370 THE RESPIRATION 



The readiness with which C0 2 can be got rid of prevents the hormone 

 which excites the respiratory activity from continuing to act after it is 

 no longer required. Provision for the removal of a hormone after its 

 activity has been displayed is of course essential to efficient correlation 

 of function, and is seen in the case of other hormones, such as epinephrine 

 and secretin, whose discontinuance of action is effected by their de- 

 struction in the blood (see page 745). 



Direct evidence that lactic acid is formed during strenuous muscular 

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

 person immediatey 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. 



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

 quotient. This ratio rises early in the muscle work (Table on page 367), 

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

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

 of C0 2 to take the place of the lactic acid that is being gradually used up 

 or excreted. A similar fall may sometimes occur in the respiratory 

 quotient during muscular exercise, if this is continued for a long time. 

 It probably indicates that a balance has been struck between the produc- 

 tion of lactic acid in the muscles and the loss of this substance by oxida- 

 tion. In any case it is a significant occurrence, for it coincides with the 

 great improvement in the subjective sensations accompanying muscular 

 exercise. It occurs, for example, at the same time as the appearance of 

 the "second wind," when the circulatory and respiratory distress expe- 

 rienced during the earlier stages of strenuous muscular exertion disap- 

 pear. The stages prior to the second wind correspond to the period when 

 considerable quantities of free C0 2 are being got rid of from the blood 

 and are probably creating a temporary maladjustment of the C H which 

 acts on the various medullary centers. If by forced breathing much of 

 this C0 2 is discharged before the muscular exercise is undertaken, the 

 initial hyperpnea is not nearly so marked. 



