HUMAN BIOLOGY

reprodUCtIVe systeMs 307

hoW do a female’s ovaries function

in reproduction?

• The ovaries are primary reproductive organs that produce
  oocytes (immature eggs) and female sex hormones.


• Female sex hormones—estrogens and progesterone—are
  released as part of a recurring menstrual cycle.


• Menopause marks the natural end of a woman’s fertility.

taKe-hoMe Message

starts flowing out through the vaginal canal. This flow is
menstruation, and it marks the first day of a new cycle.
The disintegrating endometrium is being sloughed off,
only to be rebuilt once again during the next cycle.
The menstrual cycle advances through three phases. It
starts with a menstrual phase. This is the time of menstrua-
tion, when the endometrium disintegrates. Next comes the
proliferative phase, when the endometrium begins to thicken
again. The end of this phase coincides with ovulation—the
release of an oocyte from an ovary. During the cycle’s final
phase, called the progestational (“before pregnancy”) phase,
an endocrine structure called the corpus luteum ( “yellow
body”) forms. It secretes a flood of estrogens and another
female sex hormone, progesterone. Together these hor-
mones prime the endometrium for pregnancy. Feedback
loops to the hypothalamus and pituitary gland from the
ovaries govern the menstrual cycle.
A female’s first menstruation, or menarche, usually
occurs between the ages of 10 and 16. Menstrual cycles con-
tinue until the end of menopause, which usually occurs
in a woman’s early fifties. By then, her ovaries are making
less estrogen and progesterone, and also are less sensitive
to reproductive hormones from the pituitary. Falling estro-
gen levels may trigger a range of temporary symptoms,
including moodiness, insomnia, and “hot flashes” (bouts
of sweating and uncomfortable warmth). Other changes
include reduced natural lubrication and thinning of the
vaginal wall. The fertile phase of a woman’s life ends when
her menstrual cycles stop.

In the disorder endometriosis,

endometrial tissue grows outside

the uterus. Scar tissue may form

on one or both ovaries or oviducts,

leading to infertility. Endometriosis

may develop when menstrual flow

backs up through the oviducts and

spills into the pelvic cavity. Or per-

haps some cells became situated in

the wrong place when the woman

was a developing embryo, then were

stimulated to grow during puberty,

when her sex hormones became

active. Regardless, the symptoms

include pain during menstruation,

sex, or urination. Treatment ranges from doing nothing

in mild cases to surgery to remove the abnormal tissue or

sometimes even the whole uterus.

C

Chamber in which an

embryo develops; its

narrowed entrance

(the cervix) secretes

mucus that helps sperm

move into the uterus and

bars many bacteria

Uterus

Thick muscle layers

of uterus that stretch

enormously during

pregnancy

Myometrium

anus

Inner lining of uterus;

site where early embryo

implants; becomes

thickened; gives rise to

mother’s portion of the

placenta, an organ that

supports embryonic and

fetal development

Endometrium

Clitoris

Small organ responsive

to sexual stimulation

Labium minor

One of a pair of inner skin

folds of external genitals

Labium major

One of a pair of outer

fat-padded skin folds

of external genitals

Ovary

One of a pair of primary

reproductive organs in

which oocytes (immature

eggs) form and mature;

produces hormones

(estrogens and

progesterone) that

stimulate maturation

of oocytes and forma-

tion of corpus luteum

(a glandular structure),

and prepare the uterine

lining for pregnancy

Oviduct

One of a pair of ciliated channels through

which oocytes are conducted from an ovary

to the uterus; usual site of fertilization

bladder

urethra opening

of cervix

Organ of sexual intercourse;

also serves as birth canal

Vagina

© Cengage Learning

menarche A female’s first

menstruation.

menopause The end of

menstrual cycling and a

female’s fertility.

menstruation Shedding of

the blood-rich uterine lining

(endometrium) at the start of

each new menstrual cycle.

progesterone Female

sex hormone that helps

stimulate growth of the

endometrium (uterine lining)

in preparation for a possible

pregnancy.

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