CHAPTER LXXI 



The cerebral circulation 



SEYMOUR S. KETY National Institute of Mental Health, Bethesda, Maryland 



CHAPTER CONTENTS 



Anatomy 



Methods of Study 



Normal Values and Physiological Variations of Cerebral 



Circulation 

 Control of Cerebral Circulation 



Arterial Pressure Head 



Cerebral Vascular Resistance 



Neurogenic Control 



Humoral Control 



Effects of Drugs 

 Cerebral Circulation in Human Disease 



Cerebral Arteriosclerosis 



Essential Hypertension 



there is no organ, other than thi' heart itself, more 

 completely dependent upon its supply of blood than 

 is the brain. An interruption of this circulation for 

 only 5 sec. produces remarkable changes in neuro- 

 logical function and consciousness (84), while irrevers- 

 ible damage to the brain results if cerebral ischemia 

 persists for more than a few minutes. Although the 

 brain's great need for oxygen, or some essential com- 

 ponent of air, was clearly recognized by Hippocrates, 

 it was not until 1 783, more than a century after 

 Harvey had described the circulation of the blood, 

 that serious attention was given to the physiology of 

 the cerebral circulation. At that time, Monro (73), 

 later supported by Kellie (45), elaborated a doctrine 

 that the skull being rigid and its contents incompress- 

 ible, the blood volume of the brain was fixed and, 

 therefore, so also was its circulation — that there was 

 in fact no variation possible in the flow of blood to 

 this important organ. This concept survived scientific 

 scrutiny for another century until 1896 when Hill 

 (4a) summarized his fairly extensive experimental 



studies in animals, concluding that the cerebral blood 

 flow was, in fact, variable but passively so, being; 

 governed entirely by the systemic arterial and venous 

 pressures. A few years earlier, Roy and Sherrington 

 had suggested an intrinsic control of the cerebral 

 circulation on the basis of carbon dioxide concentra- 

 tion, but Hill's concept dominated the field for 25 

 /ears 1 (9). 



Forbes (25) was among the first to direct attention 

 to the intrinsic control of the cerebral circulation, and 

 recent studies in unanesthetized man have amply 

 confirmed its importance. Wolff, in 1936, wrote an 

 excellent review on the entire field of the cerebral 

 circulation up to that time (111), while the work in 

 human cerebral circulation has been reviewed more 

 recently (48, 64a, 69, 103). 



ANAT(l\n 



The brain of almost all mammalian species may be 

 supplied with blood by means of several major 

 sources, including the internal and external carotids 

 and the vertebrals, although there exist wide varia- 

 tions among the species with respect to the relative 

 importance of these several channels. Even though 

 the internal carotid leads directly to the brain, this 

 vessel in some species is relatively unimportant or 

 even vestigial and it is the external carotid which 

 often carries the larger share of cerebral blood which 

 reaches the brain by way of the rete mirabile, a coarse 

 network of anastomoses between branches of the 

 extracerebral and the intracerebral circulation (3). 

 Among the species of special interest, laboratory- 

 rodents receive their cerebral blood chiefly by way of 

 the well developed vertebrals, with the internal 

 carotids playing a secondary role. In these species 



I75 1 



