98 



BENEFICIAL ACTION OF CERTAIN POISONS , AND 



The Effect of CO2 on the Rate of Oxidation and of the 

 Reduction of Blood Respectively. 



At 37.5°C, the time taken by bubbling hydrogen or 

 nitrogen to reduce oxygen-saturated blood to 50% satura- 

 tion is twice as long as the time to oxidize blood from zero 

 to 50% to saturation, no CO2 being present in either case. 

 The oxygen present in the oxidative process is 13.5% volumes, 

 hydrogen being the rest. Barcroft has shown that the 

 addition of CO2 (six volumes %) to the hydrogen in the 

 reduction process shortens the time (for reduction from full 

 saturation to 50%) and makes it equal to the time for oxi- 

 dation. The practical import of this is very clear; it means 

 that the velocity of the uptake of oxygen by the pulmonary 

 blood is just the same as that of its loss to the tissues. The 

 rates of pulmonary oxidation and of tissue reduction of 

 haemoglobin are the same, but only in presence of CO2. 



Another example of the beneficial action of a katabolite 

 or poison is urea. Matthews says in his recent text- 

 book: "it is found that the addition of a little urea to arti- 

 ficial perfusion solutions when one is perfusing the heart or 

 other organs, is, as a rule, advantageous. This action is not 

 so well marked in mammals as in teleostean and lasmobranch 

 fishes." 



In addition to all this, urea is it?3lf a good diuretic , 

 which circumstance is too obviously of advantage to the 

 organism to merit further remark. Ammonia is a kata- 

 bolite, but it is a valuable agent for warding off acid intoxica- 

 tion in the body. Certain amines, katabolites, are vaso- 

 dilators and wash acid out of the tissues. 



A poison may be defined in more than one way. Some- 

 thing that is not a food and not inert in the body, is one good 

 description. A poison might be described as something which 

 compromises or tends to compromise the vitality of the tissues; 

 but unless we qualify the "something," we shall have to in- 

 clude such a thing as a rope tied round the neck producing 



