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A MANUAL OF PHYSIOLOGY 



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 

 most 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 concentra- 

 tion, 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 mole- 

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

 secretory apparatus in separating it from the blood (p. 465). 

 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 '54 6 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 mole- 

 cules per unit volume (measured by A), we arrive at the concentra- 

 tion 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 constituents, especially the chlorine, since sodium 

 chloride is quantitatively the most important of the salts. Various 

 formulae have been deduced from such determinations connecting 

 the freezing-point and conductivity with other physical constants of 



^ 

 the urine. E.g., --- =K=75, where s is the specific gravity and 



K a constant with the value 75 ; =r =K=i*45, where A is the 

 specific conductivity, h the percentage of ash, and K a constant = 

 i '45. The quotient TT-, representing the ratio of the total con- 



centration to the sodium chloride concentration varies within rela- 



