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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



flow as an index of vasomotor tone in that there is 

 no involvement of the viscosity of the blood (i.e., 

 the CCP will be the same whether blood or Ringer's 

 solution is used, a fact that has been checked experi- 

 mentally). It has the disadvantage that to measure 

 CCP we need to reduce the perfusion pressure, and 

 thus the transmural pressure, until the flow is zero, 

 which may lead to physiological changes in the tone 

 of the smooth muscle, which we are attempting to 

 measure (e.g., reactive hyperemia). The act of 

 measurement may seriously disturb the quantity 

 that is to be measured. It the pressure is reduced 

 fast enough to the critical level, this may presumably 

 be avoided. This is discussed further utider the section 

 dealing with the experimental verification of the 

 theory of critical closure. 



1 8. THE FUNDAMENTAL INSTABILITY OF VESSELS UNDER 

 CONSTRICTOR TONE : THE STABILIZING ROLE OF 

 ELASTIC TISSUE AND SENSITIVITY OF CONTROL 



There are several ways of considering the origin of 

 this property of instability in cylindrical blood vessels. 

 Without any elastic tension, i.e., if there were no 

 "automatic" adjustment of the tension in the wall 

 with the degree of stretch, it has been seen (section 

 13) that there is complete instability of a cylindrical 

 vessel. There can be only a precarious equilibrium 

 between the dilating force of the transmural pressure 

 and the constricting force of the tension in the wall. 

 Departure from the equilibrium point of the slightest 

 degree in either direction will be perpetuated. It is 

 interesting to note that in the case of the glomus 

 bodies of the A-V anastomoses of the stomach vascular 

 beds (2), it has been shown (26) by the study of the 

 passage of glass beads that these "shunts" are either 

 "open" or "closed," and cannot liold intermediate 

 positions. This is what the theory would predict for 

 structures richly endowed with contractile muscle 

 bands, but without obvious elastic fibers. 



The presence of elastic tissue, which provides an 

 elastic tension automatically increasing the total 

 tension in the wall as the radius increases, removes 

 this complete instability, but over a limited range 

 only. If the transmural pressure exceeds the critical 

 value given by equation 21, related to the maximum 

 elastance of the wall, the instability reappears and 

 blowout occurs. If the transmural pressure is less 

 than that required to stretch the elastic fibers beyond 

 their unstretched length (equation 22), once more 

 the instability, under active tension, will be present. 



In section 8 of this chapter a major role of elastic 

 tissue on the walls of blood vessels was suggested; 

 that of providing the "maintenance tension" to be in 

 equilibrium with the blood pressure, without any 

 expenditure of energy. We now see a second im- 

 portant role of elastic tissue, concerned not with 

 steady conditions in the circulation, but with control 

 of the distribution of blood Elastic tissue is necessary 

 to make such control possible in a graded, stable 

 manner. 



Cybernetically considered, the arrangement of 

 function between elastic tissue and smooth muscle 

 is ideal for providing the greatest sensitivity of control. 

 We start with a completely unstable situation, where 

 sensitivity to the changes in vasomotor tone would be 

 infinite, but without any possibility of proper control. 

 By adding just enough of the automatic adjustment of 

 tension of elastic tissue, stability is added, but \ery 

 great sensitivity is still possible. This is reminiscent 

 of the analogous device in electronics, in the early 

 days of radio-receivers, having a positive feedback 

 "plate-coil" which could be brought closer and 

 closer (coupled to) a "grid-coil." To receive a faint 

 signal one increased the coupling until about at the 

 point of self-oscillation (instabilitv), which would 

 result in "howling," and interference with the re- 

 ception by one's neighbors. By just falling short of 

 the oscillating point, one could achieve a very great 

 sensitivity indeed. The disadvantage is that the 

 stability is limited, with "howling" in the radio- 

 receiver, and critical closure in the control of the 

 circulation (if indeed, this is a disadvantage in the 

 disturbed physiological cases where it occurs). Cer- 

 tainly in the case of the arterial-senous anastomoses 

 of vascular beds, the instability that leads to "open 

 or closed" character of the operation of shunts may 

 be advantageous. 



19. EXPERIMENTAL VERIFICATION OF THE THEORY OF 

 CRITICAL CLOSING PRESSURES AND CRITICAL 

 CLOSING ACTIVE TENSIONS 



Detailed discu.ssion of this is not the function of this 

 part of the Handbook. The physical theory seems 

 incontrovertible, in indicating this type of funda- 

 mental instability in small blood vessels under 

 vasomotor tone. Its prediction of CCP seems un- 

 equivocal, as being based essentially on the shape of 

 the elastic diagram of the arterioles. The mani- 

 festation of CCP in \ascular beds, howe\er, might 

 well be impo.ssible, and the phenomenon predicted 



