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A mother spotted hyena (Crocuta crocuta) and cubs.This mother hyena cares for her cubs, a behavior that is modulated by the endocrine system.© 2010 Nature Publishing Group Höner, O. et al. The fitness of dispersing spotted hyaena sons is influenced bymaternal social status. Nature Communications 1, (2010) doi:10.1038/ncomms1059. Used with permission.

139 Endocrine System

The endocrine system interacts with the nervous system to maintain homeostasis,regulate growth and development, and modulate behavior.

Topics Covered in this Module Endocrine System FunctionMajor Organs of the Endocrine SystemThe Hypothalamus and Pituitary GlandsThe Thyroid and Parathyroid GlandsThe GonadsThe Adrenal GlandThe Pineal GlandThe Pancreas

Major Objectives of this Module Give examples of how the endocrine system uses hormones to regulate homeostasis.Explain how certain behaviors are triggered by hormones.Describe how hormones regulate reproduction and the development of sex characteristics.Describe the major glands of the endocrine system.

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Figure 1: Location of major endocrine organs in the human body.© 2014 Nature Education All rights reserved.

Endocrine System FunctionThe endocrine system produces a variety of complex responses to stimuli. Some responses are immediate andtemporary, such as the increases in heart, respiratory, and metabolic rates associated with an "adrenaline rush," whileothers are long term, such as the testosterone and estrogenassociated development of secondary sexualcharacteristics during puberty. Organs of the endocrine system are ductless glands called endocrine glands. In additionto dedicated glands, specialized hormoneproducing cells within nonendocrine organs such as the intestines, heart,and kidney are also part of the endocrine system. In response to certain stimuli, endocrine cells secrete hormones intothe bloodstream. From there, hormones travel to their target cells in tissues throughout the body.

Major Organs of the Endocrine SystemThe location of major endocrine glands in the human body is summarized in Figure 1. Three of these glands, thehypothalamus, the pituitary gland, and the pineal gland, are found in the brain. The thyroid and parathyroid are locatedin the neck. The pancreas is located beneath the stomach. The adrenal glands sit on top of the kidneys. The ovaries arelocated on either side of the uterus, and the testes are located in the scrotum. Endocrine cells are also associated withthe thymus (which is part of the immune system), heart, liver, stomach, kidneys, and small intestine.

Some hormones, called tropic hormones, regulate the secretion of hormones by another endocrine gland or cell. Thehypothalamus, which is the regulatory center of the endocrine system, releases two types of tropic hormones thatregulate hormone production by the anterior pituitary: releasing hormones that stimulate hormone production andinhibiting hormones that repress hormone production. Nontropic hormones are hormones that directly produce aneffect on target cells. The hormones produced by the major endocrine glands are summarized in Table 1.


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Table 1: Hormones produced by major organs of the endocrine system.© 2014 Nature Education All rights reserved.

Endocrine System Function

Major Organs of the Endocrine System

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The Hypothalamus and Pituitary Glands

The Thyroid and Parathyroid Glands

The Gonads

The Adrenal Gland

The Pineal Gland

The Pancreas

Summary

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The Hypothalamus and Pituitary GlandsMost activities of the endocrine system are directly or indirectly regulated by the hypothalamus, a region in the braincritical for maintaining homeostasis. Importantly, the hypothalamus also serves as the primary link between the nervoussystem and the endocrine system. The pituitary gland, which hangs beneath the hypothalamus, has anterior andposterior lobes.

The posterior pituitary is an extension of the hypothalamus. Specialized neurons, called neurosecretory neurons,extend from the hypothalamus into the posterior pituitary. These neurons produce two hormones, oxytocin andvasopressin (also called antidiuretic hormone, or ADH). Oxytocin promotes maternal bonding and milk production andstimulates contractions during labor. Vasopressin regulates water and salt balance.

The anterior pituitary, which is regulated by releasing and inhibiting hormones secreted from the hypothalamus,produces both tropic and nontropic hormones. Thyroidstimulating hormone (TSH) is considered a tropic hormonebecause it stimulates the production of thyroid hormones by the thyroid gland. Thyrotropinreleasing hormone (TRH), areleasing hormone produced by the hypothalamus, stimulates TSH secretion. Growth hormone (GH), which directlystimulates growth and cell division in a wide variety of cells, is an example of a nontropic hormone.

