PHYSIOLOGY OF AORTA AND MAJOR ARTERIES 



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fig. 2. Extensibility and derived distensibil- 

 ity relations for a hypothetical tube showing a 

 linear tension-length relationship. 



10 II 



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mum extension possible. Hence /3 = L /(L m!xx — L ). 

 If a proper L max can be determined, this ratio has 

 several advantages, but it also suffers from the same 

 uncertainty as to what a proper L„ value should be, 

 particularly if, in constructing the relationship, the 

 tissue has been stretched so as to approach L max . 



It has become common to compare a "dynamic" 

 modulus, as obtained with rapidly repeated small 

 stretches, to a "static" modulus. For example, Lawton 

 (77) and Cope (22) reported a small increase in the 

 dynamic value over the static for the aorta, which 

 presumably reflects the influence of the rate-de- 

 pendent factors involved in the visco-elastic behavior. 

 But there is confusion as to how a static value should 

 be determined. Sometimes values taken from a 

 single continuous stretch curve covering the whole 

 range of physiological pressures are used if the in- 

 volved stretch has been done slowly. In other cases, 

 a pressure-length value representing the center of 

 the dynamic loop is taken as indicating the static 

 value. Only rarely does this give a value different 

 from one based on the peak values of the loop, and it 

 would appear to strain the definition to take this as a 

 static value at all. A third method is to hold a peak 

 load constant until, through creep, the length has 

 approached a final value. All three methods give 

 different values, which simply indicates again that 

 more than viscosity is concerned in tissue hysteresis. 

 This can be illustrated by an experiment shown in 

 figure 3. An isolated ring of dog thoracic aorta was 

 first subjected to a continuously increasing stretch. 



over 2 min, to a high tension. Tension was converted 

 to pressure, and half-circumference to volume. The 

 peak tension thus represented a pressure of 350 mm 

 Hg. The load was then slowly released, over 2 min, 

 and, as before, the ring did not return to the same 

 initial volume setting. A second identical stretch (in 

 terms of tension) was then made. The relations 

 obtained during this second stretch and stretch 

 release are plotted as the solid line in the figure. This 

 stretch curve is not different from those we have used 

 in the past to classify the distensibility of aortic rings. 

 Now the ring was allowed to remain in Locke- 

 Ringer's solution for 2 hours, during which time the 

 unloaded volume was very slowly decreased. It was 

 mounted on the stretching apparatus, care being 

 taken not to stretch it in the process. A small length 

 change was then made rather rapidly (0.1 sec), and 

 the stretch repeated in rapid succession ten times. 

 Stable stretch and stretch-release curves were es- 

 tablished by this time. The pressure-volume relations 

 of this stable loop are given in the figure as loop A. 

 The same stretch was then performed an 11th time, 

 but the peak value was held constant for 5 min, 

 allowing the pressure to fall to its static value, some 

 2 mm Hg lower. The ring was then returned to a 

 volume setting part way up the original loop, and a 

 new series of rapid stretches made, the last loop 

 being shown as B in figure 3. Again a static pressure 

 was obtained. The whole process was repeated 13 

 times. 



The initial volume for the loops was first smaller 



