Urine is a by-product of the kidney's activity. Volume, pH, and solute
concentration vary with the needs of the body's internal environment in a healthy
person. The characteristics of urine may change greatly when a person is ill. It is
possible to find out a great deal about the state of a body by analyzing the volume,
physical properties, and chemical properties of urine.
a. Physical Characteristics. Normal urine is yellow or amber colored and
transparent. The color, caused by pigments from the metabolism of bile, can change because
of medication or diet. A person eats beets and his urine may be a reddish color.
An individual takes large amounts of vitamin C and his urine may be deep yellow for a
time. The odor of urine varies. Stale urine develops an ammonia odor (think of a
baby's diaper), but the urine that is expelled after the digestion of asparagus will have a
completely different, but characteristic, odor.
b. Composition. About 95 percent of the total volume of urine is water. The
other 5 percent is made up of solutes that come from cellular metabolism and outside
sources such as drugs. Included in the 5 percent are nitrogenous waste products,
electrolytes, toxins, and urea. Urea, an end product formed in the liver from protein
metabolism, is the chief nitrogenous waste product in the non-water portion of urine.
One of the screening tests for renal function is the blood urea nitrogen (BUN)
test. It is the most commonly ordered test. It measures blood levels of
nitrogen in urea. This test is not sensitive to mild degrees of renal
dysfunction, but it is a good clinical indication of significant renal dysfunction.
c. Specific Gravity. This is the ratio of the weight of a volume of a substance to
the weight of an equal volume of distilled water. The specific gravity of water is 1.000.
The specific gravity of urine depends on the amount of solid materials in the urine. The
specific gravity of normal urine ranges from 1.010 to 1.020. The greater the
concentration of solutes in the urine, the higher its specific gravity.
d. Amount Voided. A normal adult eliminates about 1500 cubic centimeters of
urine daily. The amount of urine voided depends on a number of factors: blood
pressure, blood concentration, diet, temperature, diuretics, mental state, and general
e. Acidity/Alkalinity of Urine. Normal urine is slightly acid, but the acidity and
alkalinity of urine varies greatly with an individual's diet. A high-protein diet increases
the acidity of urine while a diet consisting of mostly vegetables increases the alkalinity of
urine. Other factors influencing urinary pH include high altitude, fasting, and exercise.