The quantity of urine secreted depends, of course, chiefly upon the quantity of water that has been ingested, but it is in part determined also by the activity of the skin. With free exercise, and abundant sweating in dry air and in hot weather, the quantity is diminished, whilst it is increased when the animal is at rest and exposed to cold. The quantity discharged per diem varies under these circumstances, and with the size of the animal, from four or five quarts, which is about the average, to twelve or more, or putting it in another form, from about one-quarter to one-half ounce for each one pound of the body weight of the animal. It is more abundant, as might naturally be expected, in grass-fed animals than in those whose food is confined to such dry matter as oats and hay.
When first emitted the urine of the horse is a clear yellowish fluid, but it soon becomes turbid. It has a peculiar and rather unpleasant odour, and a specific gravity of about l.042, but varying from 1.030 to 1.055 or more. Its reaction to test-paper is always alkaline in health, the alkalinity being chiefly due to the presence of potassium bicarbonate.
Fig. 134. - Urine Salts.
Various forms of calcic carbonates and ammonium urates found in normal urine of the horse.
When the urine of the horse is analysed it is found that in every 1000 parts there are about 905-910 of water, 55 of organic substances, and 40 of inorganic substances. The organic substances include urea, hippuric, benzoic, uric, and oxalic acids and their salts, with mucus and epithelial scales from the lining membrane of the bladder and urethra; whilst the inorganic substances are represented by the lactates, carbonates, sulphates, and, in very small quantity, phosphates of the metals sodium, potassium, calcium, and magnesium, with some chloride of sodium and silica. The relative proportions of these vary greatly with the nature of the food, and also according as it has been examined when taken from the animal in the fasting state or during full digestion.
The urine of the horse contains, in addition, a considerable quantity of the aromatic substances named phenolkresol and indoxyl combined with sulphuric acid, as well as the compound named Brenz- or Pyro-catechin, which is partly free and partly also combined with sulphuric acid.
Pyro-catechin exposed to the air in an alkaline solution absorbs oxygen and becomes brown, which is probably the cause of the darkening in hue which the urine of the horse undergoes after expulsion from the bladder.
Veterinary Captain F. Smith, in a paper contained in the Proceedings of the Royal Society gives the following instructive table showing the mean composition of the urine of the horse for twenty-four hours at rest and at work:-
Ammonium carbonate as urea.....
The difference in the size and breed of the animal probably accounts for the great discrepancies in the analyses of the urine of the horse that have been published. In some analyses made by Wessinger, who experimented upon animals of Hungarian breed, it was found that the specific gravity varied from 1'042 to 1.046, and that the amount of dry residue after evaporation amounted to 6176-5404 grains per diem. The quantity of chlorine varied within wide limits, but it may be taken at 400-500 grains. The quantity of sulphuric acid was 262 grains. The quantity of nitrogen varied in health from 994 grains to 1698 grains, the mean being 1193'5 grains. It is remarkable that the phosphates ingested with the food are only eliminated in small quantity by the horse in the urine, whilst in carnivora they are contained in large quantity. In the horse, the phosphates are discharged from the body by the bowels. The alkaline and earthy bases are chiefly combined with sulphuric acid, though a portion are in combination with carbon dioxide, and it is the lime carbonate in minute delicate crystals that gives the cloudy appearance to the urine of the horse, sometimes even when quite freshly drawn. A small quantity of nitrogen is eliminated in the form of ammonia and the salts of that alkali, but the proportion discharged from the system in this form, as compared with the total amount of nitrogen, is only as 1 : 214.
A few observations may be made upon each of the principal constituents of the urine. The first and most important of the organic constituents is the urea, the composition of which is CO (NIL).,. The interest attaching to this substance is that nitrogen constitutes nearly half its weight (46.6 per cent). Now nitrogen does not enter into the composition of either the fats or the starchy or saccharine components of our food, whilst it constitutes about 16 per cent of the various proteids, such as those which form the greater part of flesh, and are found in blood, milk, eggs, and in the gluten of fruits and seeds of the cereals and leguminous plants. The quantity of nitrogen which is discharged as urea rises and falls with the quantity of nitrogen-holding substances supplied in the food and absorbed in the intestines, and it thus forms a measure of the amount of proteids that have been ingested. Upon the average 16 grains of nitrogen correspond to 100 grains of dry albumen broken up and consumed in the body. Or, expressing it in another way, every grain of nitrogen in the urine corresponds to the consumption of 30 grains of flesh of the animal, to the manufacture of which it is supposed that all the gluten and albuminoids in the food are applied, and so every grain of urea (= 0.467 N) in the urine corresponds to 13.7 grains of disintegrated flesh of the animal. The quantity of urea in the urine is not materially augmented by muscular work, which is one of the facts relied upon to prove that muscular force is generated, not from the disintegration and metabolism of the muscle itself, but of the starchy and fatty constituents of the body. Just as in a locomotive, the force expended in effecting change of place is derived, not from the wear and tear of the iron framework of the machine, but from the oxygenation and combustion of the carbon of the fuel.
The origin of urea, it is believed, must be sought for in a substance named kreatin, which exists in considerable quantity in the muscles, and from which urea can be artificially obtained in the laboratory. The kreatin of the muscles, or possibly some intermediate substance as ammonium carbonate, is carried by the blood to the liver, and is there converted into urea. This is taken up by the blood and carried to the kidneys, where it is excreted by the cells of the urinary tubules and mixed with the water of the urine. That urea is not formed at or by the kidneys is demonstrated by the fact that it accumulates in the blood after the kidneys have been removed from the body by operation.
Fig. 135. - Crystals of Urea.