RESPIRATION 



203 



blood during and after pregnancy.^^ It had already been found 

 by Hasselbalch and others that the alveolar CO2 pressure is much 

 lower than normal during pregnancy. Taking advantage of this 

 fact, they determined the Ph of arterial blood before and after 

 childbirth with the results shown in the accompanying table. 



Allowing for the probable errors in determining the Ph and 

 alveolar CO2 pressure, these figures seem to show that the fall in 

 alveolar CO2 pressure compensates within the limits of accuracy 

 of the electrometric method for a fall in the Ph of the blood which 

 would otherwise occur. The mean of the first two columns shows 

 that this fall in Ph would have been 0.05, whereas the compen- 

 sating fall in alveolar CO2 pressure was 8.2 mm. as shown by the 

 mean for the second two columns. Hence a difference of 0.0 1 in 

 Ph corresponds to a difference of 1.6 mm. of CO2 pressure, or 

 0.23 per cent of CO2 in alveolar air. We have already seen, how- 

 ever, that a change of about this amount in alveolar COg pressure 

 is sufficient to cause either apnoea or doubling of the alveolar 

 ventilation according to its direction. Even under the most favor- 

 able conditions it is hardly possible at present to determine differ- 

 ences in Ph within the body to within 0.03 in single observations ; 

 but by measuring the variations in lung ventilation as compared 

 with production of CO2 we have an index of change in Ph which 

 is at least 50 times as sensitive as the existing direct electrometric 

 method, exact as this is in comparison with older methods. 



Although the measurements of Ph showed no change in the 

 alkalinity^ of the blood during pregnancy, yet the fall in alveolar 

 CO2 pressure indicated that there was an increase of 25 per cent in 

 the lung ventilation per unit of CO2 given off. This, therefore, 

 would correspond to an ''acidosis" to the extent of a Ph of 0.003 — 

 an amount far too small for direct measurement. That it was 

 acidosis which caused the increase in the breathing was shown by 

 the fact that the increase was accompanied by an increase of about 

 20 per cent in the proportion of nitrogen excreted as NH3 to total 

 nitrogen excreted in the urine. The authors conclude that there is 

 an increased acid production in the body during pregnancy (or 

 perhaps an increased drain of alkali from the body of the mother) , 

 but that it is compensated by increased breathing and formation 

 of NH3. It is true that relatively to the degree of accuracy at 

 present attainable in determining the Ph of blood the compensa- 

 tion is perfect. But if the compensation were really perfect we 

 should be landed in the position of the vitalists of assuming effects 



"Hasselbalch and Gammeltoft, Biochem. Zeitschr., 68, p. 206, 19 15. 



