine and norepinephrine dilate

(widen) airways during exercise and other

stresses

Erythropoietin regulates amount of

oxygen carried in blood by adjusting

number of red blood cells



SKELETAL

SYSTEM

Human growth hormone (hGH) and

insulinlike growth factors (IGFs) stimulate

bone growth

Estrogens cause closure of the epiphyseal

plates at the end of puberty and help

maintain bone mass in adults

Parathyroid hormone (PTH) and calcitonin

regulate levels of calcium and other

minerals in bone matrix and blood

Thyroid hormones are needed for normal

development and growth of the skeleton



DIGESTIVE

SYSTEM



MUSCULAR

SYSTEM

Epinephrine and norepinephrine help

increase blood flow to exercising muscle

PTH maintains proper level of Ca2+,

needed for muscle contraction

Glucagon, insulin, and other hormones

regulate metabolism in muscle fibers

hGH, IGFs, and thyroid hormones help

maintain muscle mass



NERVOUS

SYSTEM

Several hormones, especially thyroid

hormones, insulin, and growth hormone,

influence growth and development of the

nervous system

PTH maintains proper level of Ca2+,

needed for generation and conduction of

nerve impulses



CARDIOVASCULAR

SYSTEM

Erythropoietin (EPO) promotes formation

of red blood cells

Aldosterone and antidiuretic hormone

(ADH) increase blood volume

Epinephrine and norepinephrine increase

heart rate and force of contraction

Several hormones elevate blood pressure

during exercise and other stresses



CONTRIBUTIONS OF

THE



ENDOCRINE SYSTEM



FOR ALL BODY SYSTEMS

Together with the nervous system, circulating

and local hormones of the endocrine system

regulate activity and growth of target cells

throughout the body

Several hormones regulate metabolism,

uptake of glucose, and molecules used for

ATP production by body cells



Epinephrine and norepinephrine depress

activity of the digestive system

Gastrin, cholecystokinin, secretin, and

glucose-dependent insulinotropic

peptide (GIP) help regulate digestion

Calcitriol promotes absorption of dietary

calcium

Leptin suppresses appetite



URINARY

SYSTEM

ADH, aldosterone, and atrial natriuretic

peptide (ANP) adjust the rate of loss of

water and ions in the urine, thereby

regulating blood volume and ion content

of the blood



REPRODUCTIVE

SYSTEMS

Hypothalamic releasing and inhibiting

hormones, follicle-stimulating hormone

(FSH), and luteinizing hormone (LH)

regulate development, growth, and

secretions of the gonads (ovaries and

testes)

Estrogens and testosterone contribute to

development of oocytes and sperm and

stimulate development of secondary sex

characteristics

Prolactin promotes milk secretion in

mammary glands

Oxytocin causes contraction of the uterus

and ejection of milk from the mammary

glands



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CHAPTER 18







THE ENDOCRINE SYSTEM



Figure 18.22 Various endocrine disorders.

Disorders of the endocrine system often involve hyposecretion or hypersecretion of hormones.



(b) Acromegaly (excess hGH during adulthood)



(a) A 22-year-old man with pituitary giantism shown beside his

identical twin



(d) Exophthalmos (excess thyroid

hormones, as in Graves disease)



(c) Goiter (enlargement of

thyroid gland)



(e) Cushing’s syndrome (excess

glucocorticoids)



Which endocrine disorder is due to antibodies that mimic the action of TSH?



nose enlarge, and the skin thickens and develops furrows, especially

on the forehead and soles (Figure 18.22b).



Diabetes Insipidus

The most common abnormality associated with dysfunction of the

posterior pituitary is diabetes insipidus (DI) (dıˉ-a-BEˉ-teˉz in-SIP-i-dus;

diabetes ϭ overflow; insipidus ϭ tasteless). This disorder is due to

defects in antidiuretic hormone (ADH) receptors or an inability to

secrete ADH. Neurogenic diabetes insipidus results from hyposecretion of ADH, usually caused by a brain tumor, head trauma, or brain

surgery that damages the posterior pituitary or the hypothalamus. In

nephrogenic diabetes insipidus, the kidneys do not respond to ADH.

