CHAPTER 12
Urinary System
223
surface of the bladder is covered by
adventitia
, which is a layer
of connective tissue without a mesothelial covering.
Urethra
The
urethra
is structurally different in the male and female. The
proximal end of the
male urethra
is surrounded by an internal
urethral sphincter (smooth muscle) that functions mainly to pre-
vent seminal fl uids from entering the bladder during ejaculation.
The male urethra is about 20 cm long, and it is composed of three
segments:
prostatic
,
membranous
, and
penile
(
spongy
)
urethra
.
The
prostatic portion
is surrounded by the prostate gland and is
lined by transitional epithelium. The
membranous portion
is a
short segment surrounded by the skeletal muscle of the external
sphincter (urogenital diaphragm) and is lined by pseudostratiF ed
columnar epithelium. The
penile
(
spongy
)
urethra
(also called the
cavernous urethra
) is surrounded by the corpus spongiosum of
the penis, and its epithelial lining changes from pseudostratiF ed
columnar to stratiF ed squamous. In this region, there are many
small mucous glands called the
glands of Littré
, which secrete
mucus to coat and protect the lining of the urethra.
The
female urethra
is short, about 4 to 5 cm. It is lined
chiefl
y by stratiF
ed squamous epithelium and, in a few places,
may have patches of pseudostratiF
ed columnar epithelium.
Glands of Littré are also present in the female urethra. The
proximal end of female urethra is surrounded by skeletal
muscle (external sphincter) where it penetrates the urogenital
diaphragm. The external sphincter muscle in both male and
female is innervated by the pudendal nerves; it functions to con-
trol retention or release of the urine from the urinary bladder
through the urethra and helps maintain urinary continence. A
female does not have an internal sphincter.
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