566 THE EXCRETION OF URINE 



again taken, there is a retention of salt and of water, with a consequent 

 increase in body weight, until equilibrium is re-established on the old 

 level. While the above is the usual reaction, a considerable retention of 

 salt without an increase in the water content of the body may occur in 

 some apparently normal cases. This is due probably to the deposition 

 of salt in the tissues. 



Careful studies fail to confirm the idea that there is a fixed relation- 

 ship between the salt and the water of the body. As with the nitroge- 

 nous constituents, however, there appears to be a relationship between 

 the rate of excretion of chlorides and the amount of chloride in the blood. 

 Ambard believes that this relationship, like that of the excretion of urea 

 to the blood urea, is capable of being expressed mathematically (see 

 page 562), if allowance is made for the fact that NaCl is not excreted 

 after it falls below a certain concentration in the blood equal to about 

 5.62 gm. per 1000 c.c. This level is more or less constant for normal 

 individuals, but is considerably increased in disease of the kidney. This 

 is known as the threshold of chloride excretion. 



The amount of sodium chloride excreted in the urine in twenty-four 

 hours varies between 8 and 20 gm. a day, according to the intake. It 

 is therefore apparent that the kidney is able to concentrate the salts 

 of the plasma from ten to twenty times. 



The Sulphates. Since the inorganic sulphates do not form an im- 

 portant constituent of the food, the greater portion of the sulphates of 

 the urine are derived from the sulphur found in the protein molecule. 

 For this reason the sulphates of the urine, like the nitrogen, are a meas- 

 ure of protein metabolism. An increase in the nitrogen excretion is 

 accompanied by an increase in the sulphur excretion, the ratio being 

 about 5 to 1. The daily output of sulphur is between 1 and 3 gm. The 

 greatest output is in the form of the alkaline sulphates, about 10 per- 

 cent in combination with aromatic bodies, and a small amount in com- 

 bination with amino acids and neutral organic salts. 



The phosphates of the urine are derived from the food and from the 

 oxidation of phosphorus-containing bodies in the tissues such as 

 nuclein, lecithin, etc. The daily excretion varies between 1 and 5 gm., 

 calculated as P 2 5 . When calcium or magnesium is present in the 

 food, they are excreted by the bowel as phosphate, a,nd proportionately 

 less is found in the urine. The amount usually excreted in the feces 

 equals about 30 per cent of the total. 



Since phosphates in the urine exist as a mixture of the mono- and di- 

 sodium hydrogen phosphates, they have an important bearing on the 