The ADH receptors may be nonfunctional, or the kidneys may be

damaged. A common symptom of both forms of DI is excretion of

large volumes of urine, with resulting dehydration and thirst. Bedwetting is common in afflicted children. Because so much water is

lost in the urine, a person with DI may die of dehydration if deprived

of water for only a day or so.

Treatment of neurogenic diabetes insipidus involves hormone replacement, usually for life. Either subcutaneous injection or nasal



spray application of ADH analogs is effective. Treatment of nephrogenic diabetes insipidus is more complex and depends on the nature

of the kidney dysfunction. Restriction of salt in the diet and, paradoxically, the use of certain diuretic drugs, are helpful.



Thyroid Gland Disorders

Thyroid gland disorders affect all major body systems and are among

the most common endocrine disorders. Congenital hypothyroidism,

hyposecretion of thyroid hormones that is present at birth, has devastating consequences if not treated promptly. Previously termed cretinism, this condition causes severe mental retardation and stunted bone

growth. At birth, the baby typically is normal because lipid-soluble

maternal thyroid hormones crossed the placenta during pregnancy

and allowed normal development. Most states require testing of all

newborns to ensure adequate thyroid function. If congenital hypothyroidism exists, oral thyroid hormone treatment must be started soon

after birth and continued for life.

Hypothyroidism during the adult years produces myxedema (mixe-DEˉ-ma), which occurs about five times more often in females than

in males. A hallmark of this disorder is edema (accumulation of inter-



DISORDERS: HOMEOSTATIC IMBALANCES



Hypoparathyroidism (hıˉ-poˉ-parЈ-a-THIˉ-royd-izm)—too little parathyroid hormone—leads to a deficiency of blood Ca2ϩ, which causes

neurons and muscle fibers to depolarize and produce action potentials spontaneously. This leads to twitches, spasms, and tetany

(maintained contraction) of skeletal muscle. The leading cause of

hypoparathyroidism is accidental damage to the parathyroid glands

or to their blood supply during thyroidectomy surgery.

Hyperparathyroidism, an elevated level of parathyroid hormone, most often is due to a tumor of one of the parathyroid

glands. An elevated level of PTH causes excessive resorption of bone

matrix, raising the blood levels of calcium and phosphate ions and

causing bones to become soft and easily fractured. High blood calcium level promotes formation of kidney stones. Fatigue, personality

changes, and lethargy are also seen in patients with hyperparathyroidism.



Adrenal Gland Disorders

Cushing’s Syndrome

Hypersecretion of cortisol by the adrenal cortex produces

Cushing’s syndrome (Figure 18.22e). Causes include a tumor of

the adrenal gland that secretes cortisol, or a tumor elsewhere that

secretes adrenocorticotropic hormone (ACTH), which in turn stimulates excessive secretion of cortisol. The condition is characterized by breakdown of muscle proteins and redistribution of body

fat, resulting in spindly arms and legs accompanied by a rounded

“moon face,” “buffalo hump” on the back, and pendulous (hanging) abdomen. Facial skin is flushed, and the skin covering the

abdomen develops stretch marks. The person also bruises easily,

and wound healing is poor. The elevated level of cortisol causes

hyperglycemia, osteoporosis, weakness, hypertension, increased

susceptibility to infection, decreased resistance to stress, and

mood swings. People who need long-term glucocorticoid therapy—for instance, to prevent rejection of a transplanted organ—

may develop a cushingoid appearance.



Hyposecretion of glucocorticoids and aldosterone causes Addison’s

disease (chronic adrenocortical insufficiency). The majority of

cases are autoimmune disorders in which antibodies cause adrenal

cortex destruction or block binding of ACTH to its receptors.

Pathogens, such as the bacterium that causes tuberculosis, also may

trigger adrenal cortex destruction. Symptoms, which typically do

not appear until 90% of the adrenal cortex has been destroyed,

include mental lethargy, anorexia, nausea and vomiting, weight

loss, hypoglycemia, and muscular weakness. Loss of aldosterone

leads to elevated potassium and decreased sodium in the blood,

low blood pressure, dehydration, decreased cardiac output,

arrhythmias, and even cardiac arrest. The skin may have a “bronzed”

appearance that often is mistaken for a suntan. Such was true in

the case of President John F. Kennedy, whose Addison’s disease

was known to only a few while he was alive. Treatment consists of

replacing glucocorticoids and mineralocorticoids and increasing

sodium in the diet.



