CHAPTER XXVII 



The activities of the vasomotor center are controlled partly by hor- 

 mones and partly by afferent impulses. 



The Hormone Control 



As with the respiratory center, the chief hormone, is the hydrogen-ion 

 concentration of the blood. When this is increased, as in asphyxia, the 

 vasoconstrictor part of the vasomotor center becomes stimulated, so 

 that the blood vessels are constricted and the blood pressure rises. Tak- 

 ing, as our criterion of hydrogen-ion concentration, the tension of the 

 carbon dioxide in the blood (see page 354), we may proceed to investi- 

 gate the relationship by observing the blood pressure during changes 

 in the carbon-dioxide tension brought about by causing the animal to 

 breathe atmospheres containing known percentages of the gas (Mathi- 

 son 15 ). Thus, if a decerebrate cat is made to respire an atmosphere 

 containing 5 per cent or more of carbon dioxide, an immediate rise 

 occurs in the arterial blood pressure. That the inhaled carbon dioxide 

 acts by raising the hydrogen-ion concentration of the blood is indicated 

 by the fact that a similar rise in blood pressure can be obtained by intra- 

 venous injection of a weak solution of lactic acid (2 c.c. N/15) in a de- 

 cerebrate cat. 



Instead of injecting the lactic acid, we may cause it to be produced 

 in the muscles of the animal itself by greatly diminishing their oxygen 

 supply. When a decerebrate cat, for example, is made to breathe an 

 atmosphere of almost pure nitrogen, there is, after a latent period of 

 about 30 seconds, a sudden rise in. arterial pressure. The existence of 

 this latent period in the latter case, as compared with its absence when 

 carbon dioxide is inspired, is owing to the time taken for lactic acid to 

 be produced in the muscles on account of the oxygen deprivation. It 

 is important to note in the above experiment that decerebrate animals 

 are employed so as to avoid the necessity of using anesthesia, under 

 which the results are much less definite. The fact that oxygen depriva- 

 tion causes excitation of the vasoconstrictor center has been known for 



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