438 THE RESPIRATION 



acids, such as lactic, decrease in the oxygen tension of the plasma and 

 irradiation of impulses on the respiratory center from the cerebrospinal 

 pathways in the medulla. 



Direct evidence that lactic acid is formed during strenuous muscular 

 exercise in man has been furnished by Ryffel. 30 Blood removed from a 

 person immediately after running at full speed for about three minutes 

 contained 70.8 milligrams of lactic acid per 100 c.c. of blood, the normal 

 amount being 12.5 milligrams. Much of the lactic acid accumulating in 

 the blood is no doubt got rid of by oxidation, but a large part of it is 

 also excreted by the urine, in which it was found by Ryffel in consider- 

 able amount after strenuous muscular exertion. 



The accumulation of lactic acid in the blood must tend to raise C H as 

 well as to increase the C0 2 -tension by decomposing bicarbonates. 



With regard to the stimulation of the respiratory center by irradia- 

 tion, it is altogether likely that this can only occur provided that the 

 excitability of the center is being maintained at a certain level through the 

 existence of a proper degree of hormone stimulation, that is, by a proper 

 CH or C0 2 -tension. 



Finally, let us consider for a moment the behavior of the respiratory 

 'quotient. This ratio rises early in the muscle work (Table on page 436), 

 indicating that more C0 2 is being excreted than 2 absorbed. After the 

 work is discontinued, it usually falls below the normal because of retention 

 of C0 2 to take the place of that removed by the hyperpnea excited by 

 the other factors than increase in C0 2 -tension. A similar fall in the res- 

 piratory quotient may sometimes occur during muscular exercise, if this 

 is continued for a long time. 



Second- Wind 



When strenuous exercise is maintained, it is usually the case that the 

 breathlessness, which is severe soon after the start, more or less gradually 

 passes away, and the person feels better able to continue the effort. He 

 gets his "second- wind." Not only does the breathing become easier, but 

 any cardiac distress that may have been present is likely to disappear, 

 and usually sweating sets in. The pulse rate does not change. It is 

 difficult to explain the cause for the phenomenon, but a clue is afforded 

 by the discovery made by Pembrey and Cooke 6 * that the percentage 

 of C0 2 in the alveolar air is less after the second-wind has been ac- 

 quired than it was before it. This probably indicates that something 

 has occurred to cause a lowering of C H of the blood supplying the res- 

 piratory center,* and it becomes of interest to speculate as to the nature 



*It is highly improbable that the lessened breathing could be due to a lowering of the excitability 

 of the center. 



