410 THE RESPIRATION 



muscles, not only during the contraction, but for a considerable period 

 after it, provided 2 was present. In the absence of 2 this recovery 

 was either greatly delayed or entirely abolished. Such results favor 

 the view that 2 is used largely in the processes whereby the muscles, 

 "like an engine charging an accumulator, synthetize substances con- 

 taining a considerable amount of potential energy, which again, like the 

 accumulator, it discharges when appropriate stimuli are applied" (L. 

 V. Hill 48 ). One immediately thinks of lactic acid in connection 

 with these interesting results, for, as has already been stated, Hopkins 

 and Fletcher 29 have shown that this acid is produced in the absence of 

 2 in excised frog muscles, but when 2 is present, it is either not pro- 

 duced or, if so, quickly disappears. 



These important results lead to the further question as to whether 

 the increase in l)loodflow accompanying activity is in itself sufficient to 

 account for the increased uptake of oxygen. This question is answered 

 by finding whether the increase in total oxygen consumption during mus- 

 cular exercise (in man) is proportional to the increase in bloodflow which 

 can be determined by measurement of the output of the heart (page 

 216). Krogh 44 calls the ratio between the two the coefficient of utilization, 

 and it is obtained by dividing the amount of oxygen taken up from one 

 liter of blood during its passage round the body by the oxygen capacity 

 of the blood (per liter). The former value is ascertained by analysis of 

 the respired air and measurement of the minute volume of the heart (page 

 218). Thus, suppose 270 c.c. of O 2 is absorbed by the body in one minute 

 and the minute-volume of the heart is 4.800 c.c. (and the oxygen capacity 



70 ^fi 9^ 



of the blood 18.5 per cent then =-= 56.25 and ^ nr = 0.03. 



*.o 1.85 



The coefficient increases markedly during exercise, showing that other 

 factors besides increased blood flow come into play. This is shown in 

 the accompanying tables. 



O, CONSUMPTION OUTPUT OF HEART COEFFICIENT OF 



C. C. PER MIN. LITERS PER MIN. UTILIZATION 



1 Eest 310 5 0.30 

 a 1630 17.05 0.47 

 rk b 2089 19.65 0.55 



The unknown factors may reside in the blood itself or in the tissues. 

 In the former, the increase in C H due to the passage of acids into the 

 blood would greatly increase the rate at which oxyhemoglobin dissociates 

 (see Fig. 141) this being further assisted by the slight rise in temperature 

 which accompanies the contraction. With regard to a possible increased 

 avidity of the tissues, the recent work of Krogh, referred to elsewhere 

 (pages 252 and 414) would not seem to lend support. 



