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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



contracting spontaneously peripheral resistance and 

 arterial blood pressure might well fall to a danger- 

 ously low level. Our very lives must depend upon the 

 maintenance of basal tone in the vessels of the skeletal 

 muscular system. 



Automaticity 



This may be illustrated by experiments on the cat. 

 Folkow & Lofving (97) recorded the effect of lowering 

 and then raising the arterial pressure upon the blood 

 flow through the muscles of the leg and the following 

 results have been calculated, approximately, from 

 one of their experiments: 



When the arterial pressure was suddenly lowered 

 from 1 20 to 50 mm Hg the resistance to flow in- 

 creased slightly from 1 7 to 19 units, probably because 

 of elastic recoil of the vessels. Now over the next 

 few minutes, arterial pressure being still 50 mm Hg, 

 vascular resistance gradually fell from 19 to 13 units 

 indicating a gradual reduction in smooth muscle tone. 

 When the arterial pressure was suddenly restored to 

 120 mm the resistance fell from 13 to 8 units, due to 

 stretching of the relaxed vessels. In the course of the 

 next few minutes the resistance rose again from 8 to 

 1 7 units, its initial value, indicating a gradual restora- 

 tion of smooth muscle tone. In short, lowering the 

 arterial pressure was soon followed by decrease in 

 basal tone and vice versa. 



An even more striking example of automaticity is 

 shown in another of Folkow's (89) experiments. 

 Clamping the carotid arteries was followed by a rise 

 in arterial pressure from 100 to 150 mm Hg; blood 

 flow in the denervated muscular portions of the hind 

 parts rose initially but soon returned to its initial 

 level. In spite of the rise in arterial pressure, the lumen 

 of the muscle vessels must have decreased. 



The explanation of automaticity is not yet com- 

 plete. Plain muscle responds to stretch by increased 

 contraction. Bayliss (35) pointed out the significance 

 of this. In a well-known experiment he recorded 

 volume changes in the dog's hind leg before, during, 

 and after splanchnic nerve stimulation. He notes: 

 "As the arterial pressure rises the limb is distended 

 passively, but instead of merely returning to its 

 original volume when the blood pressure has come 

 down again it constricts much below its previous 

 level and only gradually returns." He thought this 



was probably because the plain muscle of the arterial 

 walls had responded to stretch by contraction. How- 

 ever, other factors may be involved. It will be re- 

 membered that when in one of Folkow's experiments 

 described above the arterial blood pressure was 

 raised the flow remained constant and the lumen of 

 the vessels became smaller. In that case the stimulus 

 cannot have been simply stretching the vessel walls 

 if by that is meant a maintained elongation of the 

 smooth muscle fibers. Nor can the vasoconstriction 

 have been due to the lowering of metabolite concen- 

 tration due to more rapid flow — in this experiment 

 the flow did not increase. Perhaps some of the capil- 

 lary bed shut down so that the same total quantity 

 of blood flowed faster through a restricted area. 

 Further work is needed on the fundamental signifi- 

 cance of automaticity. 



Pressure-flow relations in muscle depend a good 

 deal on the condition of the animal. As this dete- 

 riorates in the course of an experiment, automaticity 

 declines and the effect of alteration in arterial pres- 

 sure upon muscle blood flow becomes more pro- 

 nounced. 



Automaticity in Hit 



Muscle lessels 



Experiments by Greenfield & Patterson (115) show 

 that human vessels constrict when they are stretched. 

 The forearm was enclosed in a plethysmograph, for 

 measuring the rate of flow, modified so that pressures 

 of —50 and — 150 mm Hg could be applied for 30 sec 

 to the enclosed limb segment. Immediately after the 

 release of the negative pressure forearm blood flow- 

 was decreased; the vessels must therefore have con- 

 stricted. The vessels in the calf respond in the same 

 way to stretching (60). Blair and others (38) re- 

 corded the oxygen saturation changes in blood from 

 the skin and from the muscle. When suction was 

 applied oxygen saturation rose at once in the blood 

 from the skin and muscle, due to distention of the 

 vessels. This can be seen in figure 4. However, by the 

 end of the first few minutes of suction, oxygen satura- 

 tion of the blood from both skin and muscle had re- 

 turned to its initial value or was even less; thus the 

 vessels had contracted to their initial size or even 

 smaller. So much for the facts. Since the circum- 

 ferential size of the vessels was not increased, and 

 may have been decreased, the authors thought that 

 the response could not be explained simply by stretch- 

 ing. 



Stretching the vessels of normal, sympathectomized, 

 and chronicallv denervated forearms bv venous con- 



