320 



PHYSIOLOGICAL CHEMISTRY 



m 



and the contraction in volume causes the alveolar air in the lower part of y to 

 be drawn back into x. 1 At the end of 5 minutes the cock b is turned so as to 

 connect y with c, thus closing x which then contains exactly 100 c.c. of alveolar 

 air at atmospheric pressure and at the temperature of the water in which the 

 apparatus is immersed, this temperature remaining constant throughout the 

 determination. The apparatus is removed from the water, the tube c is placed 

 beneath the surface of some 10 per cent sodium hydroxide solution, some of 

 the alkali is drawn up into y, the apparatus is held in such a position that y is 

 rather depressed in order to prevent the escape of 

 small bubbles of gas from x, the cock is turned so as 

 to connect y with x, and some of the alkali is forced 

 into x. The cock b is at once turned, closing x and 

 connecting y with c through which the remainder of 

 the alkali is allowed to flow. The apparatus is inverted 

 several times during the course of half a minute which is 

 sufficient time for the absorption of all the carbon 

 dioxide. It is then returned to the water which rises 

 through c into y, after which b is turned to connect y 

 and x and the whole is allowed to remain for 5 minutes 

 to again equalize the temperature. It is then raised 

 rapidly until the water in the graduated portion of x is 

 at the same level as the water outside the apparatus, 

 i.e., when the gas within the tube x is under atmos- 

 pheric pressure. 



Calculation. The reading of the bottom of the 

 meniscus of the fluid in x is taken and represents, 

 without any further calculation or correction, the per- 

 centage of carbon dioxide in the alveolar air. If it is 

 desired to express this percentage as the partial pressure 

 of CO 2 in millimeters of mercury 2 it is multiplied by a 

 figure 40 mm. less than the prevailing barometric pres- 

 sure ; e.g., if the reading of the apparatus is 5.5 then the calculation will be 

 as follows: 5.5 per cent CO 2 or 0.055 X (760 40). This 40 mm. is the 

 tension of water vapor in the lungs at body temperature. It is sufficient for 

 clinical purposes to use the mean barometric pressure of the locality, neglect- 

 ng the daily variations. 



Interpretation. The sample of air obtained by this method (if prop- 

 erly taken) represents more nearly air whose C02 tension is the same 

 as that of arterial blood, i.e., true alveolar air, than does air obtained 

 by " rebreathing. " The results obtained by this method, then, are 

 from 5 to 10 per cent lower than those obtained by the Marriott 

 method below. The average normal value for men 3 is about 5.5 

 volumes per cent of carbon dioxide. In women and children the 

 normal value is somewhat lower. In acidosis the carbon dioxide falls 



1 Any diffusion with the outside air at the top of y will not reach to the bottom of the 

 tube owing to its length. 



2 Henderson and Morriss: Jour. Biol. Chem., 31, 217, 1917. 



3 Beddard, Pembrey, and Spriggs: Brit. Med. Jour. 2, 389, 1915. 



FIG. 103. FRIDERICIA 

 APPARATUS. 



