CHAPTER XXVIII 



THE CONTROL OF THE CIRCULATION (Cont'd) 

 CONTROL OF THE VASOMOTOR CENTER 



The activities of the vasoxnotor 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 371), 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. 



Oxygen deprivation also causes excitation of the vasoconstrictor center 

 as can be demonstrated either by causing a decerebrate cat to breathe in 

 an atmosphere of nearly pure nitrogen or by clamping the vertebral ar- 

 teries as they lie just below the centers. The rise in blood pressure is then 

 very prompt and is accompanied by hyperpnea. 



The sensitivity of the medullary center towards the hydrogen ion is many times 

 greater than that of the subsidiary centers in the spinal cord. If an animal is kept 

 alive by artificial respiration for some time after cutting the cervical spinal cord, 

 the subsidiary vasomotor centers will, as we have seen, gradually acquire a tonic 

 action, and the lowered blood pressure will gradually rise again. If, w r hen this has 

 been attained, the animal is made to breathe an atmosphere rich in carbon dioxide, 

 a sudden rise in. blood pressure will occur, but to produce it a very much greater per- 

 centage of this gas must be inspired than when the pathway between the chief and 



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