THE CEREBRAL CIRCULATION 



'755 



table 2. Mean Values for Blood Flow (ml/gm/min) 

 in Representative Areas of the Brain of 

 the Unanesthetized Cat (63) 



may be significantly increased in certain types of 

 anxiety (48). 



control of cerebral circulation 



Although a large number of factors may operate 

 to vary the cerebral circulation, these may all be 

 grouped under one or another of only two important 

 variables: the pressure head, i.e. the difference be- 

 tween the arterial and venous pressures at the level 

 of the brain; and the total resistance or hindrance 

 imposed upon the flow of blood through the vessels 

 of the brain. 



tion, in technique and in procedure. There is no 

 reason to question the carefully controlled finding of 

 a very high rate of flow in the first decade of life, 

 falling around the time of puberty toward the value 

 in the young adult (table 1). Groups who have 

 studied middle-aged and elderly hospitalized patients 

 have usually obtained values suggesting a gradual 

 progressive fall in cerebral circulation from the 

 fourth or fifth decade onward (19, 49, 91, 102). 

 Sokoloff lias reported, however, that carefully selected 

 aged volunteers in good mental and physical health 

 have a cerebral blood flow which is not significantly 

 different from that observed in healthy young men 

 (104), indicating that a reduction in this function is 

 not a necessary accompaniment of the aging process. 



The effect of temperature upon cerebral circulation 

 has not been completely studied through a wide 

 range in the same species. In man the only studies 

 using high temperatures have been made in patients 

 with central nervous system syphilis where induced 

 temperatures of the order of io6°F were not associated 

 with a significant increase in cerebral blood flow (41). 

 Some studies have been reported on the effects of 

 hypothermia in animals (60, 83). These have indi- 

 cated a decrease in cerebral circulation to a significant 

 extent, although there is some question whether the 

 circulation is reduced more than is cerebral oxygen 

 consumption. 



In normal sleep there does not appear to be any 

 significant decrease in cerebral blood flow (70); there 

 is, in fact, a slight but significant increase. This 

 argues strongly against the concept that sleep is de- 

 pendent upon the presence of cerebral ischemia. 



Whatever cerebral exertion is required in the per- 

 formance of mental arithmetic, it is not accompanied 

 by significant changes in cerebral blood flow or 

 oxygen consumption (105), although both functions 



Arterial Pressure Head 



Recent investigation does not substantiate the 

 earlier belief that cerebral blood flow passively follows 

 changes in arterial blood pressure (42). Studies in 

 intact human beings have strengthened the concept 

 ih.it a normal arterial pressure is zealously main- 

 tained by numerous honieostatic mechanisms, such as 

 the carotid sinus reflex and the central control of 

 peripheral vascular tone; and that as long as the 

 mean arterial pressure remains above a critical mini- 

 mum level, cerebral blood flow is actually regulated 

 intrinsically (95) by changes in cerebrovascular 

 resistance. This is nicely illustrated by a report of the 

 maintenance of normal levels of cerebral blood flow 

 through a markedly reduced cerebral vascular resist- 

 ance where the arterial pressure had been lowered 

 considerably by high spinal anesthesia (61). Where 

 the arterial pressure falls below a level of 60 or 70 mm 

 1 [g, then, of course, serious limitation in the cerebral 

 circulation may result (22). This may be seen in 

 carotid sinus hypersensitivity, in the Stokes-Adams 

 syndrome, in orthostatic hypotension and in surgical 

 shock. 



Under most conditions, including even cardiac 

 decompensation (77), the venous pressure constitutes 

 so small a fraction of the arterial pressure that it plays 

 a minor role in determining the total pressure gradi- 

 ent. The venous pressure, however, plays an impor- 

 tant role in combating the effects of gravity upon the 

 cerebral circulation. In the erect posture (80) or, 

 more dramatically, during exposure to high centrifu- 

 gal force (39), the effective arterial pressure at head 

 level may be moderately or severely reduced without 

 a comparable reduction in cerebral blood flow. In 

 one study under centrifugal force, cerebral blood flow- 

 appeared to be maintained at normal levels even 



