Urine, the excrementitious fluid eliminated by the kidneys, and containing the products of disintegration or physiological waste of the animal system. The physical and chemical characters of urine present a general resemblance in different classes of animals, accompanied by special variations in regard to particular ingredients. In the human subject the urine is a light amber-colored fluid, of a watery consistency, a moderately acid reaction, and an average specific gravity of 1.024. Its average daily quantity is 35 fluid ounces, or 2¼ pints; but this amount varies, within certain limits, according to the quantity of fluid taken with the food and drink, and that lost by perspiration or otherwise. If the amount of drink be unusually abundant, a part at least of the water so taken will pass off by the kidneys, and the urine will be increased in proportion. On the other hand, should the perspiration from any cause be unusually active, less fluid will be discharged by the kidneys, and the amount of urine will be consequently diminished. These variations evidently depend simply upon the fluctuation of the watery ingredients of the urine, while its solid constituents are comparatively unchanged in amount.

Accordingly, its physical qualities, particularly its color, specific gravity, and acidity, vary under these circumstances in inverse ratio to its quantity. When the urine is abundant from excess of water, it is paler than usual, its acid reaction less marked, and its specific gravity diminished. When it is lessened in amount from deficiency of water, it is more deeply colored, of a strongly acid reaction, and of a high specific gravity. Variations of this kind occur from day to day, owing to incidental causes, and are strictly within the limits of health; since the solid ingredients of the excretion, representing the products of bodily metamorphosis', are still discharged, in either case, in their due proportion. Similar fluctuations in the density, color, and acidity of the urine take place naturally at various periods of the day; that passed in the latter part of the day, during the night, and on first rising in the morning, being usually of a deep color, decidedly acid, and of a high specific gravity, often as much as 1.028; while that passed during the forenoon and middle of the day is comparatively pale, often neutral or but slightly acid, and of a specific gravity sometimes as low as 1.016 or 1.018. The specific gravity 1.024 represents the average density of all the urine passed during 24 hours in a state of health. - The average constitution of human urine is:

Water

938.00

Urea

30.00

Creatine

1.25

Creatinine

1.50

Urate of Soda

1.80

Urate of Potassa

Urate of ammonia......

Biphosphate of soda...

12.45

Phosphate of soda.....

Phosphate of soda

Phosphate of lime.....

Phosphate of magnesia

Chloride of sodium

7.80

Chloride of potassium

Sulphate of soda.......

6.90

Sulphate of potassa

Coloring matter and mucus.........................

.30

Total..................

1000.00

Of these ingredients urea is the most characteristic and important. It is a nitrogenized crystallizable substance, freely soluble in water, and exists in very minute proportion in the healthy blood of man and mammals generally. It represents one product of the physiological waste or retrograde metamorphosis of the system, though it is not yet certain from what special set of tissues or organs it is derived. It is increased by muscular exertion or by a diet of animal food, and diminished by repose or by a diet consisting of vegetable and non-nitrogenous substances. It is constantly eliminated from the blood by the kidneys, during its circulation through their vessels; and this explains the fact that, although it exists in healthy blood only in the proportion of 0.16 part per thousand, it is found in the urine in the proportion of 30 parts per thousand. It is, in fact, constantly drained away from the blood and accumulated in the urine, to be thus discharged from the body. The average quantity in which urea is produced and discharged in man during health is 500 grains a day.

If, from ligature of the renal arteries or disabling affections of the kidney, the urea is retained and allowed to accumulate in the blood, it becomes poisonous as soon as its quantity has increased to a certain point, and signs of disturbance of the nervous system and of nutrition generally come on, and become constantly aggravated until they terminate in death. This happens more particularly in both the acute and chronic form of Bright's disease. (See Albuminukia.) Creatine and creatinine are also both nitrogenous crystallizable substances, products of disintegration. They are produced in the muscular tissue, from which they are absorbed by the blood, and thence in turn eliminated by the kidneys. The urates of soda, potassa, and ammonia are combinations of these bases with a nitrogenous acid body of organic origin, namely, uric acid. Uric acid by itself is extremely insoluble in watery fluids, but its saline combinations with the above named alkaline bases are readily soluble in the proportions of water usually existing in the urine. They may however be decomposed by the addition of a free acid to the urine, or by the development of such an acid in it from the changes of decomposition.

The new acid then combines with the alkaline bases, and the insoluble uric acid thus set free is deposited in the crystalline form. It is in this way that "uric acid gravel" is formed in the urine, or that calculi composed of uric acid increase in size. The quantity of urates ordinarily discharged by the urine in health is about 25 grains a day. The acid reaction of the urine is due to the presence of the biphosphate of soda, the solutions of which are acid to test paper. The alkaline phosphates, that is, the phosphate of soda and the phosphate of potassa, are themselves soluble in water. The earthy phosphates, on the other hand, that is, the phosphate of lime and the phosphate of magnesia, require an acid fluid for their solution; consequently they are readily held in solution in the urine so long as it maintains its natural acid reaction. But if it be rendered alkaline by the addition or formation of an alkaline ingredient, it at once becomes turbid by the deposit of the earthy phosphates. These deposits may be easily recognized by their being redissolved on the addition of a small quantity of any mineral or vegetable acid. The chlorides and sulphates of the urine are all readily soluble, and seldom or never appear as a deposit.

After being discharged from the bladder, the urine, if kept exposed to the atmosphere at a moderately warm temperature, undergoes decomposition. The minute quantity of animal matter existing in the mucus becomes a ferment, and causes the urea to be gradually transformed into carbonate of ammonia. This first neutralizes the acid reaction of the urine and causes it to become turbid from a deposit of its earthy phosphates. Subsequently, as the products of decomposition increase in quantity, the urine becomes strongly alkaline and saturated with the ammoniacal salt, which forms a new crystalline combination, namely, the phosphate of magnesia and ammonia, or the so-called "triple phosphate," and finally exhales from its surface a strong ammoniacal vapor. This goes on until all the urea originally existing in the urine has been thus decomposed and returned to the atmosphere or the soil under the form of an ammoniacal combination.