Waste substances resulting from the body’s metabolic processes, selected by the KIDNEYS from the blood, dissolved in water, and excreted. Urine is around 96 per cent water, the chief waste substances being UREA (approximately 25 g/1), common salt (approximately 9 g/l), and phosphates and sulphates of potassium, sodium, calcium, and magnesium. There are also small amounts of URIC ACID, ammonia, creatinine, and various pigments. Poisons, such as MORPHINE, may be excreted in the urine; and in many infections, such as typhoid fever (see ENTERIC FEVER), the causative organism may be excreted.
The daily urine output varies, but averages around 1,500 ml in adults, less in children. The ?uid intake and ?uid output (urine and PERSPIRATION) are interdependent, so as to maintain a relatively constant ?uid balance. Urine output is increased in certain diseases, notably DIABETES MELLITUS; it is diminished (or even temporarily stopped) in acute glomerulonephritis (see under KIDNEYS, DISEASES OF), heart failure, and fevers generally. Failure of the kidneys to secrete any urine is known as anuria, while stoppage due to obstruction of the ureters (see URETER) by stones, or of the URETHRA by a stricture, despite normal urinary secretion, is known as urinary retention.
Normal urine is described as straw- to amber-coloured, but may be changed by various diseases or drugs. Chronic glomerulonephritis or poorly controlled diabetes may lead to a watery appearance, as may drinking large amounts of water. Consumption of beetroot or rhubarb may lead to an orange or red colour, while passage of blood in the urine (haematuria) results in a pink or bright red appearance, or a smoky tint if just small amounts are passed. A greenish urine is usually due to BILE, or may be produced by taking QUININE.
Healthy urine has a faint aroma, but gives o? an unpleasant ammoniacal smell when it begins to decompose, as may occur in urinary infections. Many foods and additives give urine a distinctive odour; garlic is particularly characteristic. The density or speci?c gravity of urine varies normally from 1,015 to 1,025: a low value suggests chronic glomerulonephritis, while a high value may occur in uncontrolled diabetes or during fevers. Urine is normally acidic, which has an important antiseptic action; it may at times become alkaline, however, and in vegetarians, owing to the large dietary consumption of alkaline salts, it is permanently alkaline.
Chemical or microscopical examination of the urine is necessary to reveal abnormal drugs, poisons, or micro-organisms. There are six substances which must be easily detectable for diagnostic purposes: these are ALBUMINS, blood, GLUCOSE, bile, ACETONE, and PUS and tube-casts (casts from the lining of the tubules in the kidneys). Easily used strip tests are available for all of these, except the last.
Excess of urine It is important to distinguish urinary frequency from increase in the total amount of urine passed. Frequency may be due to reduced bladder capacity, such as may be caused by an enlarged PROSTATE GLAND, or due to any irritation or infection of the kidneys or bladder, such as CYSTITIS or the formation of a stone. Increased total urinary output, on the other hand, is often a diagnostic feature of diabetes mellitus. Involuntary passage of urine at night may result, leading to bed wetting, or NOCTURNAL ENURESIS in children. Diagnosis of either condition, therefore, means that the urine should be tested for glucose, albumin, gravel (fragments of urinary calculi), and pus, with appropriate treatment.
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