lOO 



HANDBOOK OF PHYSIOLOGY '^^ CIRCULATION I 



Vertical Tube Corves on Rabbit Ear with Ringer 

 Solutions Containing ftiiviNE 



0O02 



5 « 19 BO 



FIG. 17. Proof that the closure of the vessels in the rabbit eai- at the CCP is active and not due 

 to obstruction. For explanation see text. [From the work of Nichol (19).] 



FIG. 1 8. The remarkably abrupt opening of a vascular bed of rabbit's leg in spasm, when the 

 pressure was raised above a critical value. [From the work of Girling (14).] 



PRESSl^RE — 

 L/H . .-L 



120 



160 



to several physiologists. The objections and mis- 

 understandings of the theory have been summarized 

 in a paper by Folkow & Lofving (13) that should 

 be studied by those wishing to consider both sides 

 of the question. 



20. WHICH VESSELS CLOSE AT THE CRITICAL PRESSURE.'^ 



The theory shows that the CCP will be given 

 approximately by equation 22. We would therefore 

 expect that the highest CCP, and the greatest tend- 

 ency to close, would be in those vessels where the 

 possibility of active tension, Ta, vvas greatest, and 

 where at the same time the radius was least. The 

 arterioles obviously are the most likely candidates 

 for the "critical vessels." Although the capillaries 

 are of smaller radius, their walls are not capable of 

 any active tension, since they lack any smooth 

 muscle. 



There is definite experimental evidence that 



associates "critical closure" with the "resistance 

 vessels" in a vascular circuit, indicating that the 

 vessels that close are those which offer the greatest 

 resistance to flow (at high perfusion pressures). 

 If a number of results for CCP of a perfused vascular 

 bed, produced by various vasoactive drugs and by 

 stimulation, is plotted vs. the resistance to flow of 

 these beds at a high transmural pressure, there is a 

 very high degree of correlation (correlation coeffi- 

 cient > 0.9). If the vessels which were "critical," i.e., 

 the ones that closed, were not those that offered the 

 chief item of the total resistance to flow, we would 

 not expect such a good correlation. 



21. MINIMUM V.^LUES OF CRITICAL CLOSING PRESSURE; 

 RESIDU.^L CRITICAL CLOSING PRESSURE 



The minimum value for CCP, even in vascular 

 beds without any \asomotor tone, as produced by 

 perfusion with cyanide to paralyze smooth muscle. 



