RESPIRATION 



103 



FIG. 53 



fuel of the organism, of the lungs as the chimney, etc. Such similes 

 are instructive and fairly well represent the facts so far as they go. 



A living organism, however, is very different in its working conditions 

 from a steam-engine plant. It is more automatic, less dependent on 

 external conditions, has principles and tendencies of its own, and habits 

 structural and functional, in a much greater degree than has the steam- 

 engine and its boilers. One unlikeness is fundamental and must be 

 specifically noted here. Whereas in the boiler-furnaces the draft, that is 

 the relative abundance of oxygen, determines directly the speed and 

 vigor of the combustion, in organisms the combustion or metabolism is 

 largely the controlling agent, and not the oxygen. It is not possible by 

 a "forced draft" to hasten beyond its normal maximum the combustive 

 metabolism in living tissues. They absorb no more oxygen when the 

 animal breathes only this gas than they do from the ordinary, average- 

 pure atmosphere to which the tissues are adapted by evolution. Indeed, 

 when the arterial blood has been forced to absorb one- third more than 

 its normal quantity of oxygen by subjecting the animal to an oxygen- 

 pressure of six atmospheres the animal dies, from a lowering of the 

 vital metabolism of some or other of the tissues (Paul Bert). The 

 metabolism then determines the amount 

 of oxygen needed by the tissues, rather 

 than that the supply of the gas nor- 

 mally affects the tissue-metabolism. 



Various conditions help to determine 

 the amount of oxygen consumed by the 

 tissues as also the amount of carbon 

 dioxide excreted. Anything in general 

 which increases the metabolism either 

 in intensity or in extent enlarges the 

 respiratory exchange. Voit and Pet- 

 tenkofer showed that in a man of 

 average weight, say 70 kg., about 700 

 gm. of oxygen are required in twenty- 

 four hours, while about 800 gm. of 

 carbon dioxide are given off. But 

 during rest the 700 gm. required in 

 average activity may decline to 600 

 gm. and the 800 gm. of carbon diox- 

 ide excreted may decrease even more 

 than 100 gm. On the other hand, during hard muscular work the'con- 

 sumption of oxygen may arise to 1100 gm. daily and the amount of 

 carbon dioxide to nearly 1300 gm. As an average for the adult's demand 

 for oxygen, Zuntz states about 14.5 gm. per kilo of body weight in 

 twenty-four hours. 



The Respiratory Mechanism. There is no space here for a description 

 of the gross and microscopic anatomy of the respiratory mechanism. 

 This must, however, be thoroughly kept in mind in all its details if the 



Two infundibula of human lung: a 

 and b, air-sacs; c c, two ultimate bronchi. 

 (Kolliker.) 



