Composition of Urine

THE COMPOSITION OF URINE

Urine is composed of 93% to 97% water. Urine pH ranges from 4.5–8.0. Normally, about 1,200 mL (73.2 in³) of urine is excreted each day. Urine is a sterile fluid. It only becomes contaminated with bacteria when it passes through the external genitalia. Some organic substances present include urea, creatinine, ammonia, uric acid, urobilin, and bilirubin.

Urea is derived from the metabolism of amino acids by the liver and kidneys. Its content in the urine would, therefore, increase when protein breakdown is greater than protein buildup. Creatinine is derived from the breakdown of creatine phosphate in skeletal muscle. Its excretion is proportional to the skeletal muscle mass. Ammonia is derived largely from pro-tein breakdown. Uric acid is derived from breakdown of nucleic acid, present in large amounts in the nu-cleus of cells.

Urobilin and bilirubin are breakdown products of hemoglobin and give urine its yellow color. Increased amounts are excreted when there is liver disease or when there is excessive breakdown of red blood cells. The color of urine may be altered by medications and diet (e.g., beets can cause reddish colored urine).

Urine normally does not contain glucose, or it only contains minute quantities. In conditions in which the blood glucose levels are persistently high, the kidney is unable to reabsorb all the glucose that is filtered. In this case, some glucose is lost in the urine. Glucose is absorbed from the tubules by carriers. When the level of glucose in the filtrate is higher than these carriers can absorb, glucose appears in the urine (glucosuria). The upper limit of the rate at which glucose can be transported is known as transport maximum (Tm). Tm is measured in milligrams /minute.

The most common cause of glucosuria is diabetes mellitus, in which the blood glucose levels are above normal because of the lack of insulin. Rarely, gluco-suria may be found in people whose renal tubules have a low Tm as a result of genetic mutations. One test used to detect and monitor diabetes mellitus is to test the urine for glucose.

Little, if any, protein is lost in the urine. In kidney disease or inflammation or infection of the urinary tract, protein may be detected in the urine.

The excretion of sodium, chloride, potassium, and other ions vary with diet, pH of urine, and effect of hormones.

Normally, no red blood cells can be detected in the urine, and their presence may indicate problems in the urinary tract—anywhere from the kidney to the urethra. Few white blood cells are seen in urine. The presence of an abnormal number of white blood cells indicates urinary tract infection.

The urine can be analyzed to reveal the state of the body. If body metabolism is altered, or if there is kid-ney dysfunction, substances that are not normally present in the urine may be found or the concentra-tion of normal constituents of urine may be increased to abnormal levels. The status of the kidney can also be analyzed by measuring urea and creatinine levels in blood.