urea in great abundance. This does not occur when the same experiment is performed with any other organ of the body, so that Schröder's experiments also prove the great importance of the liver in urea formation. THE INORGANIC CONSTITUENTS OF URINE The inorganic or mineral constituents of urine are chiefly chlorides, phosphates, sulphates, and carbonates; the metals with which these. are in combination are sodium, potassium, ammonium, calcium, and magnesium. The total amount of these salts varies from 9 to 25 grammes daily. The most abundant is sodium chloride, which averages in amount 10 to 13 grammes per diem. These substances are derived from two sources-first from the food, and secondly as the result of metabolic processes. The chlorides and most of the phosphates come from the food; the sulphates and some of the phosphates, as a result of metabolism. The salts of the blood and of the urine are much the same, with the important exception that, whereas the blood contains only traces of sulphates, the urine contains abundance of these salts. The sulphates are derived from the changes that occur in the proteids of the body; the nitrogen of proteids leaves the body as urea and uric acid; the sulphur of the proteids is oxidised to form sulphuric acid, which passes into the urine in the form of sulphates. The excretion of sulphates, moreover, runs parallel to that of urea. The chief tests for the various salts have been given in the practical exercises at the head of this lesson. Chlorides. The chief chloride is that of sodium. The ingestion of sodium chloride is followed by its appearance in the urine, some on the same day, some on the next day. Some is decomposed to form the hydrochloric acid of the gastric juice. The salt, in passing through the body, fulfils the useful office of stimulating metabolism and secretion. Sulphates. The sulphates in the urine are principally those of potassium and sodium. They are derived from the metabolism of proteids in the body. Only the smallest trace enters the body with the food. Sulphates have an unpleasant bitter taste (for instance, Epsom salts); hence we do not take food that contains them. The sulphates vary in amount from 1.5 to 3 grammes daily. In addition to these sulphates there is a small quantity, about onetenth of the total sulphates, that are combined with organic radicles; these are known as ethereal sulphates, and they originate from putrefactive processes occurring in the intestine. The chief of these ethereal sulphates are phenol sulphate of potassium and indoxyl sulphate of potassium. The latter originates from the indole formed in the intestine, and as it yields indigo when treated with certain reagents it is sometimes called indican. It is very important to remember that the indican of urine is not the same thing as the indican of plants. Both yield indigo, but there the resemblance ceases.. Carbonates.-Carbonates and bicarbonates of sodium, calcium, magnesium, and ammonium are generally present in alkaline urine. They arise from the carbonates of the food, or from vegetable acids (malic, tartaric, &c.) in the food. They are, therefore, more abundant in the urine of herbivora and vegetarians, whose urine is thus rendered alkaline. Urine containing carbonates becomes, like saliva, cloudy on standing, the precipitate consisting of calcium carbonate, and also phosphates. Phosphates. Two classes of phosphates occur in normal urine : (1) Alkaline phosphates—that is, phosphates of sodium (abundant) and potassium (scanty). (2) Earthy phosphates—that is, phosphates of calcium (abundant) and magnesium (scanty). The composition of the phosphates in urine is liable to variation. In acid urine the acid salts are generally present, and give the urine an acid reaction. These are chiefly Sodium dihydrogen phosphate, NaH2PO4, and calcium dihydrogen phosphate, Ca(H,PO4)2. In neutral urine, in addition, disodium hydrogen phosphate (Na2HPO4), calcium hydrogen phosphate, CaHPO4, and magnesium hydrogen phosphate, MgHPO4, are found. In alkaline urine there may be instead of, or addition to, the above the normal phosphates of sodium, calcium, and magnesium [Na3PO4, Ca3(PO4)2, Mg3(PO4)2]. The earthy phosphates are precipitated by rendering the urine alkaline by ammonia. In decomposing urine ammonia is formed from the urea : this also precipitates the earthy phosphates. The phosphates most frequently found in the white creamy precipitate which occurs in decomposing urine are (1) Triple phosphate or ammonio-magnesium phosphate (NH,MgPO,+6H2O). This crystallises FIG. 42. Ammonio in coffin-lid' crystals (see fig. 42) or feathery stars. phosphate. magnesium or triple (2) Stellar phosphate, or calcium phosphate, which crystallises in star-like clusters of prisms. Normal urine gives no precipitate when it is boiled. Neutral, alkaline, and occasionally faintly acid urines give a precipitate of calcium phosphate when boiled: this precipitate is amorphous, and is liable to be mistaken for albumin. It may be distinguished readily from albumin, as it is soluble in a few drops of acetic acid, whereas coagulated proteid does not dissolve. The phosphoric acid in the urine chiefly originates from the food, but is partly a decomposition product of the phosphorised organic materials in the body, such as lecithin and nuclein. The amount of PO, in the twenty-four hours' urine varies from 2.5 to 3.5 grammes, of which the earthy phosphates contain about half (1 to 1·5 gr.). 