RESPIRATION IN AIR 



69 



C0 2 of the respiratory center which can easily be demon- 

 strated by having a subject breathe, say, 3% C0 2 during rest 

 and during work respectively (M. Nielsen, 1936). 



Experiments on a few subjects (Krogh and Lindhard, 1913) 

 showed that in these the very beginning of muscular work 

 brought about a sudden increase in ventilation, but this is not 

 universal. During the first minute or two of constant work 

 there is some fluctuation in ventilation, but a steady state 

 is reached, usually after 3-5 minutes, in which the ventilation 

 remains remarkably constant corresponding to the rate of 

 oxygen uptake. 



2000 



1500 



500 



3 minutes 



Fig. 41. Curve showing oxygen absorption in human subject during initial 

 stages and just after work. (Krogh and Lindhard.) 



During the initial stages of muscular work the uptake of 2 

 is always lower than in the steady state and it is a character- 

 istic fact (Krogh and Lindhard, 1920) that the "oxygen-debt" 

 (A. V. Hill, 1924) thus incurred cannot be paid off while the 

 work goes on at the same or at an increasing rate, but partly 

 when the work is reduced (E. Hansen, 1934) and completely 

 when it stops (Fig. 41). This involves a gradual return of 

 respiration and circulation to the resting values over periods 

 from several minutes to about one hour, depending on the 

 severity and duration of the work. With bursts of maximal 

 work over 1/2 minute or less the maximum 2 uptake may 

 occur after the work, and athletes can reach very high figures 

 for the oxygen debt. Quite reliable figures of 9-1 1 liters O2 

 are on record (E. Hansen, 1934), corresponding to the resting 



