BENEFICIAL ACTION OF CERTAIN "POISONs". — HARRIS. 97 



its optimum tension; for a rise to 12 to 20% ciiu.scs diminisliod 

 systolic output. Conversely, a considerable diminution of 

 CO2 produces tachycardia at the same time that it stimulates 

 the vaso-constrictor centre. This naturally leads to an un- 

 satisfactory condition of the circulation, for the rapid, weak- 

 ening heart works badly against increased peripheral resis- 

 tance. Over 8% of CO2 in the blood stimulates the vasocon- 

 strictor centre. 



(3) It is now definitely known that excess of carbon diox- 

 ide in the blood dilates the coronarj^ vessels. Barcroft and 

 Dixon in 1906 wrote; "we have given reasons to show that the 

 liberation of metabolic products from the heart, the chief 

 of which is carbonic acid, controls the vaso-motor changes 

 in the coronary arterioles." This is comparable with the 

 dilator action of muscular katabolites on the arterioles 

 of muscle demonstrated long ago by Gaskell. 



(4) Both CO2 and lactic acid hasten the rate at which 

 arterial blood with low tensions of oxygen gives up its oxy- 

 gen, that is, is reduced. When hydrogen or nitrogen is 

 bubbled through arterial blood saturated with oxygen, the 

 saturation falls from 100% to 80% at the end of twenty five 

 minutes, but — 



Blood containing 



. 04% lactic acid falls to about 68% saturation in 25 minutes 



20. 00%o CO2 falls to about .. ..60%o " "" " 



0.09% lactic acid falls to about 50%o " " " " 



45.007c CO2 falls to about .... 25%, " " " " 



0.20%o lactic acid falls to about 15% " " " 



100. 00%o CO2 falls to about. .. 7% " " " " 



Both CO2 and lactic acid afe, therefore, beneficial to the 

 organism in that they accelerate the reduction of capillary 

 blood. After vigorous exercise there may be 0.07% of lac- 

 tic acid in the blood. 