Pituitary hormones and behavior.Some hormones secreted by the pituitary affect behavior. For example, oxytocin, which is secreted by the posteriorpituitary, and prolactin, which is secreted by the anterior pituitary, are both involved in parental bonding. Recent studiesindicate that prolactin may even be involved in parental bonding in nonmammals. For example, a study of the male

North American bluegill fish (Lepomis macrochirus) indicates that disruption of prolactin signaling alters behaviorassociated with paternal care. Male bluegills take exclusive responsibility for care of eggs. When a prolactin antagonist(a drug that prevents prolactin from binding its receptor) was administered to male fish caring for eggs, behaviorassociated with paternal care, such as fanning eggs, diminished.





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The Thyroid and Parathyroid GlandsThe thyroid gland, located in the neck in humans, plays an important role in metabolism. The thyroid secretes twothyroid hormones, triiodothyronine (T3) and thyroxine (T4), which increase the metabolic rate of most cells in the body.The thyroid also produces calcitonin, a hormone that reduces the concentration of calcium ions in the blood. Calcitonininhibits calcium ion absorption in the intestines and calcium ion reabsorption by the kidneys. Calcitonin also inhibits theaction of osteoclasts, cells that reabsorb (break down) bone tissue. However, in humans, calcitonin appears to play arelatively minor role in calcium ion homeostasis.

Parathyroid hormone (PTH), which is produced by four small glands, called parathyroid glands, that are located on thesurface of the thyroid, increases serum calcium ion levels. PTH promotes calcium ion absorption by the intestines andreabsorption by the kidneys and stimulates bone reabsorption by osteoclasts.

Thyroid hormones play critical roles in the development of many animals. In particular, thyroid hormones direct themetamorphosis of animals such as amphibians. For example, thyroid hormones induce metamorphosis of tadpoles intofrogs.

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What are the three ways that PTH increases blood calcium ion levels?





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The GonadsThe gonads produce steroid sex hormones that are responsible for development of both primary sexual characteristics,which are formed during embryonic development, and secondary sexual characteristics, which develop during puberty.The role of gonads in determination of primary sex characteristics was established in the 1940s by Alfred Jost, a Frenchendocrinologist. In rabbits as in humans, animals with two X chromosomes (XX) normally develop into females, andanimals with an X chromosome and a Y chromosome (XY) normally develop into males. Jost castrated male (XY)rabbits during embryonic development and found that they developed female sexual characteristics. From theseobservations, Jost hypothesized that the testes secreted a substance that "masculinized" the embryos. The substanceresponsible for masculinization was later found to contain hormones from a group of male sex steroid hormonescollectively known as androgens. Androgens, which are produced in the testes, promote sperm development as well asdevelopment of male sexual characteristics. In humans, the primary androgen is testosterone. Androgens are one ofthree classes of steroid sex hormones. The other two are estrogens, which promote development of female sexualcharacteristics, and progestins. In humans, an estrogen called estradiol and a progestin called progesterone regulatethe menstrual cycle.

Test Yourself

Based on the results of Jost's experiments, what outcome would you expect if the testes of anXY (male) fetus failed to develop normally?

Starting at puberty, the hypothalamus begins secreting gonadotropinreleasing hormone (GnRH), which mediatesdevelopment of secondary sex characteristics in both males and females (Figure 2). GnRH stimulates the anteriorpituitary to release two gonadotropins, luteinizing hormone (LH) and folliclestimulating hormone (FSH).Gonadotropins are peptide hormones that target the gonads. In males, these hormones act on two different cell typesfound in the testes. FSH stimulates Sertoli cells, which nourish sperm cells. LH causes the Leydig cells to secretetestosterone. The appearance of male secondary sex characteristics, such as facial hair, increased muscle mass, anddeepening of the voice, is primarily due to the increase in testosterone levels. Testosterone also promotes sperm celldevelopment, or spermatogenesis.

Sex steroid hormone production is regulated by a negative feedback loop. Testosterone inhibits production of GnRH bythe hypothalamus and of LH and FSH by the anterior pituitary. Sertoli cells also produce the hormone called inhibin thatinhibits anterior pituitary secretion of FSH and LH.

