1084 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



it appears reasonable to suggest that this spiral twist 

 of a distended vein may be of importance in relieving 

 kinking of the veins when the tissue is mechanically 

 distorted. 



Quite a different curve of vascular distensibility 

 was presented by MacWilliam (60) as shown in 

 figure 4. He took the precaution to collect fresh 

 tissues and observe their behavior carefully during 

 the postmortem period. Shortly after a segment of 

 living vessel was excised, it developed marked spasm. 

 This spasm persisted for many hours if the tissue was 

 kept cool. In this contracted condition a stretch curve, 

 such as that shown in the lower half of the figure, was 

 observed. When the state of contraction was elimi- 

 nated by warming the vessel, a more conventional 

 stretch curve was obtained as shown in the upper 

 portion of the figure. The marked sigmoid curve ob- 

 served originally has now been replaced by a simple 

 bow convex to the length axis. MacWilliam inter- 

 preted the lower curve as a manifestation of the 

 resistance to stretch of the smooth muscle, which 

 gradually gave way as tension increased until even- 

 tually stretch was restricted by the elastic and fibrous 

 tissue of the vessel wall. The upper curve lacked this 

 muscle component, and therefore revealed the simpler 

 manifestation of elastic tissue distension. This change 

 in the distensibility pattern was quite characteristic 

 of arteries; it was not so evident in the veins studied 

 bv MacWilliam. 



BRARY * 



Length 



fig. 4. Stepwise loading of a ring of artery which in the 

 lower section was in a contracted state from preservation in the 

 cold. In the upper tracing the identical loading sequence was 

 repeated after the vessel had been dilated by warming. [From 

 MacWiUiam (60).] 



Constricted 



Total Venous Volume *• 



Fic. 5. Distensibility patterns recorded from veins in vivo (4). 



A majority of investigators have considered this 

 spasm of the excised vessel as a postmortem artifact, 

 and many describe techniques employed to remove 

 this state of spasm in the tissue before carrying out 

 studies of its elastic behavior. The potential signifi- 

 cance of this observation of MacWilliam therefore 

 lay dormant for many years, until Capps (16) ob- 

 served the same type of curves in plethysmographic 

 recordings obtained from the human arm. The sig- 

 moid type of distensibility curve was associated with 

 constricted veins, and the smooth bow was associated 

 with dilated veins. 



More recently the significance of this distensibility 

 pattern of constricted vessels and of dilated vessels 

 has been analyzed by the author (2-4, 78). There is 

 now ample evidence that these distensibility patterns 

 are exhibited by living veins in vivo (fig. 5) and, as 

 will be discussed later, their analysis can serve as a 

 useful index to the state of contraction of the veins. 



Another feature of vascular distensibility, which has 

 been recognized since the time of Roy (79), is the 

 marked time dependency in elastic behavior (1). 

 This has been variously identified as "elastic after- 

 action," "elastic hysteresis," "delayed compliance," 

 or probably more properly by the physical phe- 

 nomena of "stress relaxation," in which pressure dissi- 

 pates following sudden distension to a constant 

 volume, and "creep," in which volume slowly in- 

 creases after sudden distension by a constant pressure. 

 With a continuous cycle of injection and withdrawal 

 of fluid, this characteristic manifests itself as a wide 

 loop of disparity between the pressure-volume rela- 

 tionships observed on injection and the pressure- 

 volume relationships found on withdrawal (figs. 6 

 and 7). To a degree, the width of this loop demon- 

 strates time dependency, in that it tends to become 



