RESPIRATION. 371 



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

 mine 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 dis- 

 charged 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 differ- 

 ent 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, of experiments with spiro- 

 metric apparatus, and from the percentage loss of oxygen and gain of 

 carbon dioxid shown by an analysis of the respired air, it can be cal- 

 culated at least approximately what the total amounts of oxygen ab- 

 sorbed and carbon dioxid exhaled must be. If it be assumed that 

 the minimum daily volume of air breathed is 10,800 liters and the 

 maximum volume 12,240 liters, and the percentage loss of oxygen is 

 4.78, then the total volume of oxygen absorbed is 516 liters (735.17 

 grams) or 585 liters (836.42 grams). By the same method the total 

 carbon dioxid exhaled daily is found to be either 473 liters (931.8 grams) 

 or 526 liters (1036 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, Petten- 

 kofer and Voit's it is only possible, however, to determine the amount 

 of carbon dioxid and water, and from these 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 (Fig. 178) consists essentially 

 of a chamber large enough to admit a man and capable of being 

 made air-tight with the exception of an inlet for air for breathing 

 purposes. The respired air is drawn through a tube and measured 

 by a large meter turned by a water or gas motor. By means of a side 

 tube a fractional quantity of the main column of air is diverted to an 

 absorption apparatus by a small pump. This air first passes into 

 a vessel containing H 2 SO 4 , by which the water is collected; then 

 into long tubes containing barium hydroxid, by which the carbon 

 dioxid is absorbed; thence into a small meter, by which its amount is 



