803
CHAPTER TWENTY
Urinary System
Other genetic conditions alter urine with-
out causing health problems. People with beet-
uria excrete dark pink urine after they eat beets.
The problem for people with
urinary excretion of
odoriferous component
of asparagus is obvious.
Parents of newborns who have inherited blue
diaper syndrome are shocked when they change
their child’s F
rst diaper. Due to a defect in trans-
port of the amino acid tryptophan in the small
intestine, bacteria degrade the partially digested
tryptophan, producing a compound that turns
blue on contact with oxygen.
Certain inherited disorders can noticeably
alter urine. The name
maple syrup urine disease
vividly describes what this inborn error of metab-
olism does to the urine. This condition, which
causes mental retardation, results from a block in
the breakdown pathways for certain amino acids.
In
alkaptonuria,
urine turns black when it is left to
stand. This condition also produces painful arthri-
tis and blackened ear tips. People with
Wilson dis-
ease
have an inherited inability to excrete copper.
If they are properly diagnosed and given the drug
penicillamine, their urine becomes the color of
copper.
U
rine has long fascinated medical minds.
As a folk remedy, urine has been used as
a mouthwash, toothache treatment, and
a cure for sore eyes. Hippocrates (460–377
B
.
C
.) was
the F
rst to observe that the condition of the urine
can re±
ect health, noting that frothy urine denoted
kidney disease. During the Middle Ages, health
practitioners consulted charts that matched urine
colors to diseases. In the seventeenth century,
British physicians diagnosed diabetes by having
their medical students taste sugar in patients’ urine.
Today, urine composition is still used as a window
on health and also to check for illicit drug use.
20.4
CLINICAL APPLICATION
Urinalysis: Clues to Health
strict and f
brous connective tissue accumulates around the
capsules. On the inside, kidney cells begin to die as early as
age twenty years, but the gradual shrinkage is not generally
noticeable until aFter age Forty. By eighty years, the kidneys
have lost about a third oF their mass.
Kidney shrinkage is largely due to the gradual loss oF
glomeruli—they may atrophy, cease Functioning, become
blocked with f
brous connective tissue, or untwist. About
5% oF glomeruli are abnormal by age Forty; 37% are abnor-
mal by age ninety. The progressive shut down oF glomeruli
decreases the surFace area For Filtration, and as a result,
glomerular f ltration rate (G±R) begins to drop in the Fourth
decade. By age seventy-f
ve, G±R is about halF that in a young
adult, Falling From about 125 milliliters/minute to about 60.
With this decline, proteins are more likely to enter the urine.
About a third oF the elderly have proteinuria.
±urther along the nephron, renal tubules thicken, accu-
mulating Fatty coats. They may shorten, Forming small out-
pouches as cell death disrupts their sleek symmetry. Urine
may become more dilute as reabsorption oF sodium and glu-
cose and other molecules becomes less eFf
cient. The renal
tubules also slow in processing certain drugs, which remain
in the circulation longer. It becomes harder to clear non-
steroidal anti-infl ammatory drugs such as aspirin, as well as
opiates, antibiotics, urea, uric acid, creatinine, and various
toxins. ThereFore, a person’s age should be considered when
prescribing drugs.
Cardiovascular changes slow the journey oF blood
through the kidneys. A college student’s kidneys may pro-
cess about a Fourth oF the cardiac output, or about 1,200
milliliters, per minute. Her eighty-year-old grandFather’s kid-
neys can handle about halF that volume. Starting at about
age twenty, renal blood fl
ow rate diminishes by about 1%
per year. The blood vessels that serve the kidneys become
PRACTICE
35
Describe micturition.
36
How is it possible to consciously inhibit the micturition re±
ex?
Incontinence is the loss of control of micturition. Stress incontinence,
caused by pressure on the bladder, is particularly common among
women who have had children, especially if they have gained weight.
An e²
ective treatment is at least two months of doing Kegel exercises,
in which a woman contracts the muscles that support the bladder,
several times daily. Treatments for severe cases include a tamponlike
cone inserted into the vagina to raise the pelvic ±
oor; a small foam
pad placed over the urethra to catch small amounts of urine; collagen
injections around the urethra to tighten it; and surgery. Many people
use absorbent pads.
Nighttime bedwetting was noted as long ago as 1500 B.C.
Treatments have ranged from drinking the broth from boiled hens’
combs, to blocking the urethra at night, to punishment and ridicule.
In many cases, this
nocturnal enuresis
is inherited. Drug treatment and
pads to absorb urine help to manage the problem in children, who
usually outgrow the condition.
20.5
LIFE-SPAN CHANGES
The urinary system is suFf
ciently redundant, in both struc-
ture and Function, to mask aging-related changes. However,
the kidneys become slower to remove nitrogenous wastes
and toxins and to compensate For changes that might alter
homeostasis.
±rom the outside, the kidneys change with age, appear-
ing scarred and grainy as arterioles serving the cortex con-
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