Biology 12

THINKING LAB

200 MHR • Unit 2 Homeostasis

only one ovum is released during each cycle. In

addition, progesterone inhibits the production of

LH. As the level of LH decreases, the corpus

luteum begins to degenerate and progesterone

levels decrease, ending the luteal stage. This

decrease in progesterone decreases the blood

supply to the endometrium, which leads to

menstruation. During menstruation, the

endometrium disintegrates, its blood vessels

rupture, and the tissues and blood flow out the

vagina. The beginning of menstruation signals the

first day of the follicular stage of the cycle.

In the Thinking Lab below, you will identify

the changes in hormone levels during the

menstrual cycle.

Menopause

With age, there is a reduction in the number of

functioning follicles, causing a decrease in the

amount of estrogen and progesterone in the blood.

The decrease in these hormones signals the onset

of menopause, which is characterized by cessation

of menstruation. During and after menopause,

cholesterollevels rise and bone mass declines.

Also during menopause, blood vessels alternately

constrict and dilate, resulting in uncomfortable

sensations known as “hot flashes.” As well, some

women experience mood changes. While some

doctors prescribe low levels of estrogen and/or

progesterone (hormone replacement therapy) to

alleviate the symptoms, this practice is the subject

of considerable debate. The benefits of hormone

replacement therapy include:

relief of menopausal symptoms such as hot

flashes, night sweats, and sleep disturbance;

prevention of bone loss (osteoporosis);

protection of cardiac health and a decreased risk

of stroke;

improved memory;

decreased chance of urinary tract infections; and

decreased rate of macular degeneration (a leading

cause of blindness).

The risks associated with hormone replacement

therapy vary with the hormone(s) prescribed. The

potential side effects of estrogen replacement

include:

irregular vaginal bleeding;

stomach upset;

severe headaches;

formation of blood clots;

increased risk of breast cancer; and

increased risk of uterine cancer.

The Menstrual Cycle

Background

Use the data table below to draw a line graph showing the

changes in hormone levels for FSH, LH, estrogen, and

progesterone during the menstrual cycle. Use different

colours to show the changes in each hormone. Be sure to

include a title and a labelled scale for each axis.

You Try It

1.Describe the pattern of change for each hormone.

2.Which hormone affects the development of the follicle

at the beginning of the cycle?

3.What causes the increase in the level of LH on day 12?

What is the effect of this increase?

4.What causes the increase in the level of progesterone

on day 16?

5.In what ways does the menstrual cycle illustrate

homeostasis?

6.Compare and contrast the menstrual cycle with the

changes in the male reproductive hormones.

Day 2468

LH

FSH

estrogen

progesterone

10

17

10

10

1

17

14

4

1

17

14

4

1

17

14

5

1

17

13

6

1

12 14 16 18 20

18

7

10

12

46

8

13

1

35

15

13

2

20

8

10

4

19

7

9

7

22 24 26 28

17

6

11

14

16

6

11

14

14

6

11

9

13

7

8

3