Pheochromocytomas

Usually benign tumors of the chromaffin cells of the adrenal

medulla, called pheochromocytomas (feˉ-oˉ-kroˉЈ-moˉ-si-TO¯-mas;

pheo- ϭ dusky; -chromo- ϭ color; -cyto- ϭ cell), cause hypersecretion of epinephrine and norepinephrine. The result is a prolonged version of the fight-or-flight response: rapid heart rate, high blood

pressure, high levels of glucose in blood and urine, an elevated

basal metabolic rate (BMR), flushed face, nervousness, sweating,

and decreased gastrointestinal motility. Treatment is surgical

removal of the tumor.



Pancreatic Islet Disorders

The most common endocrine disorder is diabetes mellitus (MEL-itus; melli- ϭ honey sweetened), caused by an inability to produce or

use insulin. Diabetes mellitus is the fourth leading cause of death by

disease in the United States, primarily because of its damage to the

cardiovascular system. Because insulin is unavailable to aid transport

of glucose into body cells, blood glucose level is high and glucose

“spills” into the urine (glucosuria). Hallmarks of diabetes mellitus are

the three “polys”: polyuria, excessive urine production due to an inability of the kidneys to reabsorb water; polydipsia, excessive thirst;

and polyphagia, excessive eating.

Both genetic and environmental factors contribute to onset of the

two types of diabetes mellitus—type 1 and type 2—but the exact

mechanisms are still unknown. Type 1 diabetes, previously known

as insulin-dependent diabetes mellitus (IDDM), occurs because the

person’s immune system destroys the pancreatic beta cells. As a result, the pancreas produces little or no insulin. Type 1 diabetes usually

develops in people younger than age 20 and it persists throughout

life. By the time symptoms of type 1 diabetes arise, 80–90% of the

islet beta cells have been destroyed. Type 1 diabetes is most common

in northern Europe, especially in Finland, where nearly 1% of the

population develops type 1 diabetes by 15 years of age. In the United

States, type 1 diabetes is 1.5–2.0 times more common in whites than

in African American or Asian populations.

The cellular metabolism of an untreated type 1 diabetic is similar to

that of a starving person. Because insulin is not present to aid the entry

of glucose into body cells, most cells use fatty acids to produce ATP.

Stores of triglycerides in adipose tissue are catabolized to yield fatty

acids and glycerol. The by-products of fatty acid breakdown—organic

acids called ketones or ketone bodies—accumulate. Buildup of ketones

causes blood pH to fall, a condition known as ketoacidosis (keˉЈ-toˉ-as¯ -sis). Unless treated quickly, ketoacidosis can cause death.

i-DO



18



Parathyroid Gland Disorders



Addison’s Disease



C H A P T E R



stitial fluid) that causes the facial tissues to swell and look puffy. A

person with myxedema has a slow heart rate, low body temperature,

sensitivity to cold, dry hair and skin, muscular weakness, general lethargy, and a tendency to gain weight easily. Because the brain has already reached maturity, mental retardation does not occur, but the

person may be less alert. Oral thyroid hormones reduce the symptoms.

The most common form of hyperthyroidism is Graves disease,

which also occurs seven to ten times more often in females than in

males, usually before age 40. Graves disease is an autoimmune disorder in which the person produces antibodies that mimic the action of

thyroid-stimulating hormone (TSH). The antibodies continually stimulate the thyroid gland to grow and produce thyroid hormones. A

primary sign is an enlarged thyroid, which may be two to three times

its normal size. Graves patients often have a peculiar edema behind

the eyes, called exophthalmos (ekЈ-sof-THAL-mos), which causes the

eyes to protrude (Figure 18.22d). Treatment may include surgical removal of part or all of the thyroid gland (thyroidectomy), the use of

radioactive iodine (131I) to selectively destroy thyroid tissue, and the

use of antithyroid drugs to block synthesis of thyroid hormones.

