Urine is a yellowish liquid that is approximately 95% water and 5% dissolved solids and gases. The pH of freshly collected urine averages 6.0, with a range of 4.5 to 8.0. Diet may cause considerable variation in pH. The amount of dissolved substances in urine is indicated by its specific gravity. The specific gravity of pure water, used as a standard, is 1.000. Because of the dissolved materials it contains, urine has a specific gravity that normally varies from 1.002 (very dilute urine) to 1.040 (very concentrated urine). When the kidneys are diseased, they lose the ability to concentrate urine, and the specific gravity no longer varies as it does when the kidneys function normally.
Some of the dissolved substances normally found in the urine are the following:
* Nitrogenous waste products, including urea, uric acid, and creatinine.
* Electrolytes, including sodium chloride (as in common table salt) and different kinds of sulfates and phosphates. Electrolytes are excreted in appropriate amounts to keep their blood concentration constant.
* Pigment, mainly yellow pigment derived from certain bile compounds. Pigments from foods and drugs also may appear in the urine.
Examination of urine, called a urinalysis (UA), is one of the most important parts of a medical evaluation.
Figure 18-1 Male urinary system, showing blood vessels.
A routine urinalysis includes observation of color and turbidity (cloudiness) as well as measurement of pH and specific gravity. Laboratories also test for a variety of abnormal components, including:
* Glucose is usually an important indicator of diabetes mellitus, in which the cells do not adequately metabolize blood sugar. The excess glucose, which cannot be reabsorbed, is excreted in the urine. The presence of glucose in the urine is known as glycosuria or glucosuria.
* Albumin. The presence of this protein, which is normally retained in the blood, may indicate a kidney disorder, such as glomerulonephritis. Albumin in the urine is known as albuminuria.
* Blood in the urine is usually an important indicator of urinary system disease, including nephritis. Blood in the urine is known as hematuria.
* Ketones are produced when fats are incompletely oxidized; ketones in the urine are seen in diabetes mellitus and starvation.
* White blood cells (pus) are evidence of infection; they can be seen by microscopic examination of a centrifuged specimen. Pus in the urine is known as pyuria.
* Casts are solid materials molded within the microscopic kidney tubules. They consist of cells or proteins and, when present in large number, they usually indicate disease of the nephrons.