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262	BRS Physiology

2. First trimester

■The corpus luteum (stimulated by HCG) is responsible for the production of estradiol and progesterone.

■Peak levels of HCG occur at gestational week 9 and then decline.

3. Second and third trimesters

■Progesterone is produced by the placenta.

■Estrogens are produced by the interplay of the fetal adrenal gland and the placenta. The fetal adrenal gland synthesizes dehydroepiandrosterone-sulfate (DHEA-S), which is

then hydroxylated in the fetal liver. These intermediates are transferred to the placenta, where enzymes remove sulfate and aromatize to estrogens. The major placental estrogen is estriol.

■Human placental lactogen is produced throughout pregnancy. Its actions are similar to those of growth hormone and prolactin.

4. Parturition

■Throughout pregnancy, progesterone increases the threshold for uterine contraction.

■Near term, the estrogen/progesterone ratio increases, which makes the uterus more sensitive to contractile stimuli.

■The initiating event in parturition is unknown. (Although oxytocin is a powerful stimulant of uterine contractions, blood levels of oxytocin do not change before labor.)

5. Lactation

■Estrogens and progesterone stimulate the growth and development of the breasts throughout pregnancy.

■Prolactin levels increase steadily during pregnancy because estrogen stimulates prolactin secretion from the anterior pituitary.

■Lactation does not occur during pregnancy because estrogen and progesterone block the action of prolactin on the breast.

■After parturition, estrogen and progesterone levels decrease abruptly and lactation occurs.

■Lactation is maintained by suckling, which stimulates both oxytocin and prolactin secretion.

■Ovulation is suppressed as long as lactation continues because prolactin has the following effects:

a. Inhibits hypothalamic GnRH secretion.

b. Inhibits the action of GnRH on the anterior pituitary and consequently inhibits LH and FSH secretion.

c. Antagonizes the actions of LH and FSH on the ovaries.

Review Test

QUesTIOns 1–5

Use the graph below, which shows changes during the menstrual cycle, to answer Questions 1–5.

						A
						B
					C	D
						D
						E
24 26	0	2	4	6	8	10 12 14 16 18 20 22 24 26	0	2	4
						Day of cycle

1. The increase shown at point A is caused by the effect of

(A)estrogen on the anterior pituitary

(B)progesterone on the hypothalamus

(C)follicle-stimulating hormone (FSH) on the ovary

(D)luteinizing hormone (LH) on the anterior pituitary

(e)prolactin on the ovary

2.Blood levels of which substance are described by curve B?

(A)Estradiol

(B)Estriol

(C)Progesterone

(D)Follicle-stimulating hormone (FSH)

(e) Luteinizing hormone (LH)

3. The source of the increase in concentration indicated at point C is the

(A)hypothalamus

(B)anterior pituitary

(C)corpus luteum

(D)ovary

(e)adrenal cortex

4.The source of the increase in concentration at point D is the

(A)ovary

(B)adrenal cortex

(C)corpus luteum

(D)hypothalamus

(e) anterior pituitary

5.The cause of the sudden increase shown at point E is

(A)negative feedback of progesterone on the hypothalamus

(B)negative feedback of estrogen on the anterior pituitary

(C)negative feedback of follicle-stimulating hormone (FSH) on the ovary

(D)positive feedback of FSH on the ovary

(e)positive feedback of estrogen on the anterior pituitary

6.A 41-year-old woman has hypocalcemia, hyperphosphatemia, and decreased urinary phosphate excretion. Injection of parathyroid hormone (PTH) causes an increase in urinary cyclic adenosine monophosphate (cAMP). The most likely diagnosis is

(A)primary hyperparathyroidism

(B)vitamin D intoxication

(C)vitamin D deficiency

(D)hypoparathyroidism after thyroid surgery

(e)pseudohypoparathyroidism

7.Which of the following hormones acts on its target tissues by a steroid hormone mechanism of action?

(A)Thyroid hormone

(B)Parathyroid hormone (PTH)

263

264	BRS Physiology

(D) β1 adrenergic agonists

(E) Glucagon

8. A 38-year-old man who has galactorrhea is found to have a prolactinoma. His physician treats him with bromocriptine, which eliminates the galactorrhea. The basis for the therapeutic action of bromocriptine is that it

(A) antagonizes the action of prolactin on the breast

(B) enhances the action of prolactin on the breast

(D) inhibits prolactin release from the hypothalamus

(E) enhances the action of dopamine on the anterior pituitary

9. Which of the following hormones originates in the anterior pituitary?

(A) Dopamine

(B) Growth hormone–releasing hormone (GHRH)

(D) Gonadotropin-releasing hormone (GnRH)

(E) Thyroid-stimulating hormone (TSH)

(F) Oxytocin

(G) Testosterone

10. Which of the following functions of the Sertoli cells mediates negative feedback control of follicle-stimulating hormone (FSH) secretion?