A goiter (GOY-ter; guttur ϭ throat) is simply an enlarged thyroid

gland. It may be associated with hyperthyroidism, hypothyroidism, or

euthyroidism (uˉ-THI¯ -royd-izm; eu ϭ good), which means normal

secretion of thyroid hormone. In some places in the world, dietary

iodine intake is inadequate; the resultant low level of thyroid hormone in the blood stimulates secretion of TSH, which causes thyroid

gland enlargement (Figure 18.22c).



655



656



CHAPTER 18







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The breakdown of stored triglycerides also causes weight loss. As

lipids are transported by the blood from storage depots to cells, lipid

particles are deposited on the walls of blood vessels, leading to atherosclerosis and a multitude of cardiovascular problems, including cerebrovascular insufficiency, ischemic heart disease, peripheral vascular disease, and gangrene. A major complication of diabetes is loss of vision

due either to cataracts (excessive glucose attaches to lens proteins, causing cloudiness) or to damage to blood vessels of the retina. Severe

kidney problems also may result from damage to renal blood vessels.

Type 1 diabetes is treated through self-monitoring of blood glucose

level (up to 7 times daily), regular meals containing 45–50% carbohydrates and less than 30% fats, exercise, and periodic insulin injections

(up to 3 times a day). Several implantable pumps are available to provide insulin without the need for repeated injections. Because they

lack a reliable glucose sensor, however, the person must self-monitor

blood glucose level to determine insulin doses. It is also possible to

successfully transplant a pancreas, but immunosuppressive drugs must

then be taken for life. Another promising approach under investigation is transplantation of isolated islets in semipermeable hollow

tubes. The tubes allow glucose and insulin to enter and leave but prevent entry of immune system cells that might attack the islet cells.

Type 2 diabetes, formerly known as non-insulin-dependent

diabetes mellitus (NIDDM), is much more common than type 1, representing more than 90% of all cases. Type 2 diabetes most often occurs

in obese people who are over age 35. However, the number of obese



children and teenagers with type 2 diabetes is increasing. Clinical

symptoms are mild, and the high glucose levels in the blood often can

be controlled by diet, exercise, and weight loss. Sometimes, drugs

such as glyburide (DiaBeta®) and metformin (Fortamet®) are used to

stimulate secretion of insulin by pancreatic beta cells. Although some

type 2 diabetics need insulin, many have a sufficient amount (or even

a surplus) of insulin in the blood. For these people, diabetes arises not

from a shortage of insulin but because target cells become less sensitive to it due to down-regulation of insulin receptors.

Hyperinsulinism most often results when a diabetic injects too

much insulin. The main symptom is hypoglycemia, decreased blood

glucose level, which occurs because the excess insulin stimulates too

much uptake of glucose by body cells. The resulting hypoglycemia

stimulates the secretion of epinephrine, glucagon, and human

growth hormone. As a consequence, anxiety, sweating, tremor, increased heart rate, hunger, and weakness occur. When blood glucose

falls, brain cells are deprived of the steady supply of glucose they need

to function effectively. Severe hypoglycemia leads to mental disorientation, convulsions, unconsciousness, and shock. Shock due to an insulin overdose is termed insulin shock. Death can occur quickly unless

blood glucose is restored to normal levels. From a clinical standpoint,

a diabetic suffering from either a hyperglycemia or a hypoglycemia

crisis can have very similar symptoms—mental changes, coma, seizures, and so on. It is important to quickly and correctly identify the

cause of the underlying symptoms and treat them appropriately.



MEDICAL TERMINOLOGY

Gynecomastia (gıˉЈ-ne-koˉ-MAS-teˉ-a; gyneco- ϭ woman; -mast- ϭ

breast) Excessive development of mammary glands in a male.

Sometimes a tumor of the adrenal gland may secrete sufficient

amounts of estrogen to cause the condition.

Hirsutism (HER-soo-tizm; hirsut- ϭ shaggy) Presence of excessive

body and facial hair in a male pattern, especially in women; may

be due to excess androgen production due to tumors or drugs.

Thyroid crisis (storm) A severe state of hyperthyroidism that can be

life-threatening. It is characterized by high body temperature,



rapid heart rate, high blood pressure, gastrointestinal symptoms

(abdominal pain, vomiting, diarrhea), agitation, tremors, confusion, seizures, and possibly coma.

