8o8 HANDBOOK OF PHYSIOLOGY -i CIRCULATION II 



terms of the shape of the stretch curve, and the total 

 stretch employed. 



Effects of Aging on Arterial Dislensihility 



The effects of aging on arterial distensibility are 

 supposed to be well established — at least the textbooks 

 so report. The actual evidence leaves much to be 

 desired. On the one hand is the story of pathologists 

 that aging is accompanied by a reduction in muscle 

 mass and in elastic tissue, with a replacement by 

 collagenous fibers. The reduction of muscle mass needs 

 documentation by actual cell counts. Chemical 

 digestion of the aortic walls, to leave only elastin, 

 left Lansing (74) unwilling to accept the dictum that 

 the elastic fibers had been reduced in number with 

 age. He would, of course, accept the possibility of a 

 chemical change which might influence the wall 

 extensibility. 



Extensibility studies made on isolated vessels taken 

 from humans of different ages suffer from our un- 

 certainty about how to compare extensibility among 

 different specimens. The repeatedly quoted studies 

 of Hallock & Benson (37), based on a small series, 

 in which only the average results of a given age group 

 were presented, showed some decrease in extensibility 

 with age, with the only truly large change seen in 

 individuals over 70 years. The comparative data were 

 expressed in terms of an elastic modulus (AP/(AV/V). 

 Here, as in all other reports (10, 62, 66, 81, 107, 127), 

 there was a progressive increase in unloaded diameter 

 with age, which in itself could increase the value of 

 this modulus. In a study of a larger series of human 

 aortas (107) we presented results taken from the 

 second of two consecutive stretch curves. Variations 

 within the age groups were large. While the group 

 averages showed a progressive increase in initial 

 diameter, it was also true that a man of 68 showed 

 the same diameter as a girl of 18. All these aortas 

 were screened so as to include none showing athero- 

 sclerosis, and any from individuals with a history of 

 hypertension were placed in a separate category. 

 The diameter increase was especially noticeable in 

 these hypertensives. The slopes (AP/AV) given by 

 the stretch curves also showed intraindividual varia- 

 tion, but they were very much more constant than 

 were the initial diameters, and there was no clear 

 trend for this slope to change with age. A changed 

 modulus value with aging was, then, predominantly 

 conditioned by a change in the initial diameter. 



Expression of Extensibility' in Terms of Moduli 



This raises the question as to just how meaningful 

 a modulus value is in expressing extensibility data. 

 Certainly having to present a whole stretch curve for 

 each specimen studied is cumbersome, and such data 

 are difficult to handle statistically. But a modulus is 

 supposed to afford insight into the architecture of the 

 specimen. Thus when a physicist wishes to describe 

 the extensibility as a property of a material, he uses 

 Young's modulus, or a related one, which is simply 

 the ratio of the applied extending force or stress, as 

 expressed per unit area of material, to the proportion- 

 ate change in length from the unloaded value. Most 

 of his materials are so stiff that the strain is small. 

 Further, the material promptly returns to the initial 

 length upon removal of the stress, and in measuring 

 extensibility he obtains a clue to the force of this 

 return. He therefore calls his modulus one of elasticity, 

 despite the fact that he is measuring extensibility and 

 not elasticity at all. Any time delay in effecting the 

 strain is usually so brief as to be inconsequential. When 

 the ratio of stress to proportional strain is constant 

 (he carefully avoids a load sufficient to cause per- 

 manent yield or plastic deformation), this ratio can be 

 calculated by using any convenient load. Because 

 most materials do not have a constant ratio, and some, 

 as cast iron, depart quite significantly from a linear 

 relation, he tries to make his applied stress as small 

 as feasible. 



When we turn to materials such as the polymers, 

 the stress-to-strain ratio is definitely not linear. 

 Further, the recorded strain is a function of the time 

 allowed under load; and the material may not 

 promptly return to the same unloaded length. Be- 

 cause of the last, it is definitely not proper to call the 

 modulus one of elasticity. As a substitute, one could 

 construct a modulus of extensibility, which would be 

 the reciprocal of what is usually called the modulus 

 of elasticity. On the thesis that there has been no 

 internal change in the material because of the stress, 

 and that the original internal architecture will be 

 precisely restored after load release, the physicist 

 continues to use a modulus as a proper expression of 

 extensibility even with polymers. To emphasize the 

 fact that the modulus calculations are based only on 

 the values seen during extension, rather than during 

 the elastic recoil, I have used the symbol S rather 

 than the conventional E in all the equations pre- 

 sented below. It should be obvious that one must 

 append to any modulus calculation a careful descrip- 



