Chapter 25
The Urinary System
981
25
capacity of the bladder is 800–1000 ml and when it is overdis-
tended, it may burst.
Urethra
Describe the general location, structure, and function of
the urethra.
Compare the course, length, and functions of the male
urethra with those of the female.
Te
urethra
is a thin-walled muscular tube that drains urine
from the bladder and conveys it out of the body. Te epithe-
lium of its mucosal lining is mostly pseudostratified columnar
epithelium. However, near the bladder it becomes transitional
Ureter
Trigone of bladder
Trigone
Prostate
Intermediate part
of the urethra
Prostatic urethra
Peritoneum
Rugae
Detrusor
Bladder neck
Internal urethral
sphincter
External urethral
sphincter
Urogenital diaphragm
Urethra
Spongy
urethra
Erectile tissue
of penis
External urethral
orifice
Ureteric orifices
Adventitia
(a) Male.
The long male urethra has three regions: prostatic,
intermediate, and spongy.
(b) Female.
External urethral
orifice
Figure 25.20
Structure of the urinary bladder and urethra.
The anterior wall of the bladder has been cut away to reveal the
position of the trigone.
epithelium, and near the external opening it changes to a pro-
tective stratified squamous epithelium.
At the bladder-urethra junction, the detrusor smooth muscle
thickens to form the
internal urethral sphincter
(Figure 25.20).
Tis involuntary sphincter, controlled by the autonomic ner-
vous system, keeps the urethra closed when urine is not being
passed and prevents leaking between voiding.
Te
external urethral sphincter
surrounds the urethra as
it passes through the
urogenital diaphragm
. Tis sphincter is
formed of skeletal muscle and is voluntarily controlled. Te
le-
vator ani
muscle of the pelvic floor also serves as a voluntary
constrictor of the urethra (see ±able 10.7, pp. 344–345).
Te length and functions of the urethra differ in the two
sexes. In females the urethra is only 3–4 cm (1.5 inches) long
and fibrous connective tissue binds it tightly to the anterior
vaginal wall. Its external opening, the
external urethral orifice
,
lies anterior to the vaginal opening and posterior to the clitoris.
In males the urethra is approximately 20 cm (8 inches) long
and has three regions.
Te
prostatic urethra
, about 2.5 cm (1 inch) long, runs
within the prostate.
Te
intermediate part of the urethra
(or
membranous ure-
thra
), which runs through the urogenital diaphragm, extends
about 2 cm from the prostate to the beginning of the penis.
Te
spongy urethra
, about 15 cm long, passes through the
penis and opens at its tip via the
external urethral orifice
.
previous page 1015 Human Anatomy and Physiology (9th ed ) 2012 read online next page 1017 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off