Virilizing adenoma (aden- ϭ gland; -oma ϭ tumor) Tumor of the

adrenal gland that liberates excessive androgens, causing virilism

(masculinization) in females. Occasionally, adrenal tumor cells

liberate estrogens to the extent that a male patient develops gynecomastia. Such a tumor is called a feminizing adenoma.



C H A P T E R R E V I E W A N D R E S O U R C E S U M M A RY

Review



Resource



Introduction



Anatomy Overview - The Endocrine

System



1. Hormones regulate the activity of smooth muscle, cardiac muscle, and some glands; alter metabolism; spur

growth and development; influence reproductive processes; and participate in circadian (daily) rhythms.



18.1 Comparison of Control by the Nervous and Endocrine Systems

1. The nervous system controls homeostasis through nerve impulses and neurotransmitters, which act

locally and quickly. The endocrine system uses hormones, which act more slowly in distant parts of the

body. (See Table 18.1.)

2. The nervous system controls neurons, muscle cells, and glandular cells; the endocrine system regulates

virtually all body cells.



18.2 Endocrine Glands

1. Exocrine glands (sudoriferous, sebaceous, mucous, and digestive) secrete their products through ducts

into body cavities or onto body surfaces. Endocrine glands secrete hormones into interstitial fluid. Then,

the hormones diffuse into the blood.

2. The endocrine system consists of endocrine glands (pituitary, thyroid, parathyroid, adrenal, and pineal

glands) and other hormone-secreting tissues (hypothalamus, thymus, pancreas, ovaries, testes, kidneys,

stomach, liver, small intestine, skin, heart, adipose tissue, and placenta).



Anatomy Overview - The Nervous

System



Anatomy Overview - The Endocrine

System



CHAPTER REVIEW AND RESOURCE SUMMARY



657



Review



Resource



18.3 Hormone Activity



Anatomy Overview - Hormones

Anatomy Overview - Lipid-Soluble

Hormones

Anatomy Overview - WaterSoluble Hormones

Anatomy Overview - Local

Hormones

Animation - Mechanisms of

Hormone Action



1. Lipid-soluble steroid hormones and thyroid hormones affect cell function by altering gene expression.

2. Water-soluble hormones alter cell function by activating plasma membrane receptors, which elicit production of a second messenger that activates various enzymes inside the cell.

3. Hormonal interactions can have three types of effects: permissive, synergistic, or antagonistic.



18.5



Homeostatic Control of Hormone Secretion



1. Hormone secretion is controlled by signals from the nervous system, chemical changes in blood, and

other hormones.

2. Negative feedback systems regulate the secretion of many hormones.



18.6 Hypothalamus and Pituitary Gland

1. The hypothalamus is the major integrating link between the nervous and endocrine systems. The

hypothalamus and pituitary gland regulate virtually all aspects of growth, development, metabolism,

and homeostasis. The pituitary gland is located in the hypophyseal fossa and is divided into two main

portions: the anterior pituitary (glandular portion) and the posterior pituitary (nervous portion).

2. Secretion of anterior pituitary hormones is stimulated by releasing hormones and suppressed by inhibiting hormones from the hypothalamus.

3. The blood supply to the anterior pituitary is from the superior hypophyseal arteries. Hypothalamic

releasing and inhibiting hormones enter the primary plexus and flow to the secondary plexus in the

anterior pituitary by the hypophyseal portal veins.

4. The anterior pituitary consists of somatotrophs that produce human growth hormone (hGH), lactotrophs

that produce prolactin (PRL), corticotrophs that secrete adrenocorticotropic hormone (ACTH) and

melanocyte-stimulating hormone (MSH), thyrotrophs that secrete thyroid-stimulating hormone (TSH),

and gonadotrophs that synthesize follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

(See Tables 18.3 and 18.4.)

5. Human growth hormone (hGH) stimulates body growth through insulinlike growth factors (IGFs).

Secretion of hGH is inhibited by GHIH (growth hormone–inhibiting hormone, or somatostatin) and

promoted by GHRH (growth hormone–releasing hormone).

6. TSH regulates thyroid gland activities. Its secretion is stimulated by TRH (thyrotropin-releasing hormone) and suppressed by GHIH.

