324 PHYSIOLOGICAL CHEMISTRY 



in equilibrium with the venous blood in the pulmonary capillaries. The tension 

 of carbon dioxide is approximately that in the venous blood. "Alveolar" air 

 collected by the Haldane or Fridericia methods is air which has come in approximate 

 equilibrium with the arterial blood, and hence is of a carbon dioxide tension from 

 10 to 20 per cent lower. 



Changes in the pulmonary epithelium such as would prevent the air in the lungs 

 from coming in equilibrium with the blood in the capillaries would, of necessity, 

 affect the composition of the alveolar air. Since very little is known as yet regarding 

 the exact effect of such changes, one is hardly justified in drawing conclusions 

 regarding acidosis from the composition of the alveolar air in patients with 

 pulmonary affections. 1 



4. Alkali Reserve. Indirect Method. Index of Acid Excretion in 

 Urine. Method of Fitz and Van Slyke. * Principle. The method de- 

 pends upon the determination of the rate of excretion of acid (NH 8 + 

 titratable acid) from which the plasma carbon dioxide capacity is 

 calculated. 



Procedure. Collect the urine for 24 hours (or if desired for a period of I 

 or 2 hours during which the subject ingests neither food nor water). In the 

 latter case the urine collection should not be too soon after a meal. Measure, 

 carefully, the volume of the urine and determine its ammonia content according 

 to the method given on page 519 and the titratable acid according to the method 

 given on page 499. Obtain the body weight of the patient. 



Calculation. The plasma bicarbonate may be calculated by substitution 

 in the following equation. 



Plasma Carbon Dioxide Capacity = 8o 3 5 \w 



D = Rate of excretion per 24 hours. 

 W = Body weight in kilograms. 



The value D is equal to the product VC, hi which V is the 24 hour volume 4 

 expressed hi liters, and C the sum of the ammonia (expressed as c.c. of N/io 

 NH 3 per liter of urine) plus the titratable acid (expressed as c.c. of N/io acid 

 per liter of urine). For practical purposes the acid excretion may, without 

 going through the calculation of the formula, be interpreted directly into terms 

 of clinical severity of acidosis, as indicated in the table on page 317, e.g., an 

 excretion exceeding 27 c.c. of N/io ammonia plus titratable acid per kilo in- 

 dicates acidosis, which usually becomes critical in severity if the excretion 

 approaches 100 c.c. per kilo. 



Interpretation. After careful investigation in which the relation- 

 ship between the carbon dioxide capacity of plasma and the excretion 

 rate and concentration of total urinary acid excreted in excess of 



1 Means, Newburgh and Porter: Boston Med. and Surg. Jour., 1915, clxxiii, 742. 



2 Fitz and Van Slyke: Jour. Biol. Chem., 30, 389, 1917; Van Slyke: Ibi4. t 33, 271, 

 1918; Barnett: Jour. Biol. Ghent., 33, 267, 1918. 



3 The value 80 represents the maximum normal value of plasma bicarbonate. Under 

 such a condition the titratable acid and ammonia excretion tend to approach zero. 



4 If the urine is collected for only i or 2 hours its volume is, of course, multiplied by 24 

 or 12 as the case may be. 



