82 DONALD D. VAN SLYKE 



and pathological cases when the plasma bicarbonate CO 2 approached 69 

 10 volumes per cent. 



The test is so simple that we can describe it in sufficient detail here. 

 It is performed by administering sodium bicarbonate dissolved in water 

 at the rate of a dose of 2 to 5 grams every half hour. Two samples of 1 c.c. 

 each of urine obtained before the first dose are diluted in test tubes to 

 25 c.c. each with distilled water. To one sample 10 drops of 0.04 per cent 

 phenolsulphonephthalein are added, to the other 10 drops of a saturated 

 solution of methyl red. Before giving each dose of bicarbonate another 

 sample of urine is drawn and treated in the same way. The first definite 

 change towards alkalinity shown by comparison with the original urine 

 samples marks the end point. Phenolsulphonephthalein serves to show 

 changes between pH 6.3 and 7.4, methyl red between 4.7 and 6.3, so that 

 the two indicators cover the entire range o urine reaction. 



Usually 0.2 gram or less of bicarbonate per kilo of body weight causes 

 a change in urinary pH when no acidosis is present. More than 0.5 

 gram per kilo indicates marked alkali deficit, and more than 1.0 gram 

 a condition that may threaten coma. 



The test would, it appears certain, be interfered with by conditions al- 

 tering the sensitiveness of the respiratory center, and therefore the blood 

 pll. For example, over-ventilation by itself is sufficient to raise the pH 

 of both blood and urine, even though the alkali content of the blood 

 is not increased, and the reverse is true of depressed respiration. In the 

 acidosis usually encountered in metabolic diseases, with diminished blood 

 alkali but normal respiratory control (Area 6 and the lower part of 9, 

 Figure 2) the bicarbonate retention test indicates simply and reliably the 

 acid-base balance of the body. Also, when alkali deficit exists, the thera- 

 peutic agent (bicarbonate) is administered as part of the procedure in the 

 required amount to correct the deficit. 



Determination of the Alveolar C0 2 Tension 3 



When the gas exchange in the lungs is normal, the alveolar air, as shown 

 by A. and M. Krogh, is in equilibrium in respect to its carbon dioxid 

 content with the arterial blood. Consequently, in accordance with the 

 law of gas solubilities, the concentration, or tension, of CO 2 in the al- 

 veolar air is directly proportional to that of the free carbonic acid (H 2 C0 3 ) 

 in the blood. And when the blood pH is normal and constant, the H 2 CO 3 



3 The "tension" is commonly used as the unit of concentration of the alveolar gases. 

 Pure dry C0 a at average atmospheric pressure has a tension of 760 mm. If the gas is 

 saturated with moisture at 38, at which temperature the vapor tension is 49 mm., the 

 total dry gas tension is 760 49 = 711 mm. Moist air containing 5.6 per cent 

 of CO 2 at 38, 760 mm. (as does air ordinary alveolar air sample) has a CO 2 tension 

 of 0.056x711 = 40 mm. 



