508 



RESPIRATION 



As far as the exchange of the oxygen is concerned, the conditions exist- 

 ing here are the same as those prevaiUng when the blood is subjected 

 to the vacuum of an air-pump. The neighboring tissues are always 

 greedy for oxygen, and abstract even the last traces of this gas from 

 the adjoining lymph. The latter in turn must replenish its oxygen 

 content by withdrawing a corresponding amount from the blood. 

 In this way, a descending scale of oxygen tensions is produced, begin- 

 ning with the red corpuscles and the plasma and lymph and terminating 

 in the interior of the cell. But while the speed of the capillary flow 

 is sufficiently slow to allow these interchanges between the blood and 

 the tissues to be completed with plenty of time to spare, the individual 

 red cells never tarry long enough at these cells to lose their entire 

 store of oxygen. Only if these corpuscles are prevented from recu- 

 perating their losses in the lungs can their oxygen store be depleted 

 further until, as occurs in asphyxia, the last traces of this gas have 

 been removed from them. It has been pointed out above ^ that the 

 evolution of the oxygen by the hemoglobin is greatly facilitated by carbon 

 dioxid, this effect being especially marked in conditions of low oxygen 

 tension. 



CHAPTER XL 



THE SEAT AND NATURE OF THE OXIDATIONS 



The Oxidative Power of the Tissues. — It is commonly accepted 

 to-day that the seat of the oxidations is in the tissues and not in the 

 blood, as has been suggested by A. Schmidt^ and Pfliiger.^ Thus, we 

 are accustomed to compare the body to a steam engine and to speak of 

 the ''burning up" of foodstuffs in a manner indicative of the processes 

 taking place during an ordinary combustion. But while it seems to be 

 true that the reductions are confined in their entirety to the cells, the 

 fact must not be lost sight of that they are not always completed by 

 the same group of cells, i.e., while a certain colony of cells may incite 



1 Barcroft, Respiratory Function of the Blood, 1914. 



2 Arbeiten aus dem physiol. Inst, zu Leipzig, li, 1867, 99. 



3 Pfltiger's Archiv, i, 1816, 98. 