7. FSH and LH regulate the activities of the gonads—ovaries and testes. Their secretion is controlled by

GnRH (gonadotropin-releasing hormone).

8. Prolactin (PRL) helps initiate milk secretion. Prolactin-inhibiting hormone (PIH) suppresses secretion

of PRL; prolactin-releasing hormone (PRH) stimulates PRL secretion.

9. ACTH regulates the activities of the adrenal cortex and is controlled by CRH (corticotropin-releasing

hormone). Dopamine inhibits secretion of MSH.

10. The posterior pituitary contains axon terminals of neurosecretory cells whose cell bodies are in the

hypothalamus. Hormones made by the hypothalamus and stored in the posterior pituitary are oxytocin

(OT), which stimulates contraction of the uterus and ejection of milk from the breasts, and antidiuretic

hormone (ADH), which stimulates water reabsorption by the kidneys and constriction of arterioles. (See

Table 18.5.) Oxytocin secretion is stimulated by uterine stretching and suckling during nursing; ADH

secretion is controlled by osmotic pressure of the blood and blood volume.



18.7 Thyroid Gland

1. The thyroid gland is located inferior to the larynx.

2. It consists of thyroid follicles composed of follicular cells, which secrete the thyroid hormones thyroxine (T4) and triiodothyronine (T3), and parafollicular cells, which secrete calcitonin (CT).



Animation – Introduction to Hormonal

Regulation, Secretion, and

Concentration

Exercise - Hormone Actions



Animation - Introduction to Hormone

Feedback Loops

Exercise - Produce That Hormone



Anatomy Overview - The Hypothalamus

and Pituitary Gland

Anatomy Overview - Hormones of the

Anterior Pituitary Gland

Anatomy Overview - The Hypothalamus

Anatomy Overview - Hormones of the

Hypothalamus

Anatomy Overview - Hypothalamic

Reproductive Hormones

Animation - hGH: Growth and

Development

Animation - GHRH/hGH

Animation - ACTH/Cortisol:

Glycogenolysis

Animation - TRH/TSH: Production

Animation - hGH: Glycogenolysis and

Lipolysis

Animation - Antidiuretic Hormonee

Figure 18.5 - Hypothalamus andd

Pituitary Gland and Their

Blood Supply



Anatomy Overview - The Thyroid

Animation - Thyroid Hormones and

Glucose and Lipid Catabolism

Animation - TRH/TSH



18



18.4 Mechanisms of Hormone Action



C H A P T E R



1. Hormones affect only specific target cells that have receptors to recognize (bind) a given hormone. The

number of hormone receptors may decrease (down-regulation) or increase (up-regulation).

2. Circulating hormones enter the bloodstream; local hormones (paracrines and autocrines) act locally on

neighboring cells.

3. Chemically, hormones are either lipid-soluble (steroids, thyroid hormones, and nitric oxide) or watersoluble (amines; peptides, proteins, and glycoproteins; and eicosanoids). (See Table 18.2.)

4. Water-soluble hormone molecules circulate in the watery blood plasma in a “free” form (not attached to

plasma proteins); most lipid-soluble hormones are bound to transport proteins synthesized by the liver.



658



CHAPTER 18







THE ENDOCRINE SYSTEM



Review

3. Thyroid hormones are synthesized from iodine and tyrosine within thyroglobulin (TGB). They are

transported in the blood bound to plasma proteins, mostly thyroxine-binding globulin (TBG).

4. Secretion is controlled by TRH from the hypothalamus and thyroid-stimulating hormone (TSH) from

the anterior pituitary.

5. Thyroid hormones regulate oxygen use and metabolic rate, cellular metabolism, and growth and development.

6. Calcitonin (CT) can lower the blood level of calcium ions (Ca2ϩ) and promote deposition of Ca2ϩ into

bone matrix. Secretion of CT is controlled by the Ca2ϩ level in the blood. (See Table 18.6.)



18.8 Parathyroid Glands

1. The parathyroid glands are embedded in the posterior surfaces of the lateral lobes of the thyroid gland.

They consist of chief cells and oxyphil cells.

