942 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 







Verfebrol Artery Pressure 

 85 



43 



79 

 47 jr 



ir°* 



b 2 5 --e^ 



Cerebral Venous Outflow 

 16 



12.5 



Autoperfused 

 Perfused with pump 



J L 



J_ 



J L 



J I L 



20 40 60 80 100 



Meon between Carotid and Vertebral Pressures - 

 Cerebral Outflow Pressure mmHg 



fig. 8. Autoregulation in the cerebral vascular bed of the 

 dog. Above: records representing vertebral arterial pressure in 

 mm Hg and cerebral venous outflow in ml/min. Below: heavy 

 solid line, relationship between the cerebral venous outflow and 

 the mean of the carotid and vertebral artery pressures minus 

 the cerebral outflow pressure during autoperfusion of the 

 brain from the carotid arteries. The perfusion pressure was 

 regulated by means of clamps on the carotid arteries. Light 

 solid line, similar relationship but during the perfusion of the 

 carotid arteries with an artificial perfusion system. 



Alitor egulatory Control of Resistance Vessels 



For the purpose of this chapter, we propose that 

 the term autoregulation be defined to include all 

 processes which operate locally in a vascular bed to 

 maintain some factor constant in the face of various 

 externally or internally induced stresses. The factor 

 which is kept constant, i.e., the controlled variable 

 (see below) may be blood flow, or the tissue concen- 

 tration or tension of some nutrient (O2, etc.) or some 

 metabolite (such as CC^)- As proposed here, the term 

 autoregulation would exclude extrinsic mechanisms 

 such as reflexes involving the central nervous system, 

 variation in arterial pressure, or changes in hormone 

 activity. 



The activity of autoregulatory mechanisms has been 

 studied by subjecting isolated or semi-isolated organs 

 to stresses such as changes in arterial perfusion pressure 

 or blood gas content, or by altering tissue metabolism 

 while recording the resulting change, or lack of change 



in blood flow, venous gas content, or the content of 

 other metabolites. 



MODIFICATION OF PASSIVE PRESSURE-FLOW RELATION- 

 SHIP by autoregulation. In many vascular beds, the 

 above-mentioned power relationship between per- 

 fusion pressure and flow in passive vascular beds is 

 modified by occurrence of autoregulation. The insert 

 in the upper portion of figure 8 shows recordings of 

 blood flow in the brain obtained during autoperfu- 

 sion (76 and unpublished data). In these studies, 

 carotid artery inflow pressure was lowered abruptly 

 from 84 to 44 mm Hg; after flow stabilized, perfusion 

 pressure was returned to the control level. Immedi- 

 ately upon lowering perfusion pressure, flow dropped 

 from 12.5 to 8.5 ml per min, then rose to a stabilized 

 level of approximately 10 ml per min. Upon restora- 

 tion of control pressure, flow rose abruptly to 16 ml per 

 min and then stabilized at approximately its original 

 level of 1 2.5 ml per min. The rise in flow following the 

 initial decline probably was due to vasodilation, and 

 the secondary decline in flow following; restoration of 

 the original perfusion pressure probably was due to 

 vasoconstriction. When data from such experiments 

 are plotted they yield a series of curves such as are 

 reproduced in figure 9. 



The heavy line in the graph in figure 8 corresponds 

 to the heavy line in figure 9 and is a plot of the 

 stabilized flows at each level of perfusion pressure from 

 85 to 15 mm Hg. This line is almost horizontal, 

 indicating an almost constant level of stabilized flow 

 over the pressure range of 80 to 35 mm Hg. In view 

 of the observations in figures 3 and 4, this finding 

 can be explained only by assuming some reactive 

 change in the diameter of the resistance vessels so as 

 to compensate for alteration of perfusion pressure 

 (76 and unpublished data). The mechanism respon- 

 sible for compensation for pressure change has been 

 termed local reaction of the arterial wall, reactive 

 vasodilation, intrinsic regulation, autoregulation and 

 ''genuine autoregulation" (3, 27-29, 36, 46, 63, 66, 



7' 2 , 9°. 9 2 . 99» I0 5» I0 9> "O- 



Observations, similar to those in the brain, were 



recorded in artificially perfused skeletal muscle 

 vascular beds (fig. 10). At pressures above normal the 

 pressure-flow relationship was curvilinear and similar 

 to that described for skin (fig. 3) and the resistance to 

 flow, after stable flow was established, increased 

 progressively as perfusion pressure was lowered. 

 However, as perfusion pressure dropped below 100 

 mm Hg the curve began bending more sharply to the 

 left so as to become approximately horizontal to the 

 pressure axis, and the stabilized resistance to flow 