In females, LH and FSH regulate the menstrual cycle. In the ovaries, FSH stimulates the maturation of follicles, whichconsist of an oocyte (egg) and support cells. Maturing follicles produce the estrogen estradiol. Estrogen causes theuterine lining, called the endometrium, to grow. As the follicle matures, it produces more estradiol. Low levels ofestradiol inhibit FSH and LH production, but high levels stimulate production of these hormones. Thus, as the folliclematures, increased estradiol levels begin to stimulate FSH and LH production. A surge in FSH and LH coincides withovulation, the rupture of the follicle that releases the egg into the reproductive tract. The remnants of the ruptured follicledifferentiate into the corpus luteum, an endocrine structure that secretes estrogen and progesterone, another female sexsteroid hormone. Together, estrogen and progesterone inhibit the secretion of FSH and LH from the anterior pituitary,preventing the maturation of another follicle.

If the egg is fertilized, the developing embryo secretes human chorionic gonadotropin (hCG), a hormone that causes thecorpus luteum to continue secreting progesterone, allowing the body to maintain the pregnancy. If the egg remainsunfertilized, the corpus luteum eventually degenerates, and estrogen and progesterone levels fall. Without thesehormones, the uterine lining sloughs off during the process of menstruation. At this point LH and FSH secretion is nolonger inhibited and the next round of ovarian follicle development begins.






Figure 2: Starting at puberty, the hypothalamus stimulates production of steroid sex hormones in the gonads.

© 2014 Nature Education All rights reserved.

At puberty, the hypothalamus begins synthesizing GnRH. GnRH stimulates the anterior pituitary to secrete LH andFSH. In males, LH and FSH stimulate the production of testosterone in testes. In females, LH and FSH stimulate theproduction of estradiol in ovaries.

Anabolic steroids.Anabolic steroids are synthetic steroid hormones that mimic testosterone. Like natural testosterone, anabolic steroidsbuild muscle mass and enhance endurance, and for this reason, they have become popular among athletes. However,to have the desired effect, anabolic steroids are often taken in very large doses, which can cause adverse side effects,such as increased aggression. In women, use of anabolic steroids, which are androgens, may disrupt the menstrualcycle and disrupt follicle maturation, resulting in infertility. In men, anabolic steroid use may result in testicular atrophy(shrinking of the testes), erectile dysfunction (inability to initiate or maintain a penile erection), and reduced sperm count.

Because anabolic steroids are androgens, it is not surprising that they impair the female reproductive cycle, but why dothey impair the male reproductive cycle? Like testosterone, anabolic steroids inhibit the production of GnRH. Without thestimulating effects of LH and FSH, testicular tissue atrophies, and both spermatogenesis and testosterone synthesis inthe testes are reduced. Usually, the negative effects of anabolic steroids are reversed a few months after drug use isstopped.





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Figure 3: The adrenal gland mediates acute and longterm stress response.

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During acute stress, neural signals from the hypothalamus stimulate the adrenal medulla to secrete epinephrineand norepinephrine. During longterm stress, the hypothalamus causes the anterior pituitary to secrete a hormonalmessage, which stimulates the adrenal cortex to secrete glucocorticoids.

The Adrenal GlandThe adrenal glands, which sit above the kidneys, mediate the response to stress. The adrenal gland has two parts: aninner adrenal medulla that mediates the response to acute stress and an outer adrenal cortex that mediates theresponse to longterm stress.

The hypothalamus regulates both parts of the adrenal glands, but the mechanism of activation differs. During an acutelystressful situation (for example, slipping at the edge of a steep cliff), the hypothalamus sends a nerve signal to theadrenal medulla via the spinal cord. In response, chromaffin cells of the adrenal medulla secrete epinephrine (alsoknown as adrenaline) and norepinephrine (also known as noradrenaline). Epinephrine and norepinephrine activate the"fightorflight" response, which increases heart and respiratory rate and causes the breakdown of glycogen and fats(Figure 3a).

During longterm stress, such as the loss of a job, the hypothalamus secretes corticotropinreleasing hormone (CRH),which in turn stimulates the release of adrenocorticotropic hormone (ACTH) from the anterior pituitary. ACTH travels tothe adrenal cortex, where it stimulates the secretion of glucocorticoids. The glucocorticoids, the most significant of whichis the steroid hormone cortisol, suppress some immune functions and increase blood glucose levels (Figure 3b).Increased levels of glucocorticoids are associated with improved memory and vigilance, which are presumably neededto get an animal through a stressful situation.

