8l2 HANDBOOK OF PHYSIOLOGY -~ CIRCULATION II 



20C- 



fig. 3. Pressure-volume relations for a ring 

 of dog thoracic aorta, in situ length 10 mm, 

 with stretch done by continuous stretch (curve 

 S) and by successive, repeated dynamic 

 stretches. The broken line, curve R, is the 

 stretch-release curve for the continuous 

 stretch. The crosses mark the mid points of the 

 successive hysteresis loops. The solid points 

 represent the postdecay (static) tension values 

 reached 5 min after completion of the stretch. 



150 



100- 



50 



0- 



V0LUME cc 



1.3 



1.5 



2.1 



2.3 



than that given by the continuous curve, but crossed 

 it and became greater at high pressure levels. The 

 continuous curve reflected the large stretch which 

 had preceded it. If less load had been used for this 

 stretch, the curve would have differed less at low 

 pressure settings, but even more at high pressures. 

 It might seem that a pressure-volume curve obtained 

 by joining the midpoints of the respective loops might 

 give a better measure of aortic distensibility. But, in 

 life, the aorta is never free from stretches, and any 

 departure from normotensive pressure levels is but 

 temporary. We would expect, then, that when the 

 pressure did fall below normal, the aortic volume 

 would be greater than indicated by this curve con- 

 structed from the loops. 



More important, the volume change (AF) for the 

 different pulse pressures was almost the same for the 

 different loops as when taken from the continuous 

 curve. This is particularly true in the normotensive 

 pressure range. Hence, the very different methods of 

 stretching produced some, but not large, changes in 

 the AP/AF value. Now let us express these distensi- 

 bility curves in terms of moduli, using equation 1 4. 

 As shown in figure 4, the dynamic S& for each of the 

 loops, using the peak value only, was greater than 



the static by an average of 10 per cent. The con- 

 tinuous stretch curve gave modulus values varying 

 from — 1 5 to +12 per cent of the static, with an 

 average difference of +2 per cent. Also shown in 

 figure 4 are the modulus values calculated for only 

 the very first part of the stretch curve for each of the 

 loops. The fit with the other moduli is erratic, but the 

 values are considerably greater than those based on 

 peak values. These results are given in detail only to 

 illustrate how difficult it is to classify the behavior of 

 the aorta on the basis of any single technique of 

 performing stretches. 



Changes in Length and Wall Thickness of Arteries 



In all modulus calculations, it is unrealistically 

 assumed that length and wall thickness remain 

 constant. Lawton (76) presented evidence that the 

 volume of the aortic wall remained unchanged during 

 a stretch. This means that as the circumference 

 increases, there should be either a shortening in 

 length or a decrease in thickness. Fenn (26) and 

 Fawcett calculated that if the wall is isotropic, there 

 should be no length change, so that only wall thick- 

 ness would be involved. A direct recording; of the 



