EXCRETION BY THE KIDNEYS 



479 



The sulphur of the inorganic sulphates is the fraction of the total 

 Sulphur which fluctuates in proportion to the total protein metabolism. 

 In this regard it follows the variations in the urea. It represents 

 * exogenous ' metabolism. The neutral sulphur occupies a position 

 analogous to that of the kreatinin: the smaller the amount of protein 

 in the food, and the smaller therefore the total protein decomposed, the 

 larger is the fraction which the neutral sulphur forms of the total 

 sulphur. The neutral sulphur accordingly represents endogenous 

 metabolism. The ethereal sulphur takes an intermediate position in 

 this regard, but upon the whole it also becomes a more prominent 

 fraction of the total sulphur when the food contains little or no protein. 

 The ethereal sulphates are therefore not entirely derived from the 

 putrefaction of protein. 



Carbonates of sodium, ammonium, calcium, and magnesium occur 

 in alkaline urine. Their source is the carbonates and the vegetable 

 organic acids of the food. In acid urine a certain amount of carbon 

 dioxide is present, although not firmly united with bases, so that m:>st 

 of it can be pumped out. 



Physico-Chemical Analysis of Urine. The freezing-point of urine is 

 often determined to obtain a measure of the molecular concentration, 

 which with the total quantity of urine secreted in a given time is an 

 index of the work of the kidney. The greater the volume of urine 

 secreted per unit of time, and the greater the number of molecules 

 dissolved in unit volume of it, the greater is the work of the secretory 

 apparatus in separating it from the blood (p. 497). Normally, A has a 

 higher value for urine than for blood i.e., the molecular concentration 

 of the urine is higher than that of the serum. But when large draughts 

 of water are taken A may be lower for urine than for blood, and -in 

 general it varies within far wider limits (from 0-115 to 2'546 C., 

 according to Koppe). The following table from Kovesi and Roth- 

 Schulz shows the changes in A under the influence of water: 



If the electrical conductivity is determined, we obtain an approxi- 

 mate measure of the number of dissociated ions in unit volume, mainly 

 the inorganic salts. Deducting this from the total number of molecules 

 per unit volume (measured by A), we arrive at the concentration of 

 the urine in non-dissociated molecules, mainly urea and other organic 

 constituents. Precision is added to such calculations by estimating 

 also in the ordinary way (by titration, e.g.) one or more of the inorganic 



