When the filtrate leaves the collecting ducts, it becomes
urine. The urine passes into the calyx, the renal pelvis, and
then the ureter. It is temporarily stored in the urinary blad-
der and is eventually excreted from the body via the ure-
thra. Urine has distinct characteristics of color, turbidity,
odor, pH, and specific gravity, as described in Table 15.1.
The composition of urine can sometimes indicate met-
abolic imbalances and disease conditions. Urine is often
tested to determine whether abnormal components are
present and to help diagnose disease (Table 15.2).
CONCEPT CHECK
1.
Which
organ of the urinary system stores urine?
2.
in
which
layer of the kidney is a renal column
found?
3.
in
which
part of the nephron is sodium reab-
sorption controlled by hormones?
4.
in
which
urine formation process does blood
pressure determine how much material crosses
the membrane?
5.
Where
does most reabsorption occur in the
nephron?
Physical characteristics of normal urine
Table 15.1
Characteristic
Description
Volume
One to two liters (about 1 to 2 quarts) in 24
hours but varies considerably.
Color
Yellow or amber, normally, is due to urochrome
(pigment produced from breakdown of bile)
and urobilin (from breakdown of hemoglobin).
Concentrated urine is darker in color. Diet,
medications, and certain diseases affect color.
Kidney stones may produce blood in urine.
Turbidity
Transparent when freshly voided, but becomes
turbid (cloudy) after awhile.
Odor
Mildly aromatic but becomes ammonia-like
after a time. Some people inherit the ability to
form methylmercaptan from digested aspara-
gus, which gives urine a characteristic odor.
pH
Ranges between pH 4.6 and 8.0; average 6.0;
varies considerably with diet. High-protein
diets increase acidity; vegetarian diets increase
alkalinity.
Specific
gravity
(density)
The ratio of the weight of a volume of a sub-
stance to the weight of an equal volume of dis-
tilled water. Urine specific gravity ranges from
1.001 to 1.035. The higher the concentration of
solutes, the higher the specific gravity.
Summary of abnormal constituents in urine Table 15.2
Abnormal Constituent
Comments
Albumin
A normal constituent of blood plasma that usually appears in only very small amounts in urine because it is too
large to be filtered. The presence of excessive albumin in the urine,
albuminuria
(al'-bu-mi-NOO-re-a), indicates
an increase in the permeability of filtering membranes due to injury or disease, increased blood pressure, or
damage to kidney cells.
Glucose
Glucosuria,
the presence of glucose in the urine, usually indicates diabetes mellitus.
Red blood cells
(erythrocytes )
Hematuria
(hem-a-TOO-re-a), the presence of hemoglobin from ruptured red blood cells in the urine, can occur
with acute inflammation of the urinary organs as a result of disease or irritation from kidney stones, tumors,
trauma, and kidney disease.
White blood cells
(leukocytes)
The presence of white blood cells and other components of pus in the urine, referred to as
pyuria
(pT-U-re-a),
indicates infection in the kidneys or other urinary organs.
Ketone bodies
High levels of ketone bodies in the urine, called
ketonuria
(ke-to-NOO-re-a), may indicate diabetes mellitus, an-
orexia, starvation, or too little carbohydrate in the diet.
Bilirubin
When red blood cells are destroyed by macrophages, the globin portion of hemoglobin is split off and the heme
is converted to biliverdin. Most of the biliverdin is converted to bilirubin. An above-normal level of bilirubin in
urine is called
bilirubinuria
(bil'-e-roo-bi-NOO-re-a).
Urobilinogen
The presence of urobilinogen (breakdown product of hemoglobin) in urine is called
urobilinogenuria
(u'-ro-bi-lin'-o-
je-NOO-re-a). Trace amounts are normal, but elevated urobilinogen may be due to hemolytic or pernicious anemia,
infectious hepatitis, obstruction of bile ducts, jaundice, cirrhosis, congestive heart failure, or infectious mononucle-
osis.
Microbes
The number and type of bacteria vary with specific infections in the urinary tract. One of the most common is
E. coli.
The most common fungus to appear in urine is
Candida albicans,
a cause of vaginitis. The most frequent
protozoan seen is
Trichomonas vaginalis,
a cause of vaginitis in females and urethritis in males.
The Urinary System Plays a Vital Role in Maintaining Homeostasis
449
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