lows the urinary bladder to expand as it fills, and contrac-
tions of the smooth muscle generate the pressure to expel
urine from the body through the urethra.
When empty, the urinary bladder looks like a deflated
balloon. As it fills, it becomes round and then pear-shaped.
The urinary bladder holds an average of 700 to 800 mL of
urine. It is smaller in females because the uterus occupies
the space just superior to the urinary bladder.
Urethral Structure Varies
Between the Sexes
The urethra (u-RE-thra) is a small tube leading from the
floor of the urinary bladder to the exterior of the body.
Around the opening to the urethra is an internal urethral
sphincter composed of smooth muscle. The opening and
closing of the internal sphincter is involuntary. Below the in-
ternal sphincter is the external urethral sphincter, which is
composed of skeletal muscle and is under voluntary control.
The structure of the urethra is different in males and
females (Figure 15.7b). In females, it lies directly behind
the pubic symphysis and is embedded in the front wall of
the vagina. The opening of the urethra to the exterior,
the
extern a l urethral orifice,
lies between the clitoris and the
vaginal opening. In males, the urethra passes vertically
through the prostate, the deep perineal muscles, and fi-
nally the penis. The male urethra also serves as the duct
through which semen is ejaculated.
The shortened length and slightly larger diameter of
the female urethra makes females susceptible to urinary
tract infections (UTIs) from bacteria outside the body.
The most common type of urinary tract infection occurs
in the urinary bladder—a condition known as
cystitis.
This
is nearly always caused by the entry of microorganisms via
the urethra. Symptoms of urinary tract infections include
painful urination, often accompanied by bleeding. Most
UTIs can be treated easily with antibiotics.
b. Differences in the urethras of males and females
Male
Prostatic
urethra
is the
segment that passes through
the prostate gland. Besides
urine, it receives secretions
containing sperm, sperm
motility and viability factors,
and substances that neutralize
the pH of the urethra.
Membranous
urethra
is the
segment that passes through
the perineum. It is the
shortest segment.
Sagittal
plane
Urinary bladder
Pubic symphysis
Prostate
Perineum
Penis
External urethral
orifice
Spongy
urethra
is the
segment that passes
through the penis. It is the
longest segment and
receives secretions
including mucus and
substances that neutralize
the pH of the urethra.
During ejaculation in the
male, the semen passes
through all segments of
the urethra to the outside.
Rectum
Vagina -
Uterus
Sagittal
plane
Urinary bladder
Female
Urethra
External urethral
orifice
Males vs. Females
• The urethra is four times longer in males than in females.
• The urethra is divided into three segments in males but is
only one short tube in females.
• The urethra is a common duct for the urinary and
reproductive systems in males. These two systems are
entirely separate in females.
The Urinary Bladder Stores Urine and Expels it from the Body
451
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