CHAPTER LXX 



Central nervous system circulation, 

 fluids and barriers — introduction 



CARL F. SCHMIDT Laboratory of Pharmacology, University of Pennsylvania, Philadelphia, Pennsylvania 



CHAPTER CONTENTS 



Cerebral Vascular Adjustments to Chemical Changes in the 



Blood 

 Cerebral Vascular Adjustments to Metabolic Requirements of 



Brain Cells 

 Role of Vasomotor Nerves in the Control of the Cerebral 



Circulation 

 Influence of Neurohumoral Agents 

 'Trigger Zones' in the Brain 



the perspective of this section is simple in principle 

 but complex in detail because these are the character- 

 istics of the physiology of the brain. It is simple for 

 the following reasons. First, the essential role of the 

 circulation in the brain is the same as thai in any 

 other tissue, viz, to meet the call of a mass of living 

 cells for an exchange of fluids and solutes, a source "I 

 energy commensurate with their metabolic require- 

 ments and a means for removal of their waste prod- 

 ucts. Second, the organ is unusually homogenous; the 

 function of most of its various components is the same 

 (viz, the origination or conduction of nerve impulses 

 or both), and the consequences of deranged function 

 are easily recognized. Finally, the blood supply also 

 is unusually homogenous and can be measured in 

 normal and diseased man with quantitative accuracy 

 at least equal to that available for the study of the 

 blood flow in other organs. The last named advantage 

 has been within reach only for the last 15 years, but 

 in that time a mass of information has been obtained 

 which is considered in Chapter LXXI by Kety in 

 this Handbook and which provides a better insight 

 into the physiology and pharmacology of the human 

 cerebral circulation than is available for any other 

 organ, excepting perhaps the kidney. 



These studies on man have substantiated the con- 

 cept, gained from earlier work on animals (37, 38), 

 thai the cerebral blood vessels possess an intrinsic 

 regulatory capacity activated 1>\ chemical products 

 of metabolism (carbon dioxide, acid and other 

 resultants ol anoxia), having the effeci (perhaps the 



purpose) of maintaining homeostasis with respect to 

 these agents in cells which are highly vulnerable to 

 departures from their optimal chemical environment 

 because they have sacrificed adaptability for special- 

 ization. The animal studies indicated that, of all the 

 common chemical product-- of metabolism, carbon 

 dioxide is most powerful and most uniform in its 

 effeci mi cerebral vascular tonus. This conclusion is 

 borne out by the subsequent work on man (23, 25), 

 with the modification thai the ability o) anoxemia to 

 dilate the human cerebral vessels turns out to be 

 relatively greater than would have been expected 

 from the animal experiments (37). Definitive well- 

 controlled estimations of the effects of acid on the 

 cerebral vasculature of man have not yet been 

 reported. 



CEREBRAL VASCULAR ADJUSTMENTS TO CHEMICAL 

 CHANGES IN THE BLOOD 



The fundamental concept remains quite simple: 

 the tonus of the blood vessels supplying a given nerve 

 cell is decreased whenever the arterial pC0 2 rises 

 and whenever the arterial pO> or pH falls. If the 

 metabolic activity of the cell remains constant, the 

 carbon dioxide and acid which it produces then will 

 be removed more rapidly, and the oxygen content of 

 the blood in the adjacent capillaries will fall less than 

 had previously been the case Thus the brain cells 



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