CHAPTER XXXVIII 

 THE CONTROL OF RESPIRATION (Cont'd) 



THE HORMONE CONTROL OF THE RESPIRATORY CENTER 



Just as the rhythmical activity of the heart is readily influenced by 

 changes in the composition of the blood supplying it, so also is that of 

 the respiratory center. In the case of the heart it is the cations cal- 

 cium, potassium and sodium that have the most pronounced effect, 

 whereas in the case of the respiratory center it is largely the relative con- 

 centration of hydrogen and hydroxyl ions the H-ion concentration 

 (C H ) of the blood. This influence can be shown in a general way by 

 injecting acid or alkaline solutions into the peripheral end of the carotid 

 artery of an anesthetized animal, or better still of one that has been 

 decerebrated. Acid injections stimulate the respiratory activity; alka- 

 line injections tend to depress it. When the acid or alkaline solutions 

 are injected intravenously in other parts of the body, so that they be- 

 come thoroughly mixed with the blood before the respiratory center is 

 reached, the effects are not nearly so pronounced, because the buffer in- 

 fluence of the blood has time to develop (see page 36). 



From the results of such injection experiments, however, one could 

 not draw the conclusion that under normal conditions the activity of 

 the respiratory center is affected by measurable changes in C H of the 

 blood, for, as we have seen, constancy of C H is one of the most remark- 

 able properties of the animal fluids. To justify the conclusion that the 

 respiratory center is affected by changes in C H , it is necessary to observe 

 the behavior of some easily measurable acid or alkaline constituent of 

 the blood that undergoes changes in amount that are proportional to an 

 alteration in C H . In order to understand what this acid or basic 

 substance may be, it will be advisable to recapitulate the main factors 

 concerned in maintaining C H at a constant level. This value is obviously 

 dependent upon the balance between basic and acid substances, so that 

 any variations which it undergoes must be caused by changes in the 

 relative amount of one of these. Changes in base may occur, exoge- 

 nously, by altering the alkali content of the food, or, endogenously, in 

 various ways but particularly by variations in the amount of ammonia 

 produced during the course of metabolism of protein. Thus, when sud- 

 den demands are made by the organism for an increased amount of base, 



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