RESPIRATION 425 



While diffusion, in response to physical and chemic conditions, thus 

 plays a large part in, and is sufficient to account for, the exchanges of gases, 

 it is possible that the alveolar or respiratory epithelium may also p^ay an 

 essential r61e. It is believed by some investigators that it is active in both 

 the absorption of oxygen and the excretion of carbon dioxid. This view has 

 been suggested as a means of interpreting the results of the experiments of 

 more recent investigators, made with a view of determining the tension of the 

 blood gases. It was found by Bohr that the tension of the oxygen in arterial 

 blood was often as high as 101 to 144 mm. Hg., and in many instances higher 

 than the tension of the oxygen in the trachea, while the carbon dioxid tension 

 in the trachea was higher than in the blood. Haldane and Smith by a dif- 

 ferent method found an oxygen tension in the arterial blood of 200 mm. Hg. 

 If these results should prove to be correct, though they are at present subject 

 to considerable criticism and not generally accepted, some other force than 

 diffusion would have to be found to explain the facts. It would then remain 

 to be determined in how far the alveolar epithelium could be regarded as 

 an active agent in both absorption and excretion in opposition to pressure. 



THE TOTAL RESPIRATORY EXCHANGE 



The total quantities of oxygen absorbed and carbon dioxid discharged 

 by a human being in twenty-four hours are measures of the intensity of the 

 respiratory process, and an indication of the extent and character of the 

 chemic changes attending all life phenomena. Their determination and 

 their relation to each other are matters of interest and importance. The 

 quantities which have been obtained by different observers are the outcome 

 of calculations based on certain groups of data and of experiments made with 

 special forms of apparatus. 



Thus from the total air breathed daily, estimated from the amounts 

 obtained during a longer or shorter period by experiments with spirometric 

 apparatus, and from the percentage loss of oxygen and gain of carbon dioxid 

 shown by an analysis of the respired air, it can be calculated at least ap- 

 proximately what the total amounts of oxygen absorbed and carbon dioxid 

 exhaled must be. If it be assumed that the minimum daily volume of air 

 breathed is 8500 liters and the maximum volume 12960 liters, and the 

 percentage loss of oxygen is 4.78, then the total volume of oxygen absorbed 

 is 406 liters (580 grams) or 620 liters (885 grams). By the same method 

 the total carbon dioxid exhaled daily is found to be either 372 liters (735 

 grams) or 567 liters (1122 grams) . The direct experiments which have been 

 made with specially devised forms of apparatus, both on human beings and 

 animals, have yielded similar results. With those forms which are 

 adapted for both human beings and animals Scharling's, Pettenkofer and 

 Voit's it is only possible, however, to determine the amount of carbon 

 dioxid and water exhaled and from them to calculate the amount of oxygen 

 absorbed. This is done by deducting the loss in weight by the man or 

 animal during the experiment from the combined weights of the carbon 

 dioxid and water discharged. The difference represents the oxygen 

 absorbed. 



The Pettenkofer- Voit apparatus consists essentially of a chamber large 

 enough to admit a man and capable of being made air-tight with the 



