PHYSIOLOGY OF AORTA AND MAJOR ARTERIES 



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fig. 4. Distensibility modulus (eq. 14) calculated from the 

 data of fig. 3. Solid line, from continuous stretch curve. Dotted 

 line with dots, from peak values for hysteresis loops. Dotted 

 line with triangles, from postdecay (static) values for loops. 

 Broken line, from initial slopes of stretch curves of hysteresis 

 loops. 



change in thickness during a stretch has not been 

 made. There are some sparse references to the relation 

 of the unloaded thickness (a) to the outside diameter 

 (£)). Thus King (62) found an a/D ratio of .09 for 

 human aortas. McDonald (84), in a survey of many 

 arteries from the dog, found a constant ratio of 

 .08. In studying the effect of age on the human 

 aorta, King (64) found a progressive decrease in 

 thickness, so that the product of thickness and radius 

 was nearly constant. On the other hand, young aortas 

 show more longitudinal retraction upon excision 

 than do those from older people (107), which might 

 account for part, at least, of the difference in wall 

 thickness. The question of how much the wall thins 

 during a stretch needs documentation, since this 

 factor will affect the derived modulus value. 



In isolated vessels subjected to a volume increase, 

 Fenn found a lengthening, from which he concluded 

 that the wall was anisotropic (26). McDonald (84) 

 is quite correct in emphasizing that the longitudinal 

 extensibility observed in isolated vessels may not be 

 a measure of changes that might take place in the 

 in situ vessel under longitudinal restraint. Hence if 

 the intact vessel is in the steep portion of the longi- 



tudinal extensibility curve, its length changes with 

 each pulse would not be large. The presence of the 

 aortic sheath might also reduce length change in the 

 in situ aorta. It is of interest here that a pulmonary 

 artery freed from surrounding connective tissue showed 

 a longitudinal thrust with a volume injection, while 

 one still bound showed but minor change (32). Yet 

 the vessel wall should not become anistropic simply 

 because it was released from its longitudinal restraint. 

 Length changes in living animals have not been 

 completely measured. Lawton (78), working with 

 serial photographs of a dog abdominal aorta, found a 

 small shortening in early systole and a lengthening 

 in diastole. This made the length and circumference 

 changes almost 180 degrees out of phase. Similar 

 length changes for the abdominal aorta were found 

 by Patel and co-workers (87). In contrast, they found 

 length and diameter changes to be in phase in the 

 thoracic aorta. 



The small length changes recorded seem in sharp 

 contrast to the sometimes rather striking longitudinal 

 thrusts seen in the aortic arch. And at times a freed 

 carotid artery, or more rarely a femoral artery, 

 visually seems to be showing a length change. These 

 thrusts might reflect factors other than a distention 

 upon invasion by the pulse wave, however. The heart 

 is anchored in the chest by the large vessels. It has 

 long been known that the base of the heart is lowered 

 in contraction, which must serve to lengthen the 

 aorta and pulmonary artery (47). Rushmer (1 13) has 

 described this movement as starting in the period of 

 isometric contraction. The motion of the arch, and of 

 the brachiocephalic arteries which serve as anchor 

 points for the arch, would reflect not only the geom- 

 etry of the vessels but the firmness of attachment of 

 the descending arch to the body wall. Further, 

 respiration displaces the aorta, which acts as though 

 it is bound rather firmly to the diaphragm. These 

 longitudinal thrusts would bear no necessary time 

 relation to the arrival of the pulse wave, and a de- 

 ciphering of the origin of length changes in a vessel 

 may not be easy. 



Considerable confusion was raised by a report (113) 

 that when diameter and pressure were simultaneously 

 recorded in the thoracic aorta, an unorthodox 

 hysteresis loop was obtained in which, during stretch, 

 the diameter change was proportionately greater 

 than the pressure change. These loops were taken 

 from an oscilloscope. Inspection of the individual 

 diameter and pressure records indicates that the 

 whole of the diameter curve simply preceded the 

 pressure curve (126). If the two were superimposed, 



