ACIDOSIS 39 



Under normal conditions the amount of NaHCO 3 in blood plasma is very 

 constant (amounting to 50-65 vols. per cent C0 2 ), and when it is reduced, 

 it indicates that an excess of fixed acid must be present. This is taken 

 by Van Slyke and others to constitute the real definition of acidosis 

 namely, "a condition in which the concentration of bicarbonate in the 

 blood is reduced below the normal level." If the respiratory center 

 for any reason should not respond promptly enough to an increase in 



TT rir\ 



the molecular ratio Vr/^/k > an( ^ ^ H consequently become greater, the 

 NaHLOg 



condition is called uncompensated acidosis, but if the center does respond 

 so that C H is held constant (although NaHC0 3 is decreased), the condition 

 is one of compensated acidosis. 



For practical reasons, therefore, the study of pathologic acidosis de- 

 pends on an estimation of the bicarbonate content of the blood or, since 

 it is simpler to carry out and is of equal value, of the plasma. When 

 plasma is obtained by removing blood from a vein of the arm and cen- 

 trifuging immediately out of contact with air (so that C0 2 may not be 

 lost from it) it contains approximately 60 vols. per cent of C0 2 . Since 

 we know that the partial pressure of C0 2 in blood is equal to 42 mm. Hg 

 (ascertained from determinations of the alveolar C0 2 ) (see page 344), 

 we can calculate how much of the 60 vols. per cent must be in simple 

 solution by application of the law of solution of gas in a liquid (page 

 336). It has been found that plasma at body temperature and at 760 



mm. Hg (atmospheric pressure) dissolves 0.54 per cent C0 2 , so that at 



42 



42 mm. it will dissolve _ pr . x 100 x 0.54 = 3 vols. per cent. Transcribing 



7bO 



[H 2 C0 3 ] 3 1 



the figures to our equation we get 



[NaHC0 3 ] 60 20 

 This definition of acidosis leaves out of regard all conditions that may 



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raise the ratio 2 3 by the addition of H 2 C0 3 without decomposing 



any of the NaHC0 3 , such, for example, as occurs Avhen an excess of free 

 carbonic acid is present in the blood plasma. Since increases in free 

 C0 2 are not infrequent in both health and disease e. g., asphyxial con- 

 ditions the above definition is not sufficiently comprehensive. When 

 we come to study the control of the respiratory center, we shall see that 



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an increase in the ratio 2 ^pof sufficient magnitude to cause an 



IN clii-vy v/3 



actual increase in C H can be produced by causing an animal to respire air 



*This agrees sufficiently with the result as calculated from the known values of the equation 

 N HCO~ ~ ~TC~ ' Thus, if we take CH as 0.35 xlO- 7 , \ as 0.605 for blood conditions, and 



L H 2 CO 3 0.605 x 0.35 x 10- 7 _ 1 



fc as 4.4 x 10- (M.chaehs and Rona),, we get = 4.4 x IQ -T = J\ 



