48 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



the excretion rate of acid as determined by an application of Ambard's 

 equation for chlorides and urea, and with curiously satisfactory results 

 (Fitz and Van Slyke). This equation is: 



Blood concentration = constant x V^VC; where D is the excretion 



rate, W the body weight, and C the concentration of excretory prod- 

 uct in the urine. For the present purpose D is therefore the number of 

 c.c. of N/10 alkali (or acid) required to bring the urine to the neutral 

 point of phenolphthalein plus the NH 3 expressed as c.c. of an N/10 solution, 

 for the twenty-four hours, and C is c.c. of N/10 alkali and of N/10 NH 3 

 per liter of urine. If we assume that the acid accumulation in the blood 

 is proportional to the fall of the plasma C0 2 figure below the maximal 

 figure of 80, the above equation becomes: 



/ L) _ 

 Retained acid = 80 - plasma C0 2 = constant x y \\fVC. 



For practical purposes it is best to make the necessary analysis on a 

 sample of urine collected over a period of one to four hours, and to col- 

 lect the blood for determination of its reserve alkalinity in the middle 

 of this period. The twenty-four-hour rate of excretion is then computed 

 (D) from the analysis. 



The value calculated by the above equation has been found to agree 

 with that of the C0 2 -combining power of the plasma to within 10 vol- 

 umes per cent, except when bicarbonate is being taken by the person, 

 when the blood bicarbonate is much higher than indicated by the urine. 



3. Determination of Alkali Retention. Another valuable criterion of 

 the alkaline reserve is the amount of alkali required to change the re- 

 action of the urine. In health the C H of the urine varies from 

 0.000,016 N (P H = 4.8) to about 0.000,000,035 N (P H = 7.46) with a mean 

 of about 0.000,001 N (P H = 6). These extremes are rarely overstepped 

 in disease, but frequently the average is considerably different. In car- 

 dio-renal disease, for example, the mean acidity may be approximately 

 0.000,005 N (P H = 5.3), or five times the normal value. A certain de- 

 gree of acidosis is therefore common enough in this condition a fact 

 which has indicated the advisability of administering sodium bicarbon- 

 ate. It has been found that 5 grams or less of soda, given by mouth to 

 a normal person, causes a distinct diminution in the C H of the urine, 

 whereas in pathologic cases it may be necessary to give more than 100 

 grams before a similar effect is observed (L. J. Henderson and Palmer 15 

 and Sellards 16 ). 



For this very large holding back of alkali, the organism and not the 

 kidney is responsible. This is indicated by the fact that, when the 

 administration of alkali is discontinued, the acidity of the urine soon 



