802 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



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i !0 i0 40 50 20 J5 76 ?0 



Circumference, mm Ci rcumference, m m 



fig. I . Stretch curves for a ring of thoracic aorta of a dog. In situ length = 10.5 mm. Curves 

 I represent the first stretch curve, made by continuous tension increase over 1 min. Curves 2 

 are the results of a second stretch identical in load and timing to the first. Curves R show the 

 curves taken during the gradual release of tension, over 1 min. Curves C show the effect of 

 muscle contraction by immersion of the ring in epinephrine. 



I 2 



Volume, cc 



or has been dead for some time, and whether the 

 voluntary muscles are in a state of rigor mortis or not. 

 The change develops no matter how* carefully the 

 removal is done. The interpretation placed upon this 

 change in the past is that it reflects a strong contrac- 

 tion of the smooth muscle contained in the wall. 

 Aside from the speed of its development, which con- 

 trasts with the slower time courseof muscle contraction, 

 and the lack of correlation between the amount of 

 longitudinal shortening and the proportionate amount 

 of muscle in the wall (ascending aorta, for example, 

 contracts to the greatest degree), there are other 

 features which do not fit too well with this interpre- 

 tation. 



When such a tensed ring is subjected to stretch, 

 and the load is then removed, the walls are no longer 

 so tense, and the circumference is about 30 per cent 

 greater than before the stretch. What was not real- 

 ized in the earlier studies was that the amount of this 

 diameter increase bears a direct relation to the total 

 stretch imposed (96). Now, if a second stretch of the 

 same size is made, the stretch curve starts from the 

 greater initial value and courses almost parallel to 

 the first through the region of greatest extensibility 

 (fig. 1 A). Then, as the wall stiffens, the second curse 

 becomes enough steeper that it merges with the first 

 some time before the peak load is reached. This 

 merging argues against a conclusion that the first 

 stretch had caused some irreparable tissue damage, 

 such as an internal tearing. If, after the first stretch 

 and stretch release is completed, the tissue is allowed 



to remain unloaded for a long period (several hours), 

 the original small diameter may be almost, if not 

 fully, restored. If the first stretch had ""pulled out" an 

 existing muscle spasm, why should it reform and take 

 so long in doing so? Experiments designed to test this 

 "pulling out" idea with smooth muscle organs, such as 

 the gut, have given little indication that an active 

 contraction itself is eliminated by an extension of the 

 whole tissue under load (6, 101). 



With a relatively large load, a third stretch made 

 after the second usually gives a stretch curve identical 

 to that of the second. This fact has been recognized 

 by many past workers who were aware that a large 

 initial stretch, which was not quantitated, made 

 subsequent stretch curves more reproducible. Such 

 reproducibility does not mean that they are necessarily 

 more descriptive of the behavior of the vessel in vivo. 

 With a similar load, successive stretches may start 

 Irom progressively increasing initial diameters, but a 

 reproducible stretch curve is reached within 5 to 10 

 stretches. Unfortunately, when examining past 

 studies, it is impossible to be sure whether a prelimi- 

 nary stretch was used, and, if so, how much tension 

 was involved and how long a time interval was al- 

 lowed before the recorded stretch was made. 



The Hysteresis Loop 



Until recently, too, little attention was paid to the 

 fact that as the loads were removed the length curve 

 did not follow, during this stretch release, the previous 