(A) Synthesis of inhibin

(B) Synthesis of testosterone

(D) Maintenance of the blood–testes barrier

11. Which of the following substances is derived from proopiomelanocortin (POMC)?

(A) Adrenocorticotropic hormone (ACTH)

(B) Follicle-stimulating hormone (FSH)

(D) Cortisol

(E) Dehydroepiandrosterone

12. Which of the following inhibits the secretion of growth hormone by the anterior pituitary?

(A) Sleep

(B) Stress

(D) Somatomedins

(E) Starvation

(F) Hypoglycemia

13. Selective destruction of the zona glomerulosa of the adrenal cortex would produce a deficiency of which hormone?

(A) Aldosterone

(B) Androstenedione

(D) Dehydroepiandrosterone

(E) Testosterone

14. Which of the following explains the suppression of lactation during pregnancy?

(A) Blood prolactin levels are too low for milk production to occur

(B) Human placental lactogen levels are too low for milk production to occur

(D) Blood levels of estrogen and progesterone are high

(E) The maternal anterior pituitary is suppressed

15. Which step in steroid hormone biosynthesis, if inhibited, blocks the production of all androgenic compounds but does not block the production of glucocorticoids?

(A) Cholesterol → pregnenolone

(B) Progesterone → 11-deoxycorticosterone

(D) Testosterone → estradiol

(E) Testosterone → dihydrotestosterone

16. A 46-year-old woman has hirsutism, hyperglycemia, obesity, muscle wasting, and increased circulating levels of adrenocorticotropic hormone (ACTH). The most likely cause of her symptoms is

(A) primary adrenocortical insufficiency (Addison disease)

(B) pheochromocytoma

(D) treatment with exogenous glucocorticoids

(E) hypophysectomy

17. Which of the following decreases the conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol?

(A) A diet low in Ca2+

(B) Hypocalcemia

(D) Hypophosphatemia

(E) Chronic renal failure

18. Increased adrenocorticotropic hormone (ACTH) secretion would be expected in patients

(A) with chronic adrenocortical insufficiency (Addison disease)

(B) with primary adrenocortical hyperplasia

(D) with elevated levels of angiotensin II

19. Which of the following would be expected in a patient with Graves disease?

(A) Cold sensitivity

(B) Weight gain

(D) Decreased cardiac output

(E) Drooping eyelids

(F) Atrophy of the thyroid gland

(G) Increased thyroid-stimulating hormone (TSH) levels

(H) Increased triiodothyronine (T3) levels

20. Blood levels of which of the following substances is decreased in Graves disease?

(A) Triiodothyronine (T3)

(B) Thyroxine (T4)

(D) Thyroid-stimulating hormone (TSH)

(E) Iodide (I−)

21. Which of the following hormones acts by an inositol 1,4,5-triphosphate (IP3)-Ca2+ mechanism of action?

(A) 1,25-Dihydroxycholecalciferol

(B) Progesterone

(D) Parathyroid hormone (PTH)

(E) Gonadotropin-releasing hormone (GnRH)

22. Which step in steroid hormone biosynthesis is stimulated by adrenocorticotropic hormone (ACTH)?

	Endocrine Physiology	265
Chapter 7

(A) Cholesterol → pregnenolone

(B) Progesterone → 11-deoxycorticosterone

(D) Testosterone → estradiol

(E) Testosterone → dihydrotestosterone

23. The source of estrogen during the second and third trimesters of pregnancy is the

(A) corpus luteum

(B) maternal ovaries

(D) placenta

(E) maternal ovaries and fetal adrenal gland

(F) maternal adrenal gland and fetal liver

(G) fetal adrenal gland, fetal liver, and placenta

24. Which of the following causes increased aldosterone secretion?

(A) Decreased blood volume

(B) Administration of an inhibitor of angiotensin-converting enzyme (ACE)

(D) Hypokalemia

25. Secretion of oxytocin is increased by

(A) milk ejection

(B) dilation of the cervix

(D) increased extracellular fluid (ECF) volume

(E) increased serum osmolarity

26. A 61-year-old woman with hyperthyroidism is treated with propylthiouracil. The drug reduces the synthesis of thyroid hormones because it inhibits oxidation of

(A) Triiodothyronine (T3)

(B) Thyroxine (T4)

(D) Thyroid-stimulating hormone (TSH)

(E) Iodide (I−)

27. A 39-year-old man with untreated diabetes mellitus type I is brought to the emergency room. An injection of insulin would be expected to cause an increase in his

(A) urine glucose concentration

(B) blood glucose concentration

(D) blood pH

(E) breathing rate

266	BRS Physiology

28. Which of the following results from the action of parathyroid hormone (PTH) on the renal tubule?

(A) Inhibition of 1α-hydroxylase

(B) Stimulation of Ca2+ reabsorption in the distal tubule

(D) Interaction with receptors on the luminal membrane of the proximal tubular cells

(E) Decreased urinary excretion of cyclic adenosine monophosphate (CAMP)

29. Which step in steroid hormone biosynthesis occurs in the accessory sex target tissues of the male and is catalyzed by 5α-reductase?

(A) Cholesterol → pregnenolone

(B) Progesterone → 11-deoxycorticosterone

(D) Testosterone → estradiol

(E) Testosterone → dihydrotestosterone

30. Which of the following pancreatic secretions has a receptor with four subunits, two of which have tyrosine kinase activity?

(A) Insulin

(B) Glucagon

(D) Pancreatic lipase

31. A 16-year-old, seemingly normal female is diagnosed with androgen insensitivity disorder. She has never had a menstrual cycle and is found to have a blind-ending vagina; no uterus, cervix, or ovaries; a 46 XY genotype; and intra-abdominal testes. Her serum testosterone is elevated. Which of the

following characteristics is caused by lack of androgen receptors?

(A) 46 XY genotype

(B) Testes

(D) Lack of uterus and cervix

(E) Lack of menstrual cycles