624 



SCIENCE 



[N. 8. Vol. XLIV. No. 1140 



is so arranged that there is no fall in the 

 alveolar percentage of carbon dioxide, no 

 apnea follows. There is thus no such 

 thing as the so-called vagus apnea. Apnea 

 is simply due to excessive removal of car- 

 bon dioxide from the alveolar air. 



When the barometric pressure is varied 

 it becomes evident that the normal which 

 dominates the regulation of breathing is 

 not the percentage of carbon dioxide in 

 the alveolar air, but the partial pressure 

 or molecular concentration. At the normal 

 atmospheric pressure of 30 inches there is 

 about 5.6 per cent, of carbon dioxide in 

 the alveolar air, but only 2.8 at 60 inches 

 barometric pressure, and 1.4 at 120 inches. 

 In these three cases the percentage of C0 2 

 varies widely, but the partial pressure is 

 the same. It is only with constant baro- 

 metric pressure that the normal percentage 

 is steady. 



When the breathing is increased by ex- 

 cess of C0 2 in the inspired air, or increased 

 production of CO, in the body, there is, as 

 might be expected, a slight rise in the 

 alveolar C0 2 percentage. It is this slight 

 rise that is the stimulus to increased 

 breathing. Roughly speaking, a rise of 0.2 

 per cent, increases the resting breathing by 

 100 per cent., while a fall of 0.2 per cent, 

 produces apnea. The stimulus of the in- 

 creased C0 2 percentage is conveyed to the 

 respiratory center by the blood. Under 

 ordinary average conditions the center re- 

 sponds with normal breathing when the 

 blood leaving the lungs is saturated with 

 air containing 5.6 per cent, of C0 2 , but does 

 not respond at all when the blood is satu- 

 rated with 5.4 per cent, of CO, or less. The 

 threshold value of C0 2 is, however, greatly 

 lowered by excessive administration of 

 acids or in any condition of so-called 

 acidosis, and is raised by alkalies or an 

 alkaline diet. This ajid other evidence 

 points to the fact that C0 2 acts on the 



respiratory center in virtue of its acid 

 properties when in solution. 



According to modern ideas the acidity 

 or alkalinity of a liquid depends on its 

 hydrogen ion concentration. The accurate 

 measurement of the hydrogen ion concen- 

 tration of blood by the electrometric 

 method is attended with great difficulties; 

 but these have been to a large extent over- 

 come by Hasselbalch of Copenhagen, who 

 has obtained measurements of the effects of 

 saturation with different partial pressures 

 of CO, on the hydrogen ion concentration 

 of blood. He has also shown experi- 

 mentally that when the alveolar C0 2 

 threshold is lowered or raised by an acid 

 ■or alkaline diet this raising or lowering is 

 .just sufficient to keep the hydrogen ion 

 concentration of the arterial blood sensibly 

 steady. It is now certain, therefore, that 

 what the respiratory center is reacting to 

 when it reacts to a slight increase in the 

 alveolar C0 2 percentage is the consequent 

 slight increase in the hydrogen ion concen- 

 tration of the blood. 



The latter increase is so minute that 

 it can only be detected electrometrically 

 when it is of sufficient extent to produce 

 very gross changes in the breathing. The 

 respiratory center is enormously more deli- 

 cate as an index of change in hydrogen ion 

 concentration of the blood than any exist- 

 ing physical or chemical method. 



As already remarked, the alveolar C0 2 

 percentage is extremely steady under ordi- 

 nary resting conditions. This implies that 

 the hydrogen ion concentration of the blood 

 is regulated with almost incredible deli- 

 cacy, and must be so regulated apart al- 

 together from the breathing. The breath- 

 ing simply regulates the rapid disturbances 

 in hydrogen ion concentration caused by 

 variations in the production of CO, : other 

 disturbances are regulated otherwise than 

 by the breathing. There is clear evidence 



