THE HEPATIC CIRCULATION 



1409 



isolated arterial segments. Owing to the questionable 

 application of these observations to the situation in 

 intact animals and the uncertain role of local vaso- 

 active materials, the "myogenic theory of tone" 

 was not readily accepted, but recent work by Folkow 

 and others appears to have put it upon a sounder 

 basis. Folkow & Lofving (131) worked with the 

 denervated hind limb, skin, and mesenteric arterial 

 bed in anesthetized cats, dogs, and rabbits. They 

 found that lowering the arterial pressure for as short a 

 period as 3 to 5 sec by arterial or aortic occlusion 

 produced dilation, whereas raising the pressure by 

 bilateral carotid occlusion for 20 to 60 sec elicited a 

 vasoconstriction. Neither anoxia nor hypercapnia 

 altered the response. Denervation apparently elim- 

 inated neither vascular tone nor responsiveness to 

 vasoactive drugs such as acetylcholine, epinephrine, 

 norepinephrine, serotonin, vasopressin, and angioten- 

 sin. From these results and from direct study of 

 isolated arteries and minute blood vessels Folkow 

 concluded that ''vascular tone" is created by a 

 rhythmic unsynchronized activity of the smooth 

 muscle of the resistance vessels. Conclusive demon- 

 stration of myogenic autoregulation within the mesen- 

 teric vasculature has proved somewhat difficult, 

 though Johnson (180) has been successful in finding 

 it in 2 1 of 26 experiments. The response, he observed, 

 was not eliminated by infusion of enough procaine 

 to block a possible local autonomic reflex arc and 

 did not appear to depend upon a change in tissue 

 fluid content, oxygen consumption, or lactic acid 

 production. The use of a suitable perfusion system 

 and enough time to permit recovery from surgery, 

 venous cannulation, and denervation may have been 

 important in Johnson's success in demonstrating the 

 phenomenon. Study of pressure-flow relationships 

 in the portal venous drainage tract has been less 

 clear cut in showing evidence of myogenic mainten- 

 ance of tone. Although the data obtained by Riecker 

 (250) with perfusion of the canine liver via the portal 

 vein in situ are not marred by the effects of the trauma 

 and disorganization, inevitable during excision and 

 study in vitro, they exhibit considerable variance; 

 opposing, on the one hand, the view that the porto- 

 hepatic vasculature is a simple, passive elastic svstem 

 and failing to support, on the other hand, intrinsic 

 control of vascular cross section. 



Although the data indicate that closure by col- 

 lapse may occur in the hepatic and splanchnic vascula- 

 ture, the role of a definite critical closing pressure 

 remains uncertain. Confusion arises particularly in 

 connection with the character of closure. According 



to the myogenic theory, the "unstretched radius" 

 is reached after the complete contraction of elastic 

 recoil and is therefore zero. In this view, closure 

 consists in a concentric constriction, but it may also 

 be regarded as collapse to form a closed slit from 

 some finite value for the unstretched radius. Since 

 critical closing has apparently escaped direct observa- 

 tion, it is impossible to say which, if either, state 

 obtains. The fact (70) that the pressure at which 

 closure occurs does not differ from that at which the 

 vessels re-expand (critical opening pressure) favors 

 the first, at least so far as the resistance vessels are 

 concerned. There is ample evidence that the critical 

 opening pressure for the large veins greatly exceeds 

 their critical closing pressure. A response similar to 

 that of the large veins — and slit formation on closure — 

 seems more likely also at the level of the tenuous 

 venular channels and capillaries and at arteriovenous 

 communications. However, the liver plates must 

 move with expansion or deflation of the sinusoids to 

 impose special plastic properties quite unlike those 

 characteristic of other capillary nets. Critical closing 

 pressures in the depth of a lobe probably differ 

 markedly from those characteristics of sinusoids close 

 to the surface not only because deformation must 

 affect the periphery more easily but also because the 

 distance from the afferent vessels is shorter in the 

 central regions. No matter what the mechanism of 

 closure may be, it effectively changes resistance to 

 flow by reducing the vascular cross section. In addi- 

 tion, the distribution of collapse may affect the re- 

 sistance by altering the mean length of the resisting 

 circuits. 



Path Length and Distributional Pattern 



Resistance is directly related to path length and 

 though the length of the conduit contributes much 

 less to frictional loss of the energy head at any level 

 than does the radius, it figures importantly in the 

 total resistance from artery to vein. Arteriovenous or 

 veno-venous shunting is the most obvious means of 

 shortening the vascular bed. Arteriovenous anas- 

 tomoses (A-V) occur prominently in the wall of the 

 stomach (22, 39) and may be operative elsewhere in 

 the gastrointestinal tract, but there is little evidence 

 that they are significant hemodynamically. Even 

 when the capillaries of the perfused stomach are 

 completely blocked with starch granules, no more 

 than 5 per cent of the total flow passes through the 

 A-V anastomosis. Few or none are demonstrable in 

 the liver though Prinzmetal et al. (233) have re- 