Glucocorticoids are members of a larger class of steroid hormones known as the corticosteroids, so named becausethey are produced primarily by the adrenal cortex. Corticosteroids also include the mineralocorticoids, which regulatewater balance and concentration of sodium and potassium "mineral" ions. A mineralocorticoid called aldosterone issecreted in response to increased serum potassium levels or the presence of a hormone called angiotensin II.Angiotensin II is produced in the kidneys in response to a decrease in blood pressure or blood volume. The adrenalgland also produces small amounts of androgens.

Glucocorticoids and behavior.A 2004 study led by Menno Kruk of the Leiden/Amsterdam Center for Drug Research in the Netherlands indicates thatthe adrenal gland may be involved in regulation of aggressive behavior. Kruk found that electrical stimulation of aregion of the rat hypothalamus associated with aggression caused aggressive behavior. After the electrical stimuluswas applied, serum levels of corticosterone, which is produced by the adrenal cortex, increased. Next, Kruk removedthe adrenal glands so that the rats were unable to produce corticosterone. In rats whose adrenal glands were removed,electrical stimulation of the hypothalamus did not result in aggressive behavior. However, aggressive behavior in theserats could be induced by corticosterone injections. Taken together, these results suggest that corticosterones areinvolved in aggressive behavior. Stress is associated with aggression in humans, and the results of this study provide a








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Addison's Disease Is Caused by Adrenal InsufficiencyA woman is rushed to the emergency room suffering from extreme fatigue, severe vomiting and diarrhea, low bloodpressure, and low blood glucose. A blood test reveals that the woman has abnormally elevated ACTH, extremely lowcortisol, and an increase in the number of inflammatory immune cells. These symptoms support a diagnosis ofAddison's disease, a disorder that is typically caused by autoimmune destruction of cells of the adrenal cortex. Theadrenal cortex produces cortisol and other glucocorticoids, which regulate glucose metabolism and suppress theimmune system, and mineralocorticoids, which regulate salt and water balance. Thus, the observed symptoms of lowblood pressure, low blood glucose, and increased inflammatory cells result from reduced levels of glucocorticoids. Theanterior pituitary produces more ACTH in an attempt to increase cortisol production, resulting in the observed highserum ACTH levels. Addison's disease progresses slowly with symptoms generally not appearing until approximately90% of the adrenal cortex has been destroyed. Patients treated with steroid supplements can live a reasonably normallife.

Test Yourself

Addison's disease is a form of adrenal insufficiency, or inability of the adrenal cortex toproduce steroid hormones. Given your understanding of the triggers for cortisol secretion,can you predict another cause of adrenal insufficiency that does NOT involve destruction ofthe adrenal cortex?





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The Pancreas

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The Pineal GlandThe pineal gland is a peasized endocrine gland located deep within the brain that produces the hormone melatonin,which is synthesized from serotonin. Light inhibits melatonin production, and melatonin secretion correlates closely withthe 24hour diurnal cycle of day and night. Melatonin controls circadian cycles such the sleep cycle and daily changesin body temperature.

The PancreasThe pancreas produces two hormones, insulin and glucagon, that regulate blood sugar levels. Insulin promotes theuptake of glucose by cells and induces the liver to make glycogen, thereby lowering blood sugar levels. Glucagoninhibits the uptake of sugar by cells and induces the liver to break down glycogen, thereby raising blood sugar levels.





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1/29/2015 Summary of Endocrine System | Principles of Biology from Nature Education




OBJECTIVE Describe the major glands of the endocrine system.The hypothalamus, located in the brain, is the regulatory center of the endocrine system and integrates the nervous andendocrine systems. The hypothalamus secretes hormones, including oxytocin and antidiuretic hormone (ADH), into theposterior pituitary. The hypothalamus also secretes releasing and inhibiting hormones that regulate hormone secretionby the anterior pituitary. The anterior pituitary secretes both tropic hormones, which regulate secretion of hormones byother endocrine glands, and nontropic hormones, which directly affect cells of the body. The thyroid, located in the neckin humans, secretes hormones that regulate metabolism, development and calcium levels. The parathyroid glands,located on the thyroid gland, secrete a hormone that regulates blood calcium levels. The pancreas, located beneath thestomach, secretes hormones that regulate blood glucose levels. The adrenal glands secrete hormones that regulatestress, aggression, and salt balance. The gonads, including the testes and ovaries, secrete steroid sex hormones andproduce gametes. The pineal gland secretes melatonin, which regulates circadian rhythms.