2. Parathyroid hormone (PTH) regulates the homeostasis of calcium, magnesium, and phosphate ions by

increasing blood calcium and magnesium levels and decreasing blood phosphate levels. PTH secretion

is controlled by the level of calcium in the blood. (See Table 18.7.)



18.9 Adrenal Glands

1. The adrenal glands are located superior to the kidneys. They consist of an outer adrenal cortex and inner

adrenal medulla.

2. The adrenal cortex is divided into a zona glomerulosa, a zona fasciculata, and a zona reticularis; the

adrenal medulla consists of chromaffin cells and large blood vessels.

3. Cortical secretions include mineralocorticoids, glucocorticoids, and androgens.

4. Mineralocorticoids (mainly aldosterone) increase sodium and water reabsorption and decrease potassium reabsorption. Secretion is controlled by the renin–angiotensin–aldosterone (RAA) pathway and by

Kϩ level in the blood.

5. Glucocorticoids (mainly cortisol) promote protein breakdown, gluconeogenesis, and lipolysis; help

resist stress; and serve as anti-inflammatory substances. Their secretion is controlled by ACTH.

6. Androgens secreted by the adrenal cortex stimulate growth of axillary and pubic hair, aid the prepubertal

growth spurt, and contribute to libido.

7. The adrenal medulla secretes epinephrine and norepinephrine (NE), which are released during stress and

produce effects similar to sympathetic responses. (See Table 18.8.)



18.10 Pancreatic Islets

1. The pancreas lies in the curve of the duodenum. It has both endocrine and exocrine functions.

2. The endocrine portion consists of pancreatic islets or islets of Langerhans, made up of four types of

cells: alpha, beta, delta, and F cells.

3. Alpha cells secrete glucagon, beta cells secrete insulin, delta cells secrete somatostatin, and F cells

secrete pancreatic polypeptide.

4. Glucagon increases blood glucose level; insulin decreases blood glucose level. Secretion of both hormones is controlled by the level of glucose in the blood. (See Table 18.9.)



18.11 Ovaries and Testes

1. The ovaries are located in the pelvic cavity and produce estrogens, progesterone, and inhibin.

These sex hormones govern the development and maintenance of female secondary sex characteristics, reproductive cycles, pregnancy, lactation, and normal female reproductive functions. (See

Table 18.10.)

2. The testes lie inside the scrotum and produce testosterone and inhibin. These sex hormones govern

the development and maintenance of male secondary sex characteristics and normal male reproductive

functions. (See Table 18.10.)



18.12 Pineal Gland and Thymus

1. The pineal gland is attached to the roof of the third ventricle of the brain. It consists of secretory cells

called pinealocytes, neuroglia, and endings of sympathetic postganglionic axons.

2. The pineal gland secretes melatonin, which contributes to setting the body’s biological clock (controlled

in the suprachiasmatic nucleus). During sleep, plasma levels of melatonin increase.

3. The thymus secretes several hormones related to immunity.

4. Thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin promote the maturation

of T cells.



Resource

Animation - Calcitonin

Figure 18.11 - Steps in the

Synthesis and Secretion of

Thyroid Hormones

Exercise - Calcium Homeostasis



Anatomy Overview - Parathyroid Glands

Anatomy Overview - Hormones of the

Parathyroid

Animation - Parathyroid Hormone



Anatomy Overview - Adrenal Glands

Anatomy Overview - Hormones of the

Adrenal Glands

Animation - Epinephrine/NE

Figure 18.15 - The Adrenal

Glands

Figure 18.16 - Regulation of

Aldosterone Secretion



Anatomy Overviews: The Pancreas;

Hormones of the Pancreas

Animations: Glucagon; Insulin

Figure 18.18 - The Pancreas

Figure 18.19 - Secretion of

Glucagon and Insulin

Exercise - Glucose Regulation

Feedback Loop

Concepts and Connections - Blood

Glucose Regulation

Anatomy Overviews: The Ovaries;

Ovarian Hormones; The Testes;

Testicular Hormones

Animations: Hormonal Control of Male

Reproductive Function; Hormonal

Regulation of Female Reproductive

System

Exercises: Match the Female Hormones;

Match the Male Hormones



Anatomy Overview - The Pineal Gland



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