EXCRETION AND THE EXCRETORY ORGANS 525 



colored liquid, of a slightly acid reaction; its specific gravity is 

 about 1,022, being higher when the total quantity excreted is 

 small than when it is greater, since the amount of solids dissolved 

 in it remains nearly the same in health; the changes in its bulk 

 being dependent mainly on changes in the amount of water 

 separated from the blood by the kidneys. 



Normal Urine consists of about 96 per cent water and 4 per cent 

 dissolved solids. Chemically it is a very complex liquid, the 4 

 per cent of dissolved materials including a large variety of dif- 

 ferent substances. This is to be expected when, we recall that the 

 kidney is the excretory channel, not only for the chief part of the 

 endogenous excreta, but also for virtually all the exogenous waste 

 materials that are absorbed into the blood-stream. Among these 

 latter are found the substances that lend flavor to our food; like- 

 wise most drugs that are taken find their way ultimately into the 

 urine. One group of exogenous urinary substances, the ethereal 

 sulphates, are interesting since they are derived from compounds 

 formed in the large intestine in the course of the putrefactive 

 processes which normally go on there; these compounds are ab- 

 sorbed into the blood-stream and are excreted by the kidney. 

 The extent of their occurrence in the urine measures the amount 

 of putrefaction in the large intestine. These substances are toxic 

 if present in quantity and it may be that the ill feeling which 

 often accompanies constipation is the result of their presence in 

 considerable concentration in the blood. 



Urea is the constituent of urine most abundant next to the 

 water. About two per cent of urine, half of all the dissolved ma- 

 terials, is urea. The greater part of this is of exogenous origin, 

 being formed in the liver from the ammonia residues of fuel- 

 protein. The amount of exogenous urea varies from time to 

 time according as the amount of protein undergoing absorption 

 varies. It is thought that a certain amount of endogenous 

 urea is produced during the course of cell metabolism. How 

 much of the total urea of the excretion is of this origin cannot 

 be told. 



Creatinin. In some respects the most interesting of the en- 

 dogenous excreta found in the urine is the compound creatinin. 

 This substance, as stated in Chap. I, is excreted during health at 

 a rate which is practically constant for a given individual, and 



