channels the urine toward the URETER, which will
carry the urine, like a drain, into the bladder.
Though small in diameter the ureter has relatively
thick, sturdy walls that contract in rhythmic
waves to move urine in a steady flow. The ureter’s
peristaltic action also helps prevent urine from
flowing back up into the kidney. Each ureter
inserts into the back of the bladder wall, tunneling
through the detrusor muscle for a short distance
before emerging into the urothelium (inner
epithelial layer of the bladder). The tunnel is
another safeguard to keep urine from backflowing
to the kidney, flattening unless pressure from
flowing urine causes it to open.
The urethra carries urine from the base of the
bladder to outside the body. A woman’s urethra is
less than two inches long and exits her body
between the CLITORISand the VAGINA. A man’s ure-
thra is about eight inches long and exits his body
at the tip of the PENIS. A ring of muscle, the ure-
thral sphincter, encircles the urethra at the neck of
the bladder. When contracted the sphincter holds
the urethra closed and urine remains in the blad-
der; when relaxed the sphincter allows the ure-
thra to open and urine to leave the bladder.
Health and Disorders of the Urinary System
The kidneys have remarkable capacity. Each kid-
ney contains more than a million nephrons.
Though the normal design of the human body
features two kidneys, one healthy kidney is per-
fectly able to meet the needs of the body. The kid-
neys can lose as much as 65 to 70 percent of their
ability to function and still maintain the health of
the body. When kidney function reaches 25 per-
cent, however, the filtration workload over-
whelms the nephrons and symptoms of kidney
failure begin to manifest. And when kidney func-
tion drops to 15 percent or lower, the kidneys can
no longer perform at a level that sustains life.
The most significant health challenges that con-
front the kidneys are DIABETESand hypertension,
which are especially dangerous when they occur
in combination as they do in about half of people
who have diabetes as hypertension is a complica-
tion of diabetes. These two conditions place inor-
dinate stress on the glomeruli, hypertension
because it increases the pressure under which
blood enters the glomeruli and diabetes because
the elevated levels of GLUCOSEin the blood damage
capillaries throughout the body. The glomeruli,
being among the most intensely concentrated cap-
illary networks in the body, bear the brunt of such
damage. About 20 million Americans live with
some degree of kidney failure and another 10 mil-
lion are at risk of kidney failure because of health
conditions such as diabetes and hypertension as
well as conditions that directly affect the kidneys.HEALTH CONDITIONS THAT AFFECT
THE URINARY SYSTEM
ALPORT’S SYNDROME BLADDER CANCER
BLADDER EXSTROPHY CYSTINURIA
CYSTITIS CYSTOCELE
END-STAGE RENAL DISEASE(ESRD) EPISPADIAS
FANCONI’S SYNDROME GLOMERULONEPHRITIS
GLOMERULOSCLEROSIS GOODPASTURE’S SYNDROME
HEMOLYTIC UREMIC SYNDROME HEPATORENAL FAILURE
HORSESHOE KIDNEY HYDRONEPHROSIS
HYPOSPADIAS NEPHRITIS
NEPHROLITHIASIS NEPHROPATHY
NEPHROTIC SYNDROME POLYCYSTIC KIDNEY DISEASE
RENAL CANCER RENAL CYST
RENAL FAILURE RENAL TUBULAR ACIDOSIS
UREMIA URETHRAL STRICTURE
URETHRITIS URINARY INCONTINENCE
URINARY TRACT INFECTION(UTI) UROLITHIASIS
VESICOURETERAL REFLUX WILMS’S TUMORTraditions in Medical History
Among the earliest known medical treatments are
those for kidney stones and bladder stones.
Ancient healers across cultures documented vari-
ous remedies, including surgical removal, for the
painful conditions known today as NEPHROLITHIASIS
and UROLITHIASIS, respectively. Though early physi-
cians could not examine the urinary structures
themselves in any great detail, these structures
abundantly produced a substance that many
physicians turned into a diagnostic oracle: the
urine. The gifted physician was one who could
study the color, cloudiness, consistency, odor, and
even taste of the urine to diagnose conditions
ranging from HEART FAILUREto PREGNANCYto SYPHILIS
and, of course, diabetes. This was the practice of
uroscopy, the forerunner of modern urinalysis.
Greek philosopher and scientist Aristotle
(384–322 B.C.E.), whose father was a physician,The Urinary System 173