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In the male, the urethra (yoo-REE-thrah) passes
through the prostate gland, the urogenital diaphragm, and
the penis. It is about 8 inches long and is subdi-vided into
three parts: the prostatic urethra, which is sur-rounded
by the prostate gland and is about 1 inch long; the
membranous urethra about 1/2 inch long, which runs
from the prostatic urethra to the penis; and the spongy or
cavernous urethra, found within the penis and about 6
inches long, but varies according to the size of the penis.
The spongy urethra enters the head or bulb of the penis and
terminates at the male urethral orifice.
The Accessory Glands
The accessory glands include the two seminal vesi-cles, the
prostate gland and the paired bulbourethral glands. These
glands secrete the liquid portion of the semen, the sperm-
containing fluid that is produced during ejaculation.
The paired seminal vesicles (SEM-ih-nal VESS-ih-
kulz) are convoluted pouchlike structures approximately 2
inches in length. They are located posterior to and at the
base of the urinary bladder in front of the rectum (see
Figure 19-1). They produce the alkaline, viscous
component of semen that is rich in the sugar fructose and
other nutrients for the sperm cells and pass it into the
ejaculatory duct. They produce about 60% of the volume of
semen. Because the duct of each seminal vesicle joins the
ductus deferens on each side to form the ejaculatory duct,
sperm and seminal fluid together enter the urethra during
ejaculation.
The prostate gland is a single, doughnut-shaped
gland about the size and shape of a chestnut. It surrounds
the superior portion (the prostatic urethra) of the urethra
just below the bladder. It also secretes an alkaline fluid that
makes up about 13% to 33% of semen. Its fluid plays a role
in activating the sperm cells to swim. The fluid en-ters the
prostatic urethra through several small ducts. The prostate
gland is located anterior to the rectum and a physician can
feel its size and texture by digital examina-tion through the
anterior wall of the rectum (to check for an enlarged
prostate, which could indicate cancer).
The paired bulbourethral (BUL-boh-yoo-REE-thral)
glands, also known as Cowper’s glands, are about the
size of peas. They are located beneath the prostate gland on
either side of the membranous urethra. They secrete thick,
viscous, alkaline mucus. Their ducts connect with the
spongy urethra. This secretion is the first to move down the
urethra when a man becomes sexually aroused and
develops an erection. It functions as both a lubricant for
sexual intercourse and as an agent to clean the ure-thra of
any traces of acidic urine.
Chapter 19Semen
Semen, also known as seminal fluid, is a mixture of
sperm cells and the secretions of the seminal vesicles, the
prostate, and the bulbourethral glands. The fluid is milky in
color and sticky, due to the fructose sugar that provides the
energy for the beating flagellum of each sperm cell. The
semen is alkaline, with a pH of 7.2 to 7.6. This neutralizes
the acidity of the female vagina and the male urethra and
helps protect the sperm cell. The semen provides a
transport medium for the swimming sperm cells.The average volume of semen per ejaculation is 2.5 to
6 mL, and the average range of spermatozoa ejaculated is
50 to 100 million/mL. If the number of spermatozoa falls
below 20 million/mL the man is considered to be sterile.
Semen contains enzymes that activate sperm after
ejaculation. The semen also contains an antibiotic called
seminalplasmin (SEM-ih-null-PLAZ-min), which has
the capability of destroying certain bacteria. Because the
female reproductive tract and the semen contain bacteria,
the seminalplasmin helps keep these bacteria under control
and thus helps protect the sperm and ensure fertilization.Media Link
Watch an animation on sperm formation on
the Student Companion
Website..The Penis
The penis (PEE-nis) is used to introduce or deliver sper-
matozoa into the female reproductive tract by being
inserted into the vagina. The penis consists of a shaft
whose distal end is a slightly enlarged region called the
glans penis or head, which means “shaped like an
acorn.” Covering the glans penis is a section of loose skin
called the prepuce (PRE-pyoose) or foreskin.
Occasionally, the foreskin is removed at birth by a surgical
procedure called circumcision. This will be done if the
foreskin does not pull back completely over the glans penis,
resulting in future hygiene problems. If the skin is loose
and pulls back, circumcision is not necessary. It is also
done as a rite in certain religions.
Internally, the penis is composed of three cylindrical
masses of spongy tissue containing blood sinuses (see
Figure 19-1). During sexual stimulation, the arteries that