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tube. The ovum is then moved along the uterine tube by the
action of the cilia and by wavelike peristaltic contractions
of the smooth muscle of the uterine tube. If the egg is
fertilized, it usually occurs in the upper third of the uterine
tube. This means that the sperm must swim up through the
vagina, into the uterus, and then up two-thirds of the uterine
tube. Fertilization may occur at any time up to about 24
hours following ovulation. If fertilized, the ovum will make
its journey down the uterine tube and enter the uterus
within 7 days.
Media Link
Watch an animation about ovulation
on the Student Companion Website.The Uterus
The uterus (YOO-ter-us) or womb is located in the pelvic
cavity between the rectum and the urinary bladder (see
Figure 19-5). It is held in position by a series of ligaments,
and it is the site of menstruation. The uterus is where the
fertilized egg is implanted, where the fetus develops dur-
ing pregnancy, and where labor begins during delivery. It is
shaped like an inverted pear and can greatly increase in size
during pregnancy to accommodate the develop-ing fetus. It
will extend well above the navel or umbilicus in the late
stages of pregnancy.
Its anatomic divisions include the dome-shaped por-
tion above the uterine tubes called the fundus (FUN-dus)
(see Figure 19-6). Its major portion is the central tapering
region known as the body of the uterus. The narrow
infe-rior portion that opens into the vagina is called the
cervix. Between the body and the cervix is a small
constricted region called the isthmus (ISS-mus). The
interior of the body of the uterus is known as the uterine
cavity; the in-terior of the narrow cervix is known as the
cervical canal. The junction of the uterine cavity with the
cervical canal is called the internal os, and the opening of
the cervix into the vagina is called the external os.
The wall of the uterus is made of three layers of tis-
sue. The innermost layer is the endometrium (endoh-
MEE-tree-um). This mucosal layer is where the fertilized
egg burrows into the uterus, a process called implanta-tion.
The middle layer of the uterus is called the myome-trium
(my-oh-MEE-tree-um), which consists of smooth muscle
important during delivery to move the child out of the
womb. The outermost layer is the perimetrium (pair-ih-
MEE-tree-um) made of serous membrane and
Chapter 19also known as the visceral peritoneum. When a woman is
not pregnant, the endometrial lining of the uterus is shed
approximately every 28 days in the process called
menstruation.The Menstrual Cycle
The menstrual (MEN-stroo-al) cycle, also known as the
menses (MEN-seez) or menstruation (men-stroo-AY-
shun), is the cyclical shedding of the lining of the uterus in
response to changes in hormonal levels. The cycle varies
from woman to woman within a range of 24 to 35 days. To
discuss the events occurring during the cycle, we will
assume an average duration of 28 days. Events occurring
during the cycle can be divided into three phases: the
menstrual phase, the preovulatory or prolif-erative phase,
and the postovulatory or secretory phase (Figure 19-9).The menstrual phase. This phase is also known as
menstruation or menses. It lasts from day 1 to 5. During
this time, the thick endometrial lining of the uterus is shed
along with tissue fluid, blood, mucus, and epithe-lial cells.
Bleeding during this period can last from 3 to 5 days. The
detached tissues and blood exit through the vagina as the
menstrual flow.
During this phase, the ovarian cycle is also in
operation. The ovarian follicles, known as primary follicles,
begin their development. During the early phase of each
menstrual cycle, 20 to 25 primary follicles begin to produce
very low levels of estrogen. A clear membrane, the zona
pellucida, also develops around the eggs. Later on in the
phase at day 4 to 5, about 20 of the primary follicles
develop into secondary follicles. These secrete a follicular
fluid that forces the ovum to the edge of the secondary
follicle. Although a number of follicles begin development
during each cycle, only one attains maturity through the
process of meiosis. The other follicles undergo cellular
death or atresia.
The preovulatory or proliferative phase. This phase is
more variable in length. It will last from day 6 to 14 in our
28 - day cycle. During this phase, only one of the secondary
follicles in the ovary matures into a graafian follicle. This
follicle contains a mature egg and will discharge the egg in
a process called ovulation. Rising estrogen levels produced
by the follicles cause the endometrial lining to thicken
during this phase.
Ovulation is the rupturing of the graafian follicle.
Refer to Figure 19-7 as well as Figure 19-9. The ovum is
released into the pelvic cavity and this process occurs on
day 14 in our 28-day cycle. After ovulation, the graafian
follicle collapses and blood within it forms a clot called the
corpus hemorrhagicum