452 PHYSIOLOGICAL CHEMISTRY. 



55. URINE AND ITS NORMAL CONSTITUENTS. 



Secretion of urine. It has been explained in a former chapter 

 how blood absorbs the digested food as chyle, how this is acted upon 

 by the atmospheric oxygen in the lungs ; and how this arterial blood, 

 whilst passing through the system, deposits proteids and other sub- 

 stances, receiving in exchange the products formed by the oxidation of 

 the various tissues. These products are either gases (chiefly carbon 

 dioxide), liquids (chiefly water), or solids held in solution by the 

 water. These waste solids must necessarily be eliminated from the 

 system, and the organs which accomplish this result are the kidneys. 



The process of separating the waste materials from the blood is chiefly of a 

 physical nature, partly a transudation or nitration, and partly a diffusion or 

 osmose. The conditions essential for such an exchange are given in the 

 kidneys. Blood is separated by delicate membranes from a thin, aqueous, 

 saline solution ; the interchange taking place is chiefly a passage of the waste 

 crystalline products of the blood into the aqueous solution, which is thereby 

 gradually converted into urine, that liquid, which is finally discharged, carry- 

 ing off nearly the total quantity of all the nitrogen taken into the system in the 

 form of nitrogenous food. 



General properties. Normal human urine, when in a fresh state, 

 is a clear, transparent aqueous liquid, of a lighter or deeper amber 

 color, having a peculiar, faintly aromatic odor, a bitter, saline taste, 

 a distinct acid reaction on blue litmus-paper, and a specific gravity 

 heavier than water (average about 1.020). When urine is kept in a 

 clean vessel it may remain unchanged for several days, provided the 

 temperature be not too high, and the amount of total solid con- 

 stituents not too small. 



In urine, shortly after cooling, especially if it be concentrated, a 

 light, cloudy film of mucus is formed, which slowly sinks to the 

 bottom ; the acid reaction gradually increases, small yellowish-red 

 crystals of acid urates, or uric acid, are deposited. In this condition 

 the urine may often continue unchanged for several weeks, provided 

 the temperature be low. If, however, the urine be very dilute, and 

 the temperature above the mean, decomposition speedily takes place. 

 The urine is then found to be covered with a thin, shining, and fre- 

 quently iridescent membrane, fragments of which sink gradually to 

 the bottom. The urine then becomes turbid, acquires a pale color, 

 its reaction becomes alkaline, and it begins to develop a nauseous 

 ammoniacal odor, due to the products formed by the decomposing 

 action of certain microorganisms (chiefly bacterium urese and micro- 