2 5 LESSON XI URINE (continued) 1. Urea Nitrate.-Evaporate some urine to a quarter of its bulk. Pour the concentrated urine into a watch-glass; let it cool, and add a few drops of strong, but not fuming, nitric acid. Crystals of urea nitrate separate out. Examine these microscopically. 2. Uric Acid. Examine microscopically the crystals of uric acid in some urine, to which 5 per cent. of hydrochloric acid has been added twenty-four hours previously. Note that they are deeply tinged with pigment, and to the naked eye look like granules of cayenne pepper. Dissolve the crystals in caustic potash and then carefully add excess of hydrochloric acid. Small crystals of uric acid again form. Place a little uric acid, or a urate (for instance, serpent's urine), in a capsule; add a little dilute nitric acid and evaporate to dryness. A yellowishred residue is left. Add ammonia. This is best done by holding the capsule upside down over the mouth of a bottle of ammonia. The residue turns to violet. This is due to the formation of murexide or purpurate of ammonia. On the addition of potash the colour becomes bluer. 3. Deposit of Urates or Lithates (Lateritious Deposit).-The specimen of urine from the hospital contains excess of urates, which have become deposited on the urine becoming cool. They are tinged with pigment, and have a pinkish colour, like brick-dust; hence the term 'lateritious.' Examine microscopically. The deposit is usually amorphous-that is, non-crystalline. Sometimes crystals of calcium oxalate (envelope crystals -octahedra) are seen. The deposit of urates dissolves on heating the urine. 4. Deposit of Phosphates.--Another specimen of pathological urine contains excess of phosphates, which have formed a white deposit on the urine becoming alkaline. This precipitate does not dissolve on heating; it may be increased. It is, however, soluble in acetic acid. Examine microscopically for coffin-lid crystals of triple phosphate (ammonio-magnesium phosphate), for crystals of stellar (calcium) phosphate, and for mucus. Mucus is flocculent to the naked eye, amorphous to the microscope. N.B.—On boiling neutral or alkaline urine it may become turbid from deposition of phosphates. The solubility of this deposit in a few drops of acetic acid distinguishes it from albumin, for which it is liable to be mistaken. Some of the facts described in the foregoing exercises have been already dwelt upon in the preceding lesson. They are, however, conveniently grouped together here, as all involve the use of the microscope. We have now studied urea, the principal nitrogenous constituent of urine, at some length. There are still left for our consideration a H number of other nitrogenous constituents, the most important of which are uric acid, hippuric acid, and creatinine. URIC ACID Uric Acid (C,N1H1О ̧) is in mammals, next to urea, the medium by which the largest quantity of nitrogen is excreted from the body. It is, however, in birds and reptiles the principal nitrogenous constituent of their urine. It is not present in the free state, but is combined with bases to form urates.] It may be obtained from human urine by adding 5 c.c. of hydrochloric acid to 100 c.c. of the urine, and allowing the mixture to stand for twelve to twenty-four hours. The crystals which form are deeply tinged with urinary pigment, and though by repeated solution in FIG. 43.-Uric acid crystals. a caustic soda or potash, and reprecipitation by hydrochloric acid, they may be obtained fairly free from pigment, pure uric acid is. more readily obtained from the solid urine of a serpent or bird, which consists principally of the acid ammonium urate. This is dissolved in soda, and then the addition of hydrochloric acid produces as before the crystallisation of uric acid from the solution. The pure acid crystallises in colourless rectangular plates or prisms. In striking contrast to urea it is a most insoluble substance, requiring for its solution 1900 parts. of hot and 15000 parts of cold water. The forms which uric acid assumes when precipitated from human urine, either by the addition of hydrochloric acid or in certain pathological processes, are very various, the most frequent being the whetstone shape; there are also bundles of crystal resembling sheaves, barrels, and dumb-bells (see fig. 43). The murexide test which has just been described among the practical exercises is the principal test for uric acid. The test has received the name on account of the resemblance of the colour to the purple of the ancients, which was obtained from certain snail-like creatures of the genus Murex. Another important reaction that uric acid undergoes (though it is not applicable as a test) is that on treatment with certain reagents urea and oxalic acid can be obtained from it. The importance of this fact is that probably uric acid may be in the body an antecedent of Excess of uric acid and of oxalic acid generally go together; and urea. |