OBJECTIVE Give examples of how the endocrine system uses hormones to regulate homeostasis.A major function of the endocrine system is to maintain homeostasis, often through negative feedback loops. Incoordination with the hypothalamus and anterior pituitary, the thyroid gland secretes thyroid hormones to regulatemetabolic rate. The thyroid also secretes calcitonin in response to elevated serum calcium ion levels; the actions ofcalcitonin are opposed by parathyroid hormone. The production of testosterone operates through a similar negativefeedback system; increased testosterone levels inhibit the production of gonadotropinreleasing hormone in thehypothalamus and the production of FSH/LH in the anterior pituitary, which prevents further stimulation of testosteroneproduction in the Leydig cells of the testes.

OBJECTIVE Explain how certain behaviors are triggered by hormones.Environmental and internal stimuli can induce the production of hormones, which in turn can drive behavior. Stressgenerates two responses in the adrenal gland: a rapid response to acute stress mediated by epinephrine andnorepinephrine produced in the adrenal medulla, and a slower, longterm response to chronic stress mediated bycorticosteroids produced in the adrenal cortex. Prolactin and oxytocin, which are secreted from the pituitary, are involvedin the development of parental behavior.

OBJECTIVE Describe how hormones regulate reproduction and the development of sex characteristics.Androgens, the male sex steroid hormones, and estrogens, the female sex steroids, control the development of bothprimary and secondary sex characteristics. During embryonic development, testosterone stimulates the development ofmale sex characteristics. At puberty, sex hormones drive the development of secondary sex characteristics in both malesand females. Sex hormone production is regulated by gonadotropinreleasing hormone produced by the hypothalamus,which stimulates production of the gonadotropins FSH and LH from the anterior pituitary. FSH and LH stimulate theproduction of gametes and sex hormones in the gonads (testes in males and ovaries in females). Cyclic fluctuations inFSH, LH, and female sex steroids are responsible for the menstrual cycle in women.

adrenal cortexThe outer layer of the adrenal gland; produces cortisol, aldosterone, and a small amount of sex hormones;biologically distinct from the adrenal medulla and produces different hormones.

adrenal medullaThe inner layer of the adrenal gland; produces epinephrine and norepinephrine; biologically distinct from theadrenal cortex and produces different hormones.

aldosteroneA steroid hormone produced in the adrenal cortex that increases the reabsorption of sodium ions by the distalconvoluted tubule and collecting duct; indirectly promotes the reuptake of water by the kidney.

androgenAny of the male sex steroid hormones that influence the development of male primary and secondary sexcharacteristics; examples include testosterone and androsterone.

anterior pituitaryOne of two lobes of the pituitary gland; secretes various hormones in response to releasing hormones from thehypothalamus.

calcitoninA peptide hormone produced in the thyroid gland that lowers circulating calcium levels by inhibiting osteoclastactivity and calcium absorption in the kidneys and intestines.

corticosteroidAny of the steroid hormones produced primarily by the adrenal cortex; includes the glucocorticoids, most notablycortisol, and the mineralocorticoids, most notably aldosterone.

endocrine glandDuctless organ of the endocrine system that secretes hormones.

estrogenA class of steroid sex hormones that promote development of secondary female sexual characteristics and

Summary

Key Terms






stimulate growth of the uterine lining; estrogens are produced in the ovaries.

folliclestimulating hormone (FSH)A peptide hormone produced in the anterior pituitary that stimulates growth and development of ovarian follicles infemales and stimulates maturation of sperm cells in males.

gonadAn organ that produces gametes and sex steroid hormones; the male gonads are the testes, and the femalegonads are the ovaries.

gonadotropinAny of several peptide hormones that target the gonads; examples include folliclestimulating hormone (FSH),luteinizing hormone (LH), and human chorionic gonadotropin (hCG).

gonadotropinreleasing hormone (GnRH)A peptide hormone produced by the hypothalamus that stimulates the production of the gonadotropins folliclestimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary.

hormoneA chemical produced by glands or cells of the endocrine system that is transported in blood or hemolymph todistant target cells.

hypothalamusEndocrine gland located in the brain that integrates information from the nervous system and regulates endocrinefunction.

inhibiting hormoneA hormone produced by the hypothalamus that inhibits release of hormones from the anterior pituitary.

Leydig cellA cell type in the testes that synthesizes and secretes testosterone in response to LH stimulation; sometimesknown as an interstitial cell.

luteinizing hormone (LH)A peptide hormone produced in the anterior pituitary that stimulates ovulation and development of the corpusluteum in females and stimulates testosterone production in the Leydig cells in males.

nontropic hormoneA hormone that directly produces a response from cells of the body.

parathyroid glandAny of four endocrine glands on the posterior surface of the thyroid gland that secrete parathyroid hormone (PTH)in response to low blood calcium levels.

pineal glandAn endocrine gland located deep in the brain between the cerebral hemispheres; secretes melatonin, whichregulates sleep patterns and circadian rhythm.

pituitary glandA gland that hangs down from the hypothalamus; secretes a variety of tropic and nontropic hormones.

posterior pituitaryOne of two lobes of the pituitary gland; directly connected to the hypothalamus; directly responsible for secretion ofoxytocin and antidiuretic hormone (ADH).

progestinA steroid sex hormone that promotes growth of the uterine lining in women.

prolactinA peptide hormone produced in the anterior pituitary that stimulates the production of milk in the mammary glandsof lactating mammals; may also be involved in parental behaviors in males and nonmammalian species.

releasing hormoneAny of several hormones produced by the hypothalamus that stimulate the release of specific hormones from theanterior pituitary; examples include corticotropinreleasing hormone (CRH, which induces ACTH secretion) andgonadotropinreleasing hormone (GnRH, which induces FSH and LH secretion).

Sertoli cellA cell type in the testes that supports the growth and maturation of sperm cells; activated by FSH stimulation; alsostimulates the hormone inhibin to regulate FSH secretion by the anterior pituitary.

thyroid glandGland located in the neck in humans that regulates metabolism, growth, and calcium levels.

tropic hormoneAny hormone that stimulates the production of hormones in another endocrine gland; examples include thehypothalamic releasing hormones (which stimulate the anterior pituitary) and thyroidstimulating hormone (whichstimulates the thyroid).




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1.

hypothalamus and pituitarythyroid and thymusthymus and parathyroidthyroid and parathyroidpituitary and adrenal

Which two glands regulate the level of calcium in the blood?

2.

The animal would be less able to survive sudden attacks and emergencies.The animal would exhibit less aggressive behavior.The animal would be less able to deal with longterm stress.The "adrenaline rush" would be much more pronounced.Water balance could not be maintained.

If the adrenal medulla were removed from an animal, which of the following statements would be true?

3.

administration of supplemental corticosteroids on a longterm basisadministration of supplemental PTH (parathyroid hormone) on a longterm basisremoval of the thyroid glandremoval of the adrenal cortexThere is no treatment for Addison's disease.

What is the appropriate treatment for a person with Addison's disease?

4.

The anterior pituitary secretes releasing and inhibiting hormones that regulate hormone production bythe hypothalamus.The hypothalamus has two lobes, an anterior lobe and a posterior lobe.The anterior pituitary is an extension of the hypothalamus that contains neurosecretory neurons.The hypothalamus secretes releasing and inhibiting hormones that regulate hormone production bythe posterior pituitary.The hypothalamus secretes releasing and inhibiting hormones that regulate hormone production bythe anterior pituitary.

Which of the following statements about the hypothalamus and pituitary glands are true?

5.

epinephrine and norepinephrinetriiodothyronine and thyroxineinsulin and glucagoncortisol and aldosteroneluteinizing hormone and folliclestimulating hormone

Which of the following pairs of hormones are released by the adrenal cortex?

6.

FSH is stimulated by CRH from the hypothalamus.FSH stimulates production of testosterone in males and estrogen in females.FSH stimulates the development of gametes.FSH is stimulated by LH from the hypothalamus.FSH directly promotes the development of secondary sex characteristics.

Which of the following is true of the pituitary hormone FSH?

7.

In the absence of testes, male (XY) rabbits appear male but are infertile.Ovaries are necessary for the development of female primary sex characteristics in rabbits.Testosterone causes development of male sexual characteristics.Male primary sex characteristics develop in the absence of testes as long as the rabbit has a Ychromosome.Testes produce a factor that stimulates development of male sex characteristics.

Alfred Jost removed the testes from rabbit embryos in order to study the role of these gonads in sexualdifferentiation. Which of the following conclusions can be drawn from his experiment?

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