PERIPHERAL VENOUS SYSTEM 



I08 5 



2 4 6 



fig. 6. Continuous injection (upper dashed line) and with- 

 drawal Kloiver dashed line) of blood into mesenteric veins of a 

 dog at the rate of 37.5 cm 3 /min, compared with an injection 

 performed at the same rate but interrupted at three points for 

 intervals of 10 sec. Note that the stress relaxation, demonstrated 

 by the loop in the continuous curve, is significantly greater as 

 the higher pressures are reached in the interrupted curve (38). 



narrower at very fast and very slow rates of injection, 

 being widest when the rate of injection and with- 

 drawal permits the greatest degree of stress relaxation 

 to occur between the injection phase and the with- 

 drawal phase. In addition to this simple visco-elastic 

 type of behavior, moreover, veins also exhibit plastic 

 properties, in that the stress relaxation is minimal at 

 very low pressures, and becomes disproportionately 

 exaggerated at higher pressures, as indicated in 

 figure 6. It appears that certain "yield pressures" 

 are required to produce the transformation which 

 gives rise to the stress relaxation. 



The complex problem of the exact basis for these 

 properties of blood vessels (78) is beyond our scope 

 here, but it should be appreciated that these charac- 

 teristics have two implications of importance in 

 evaluating distensibility determinations. First, pres- 

 sure-volume data obtained from veins are strongly 

 influenced by the exact conditions of venous disten- 

 sion, and data are never rigorously comparable unless 

 identical rates and magnitudes of distension are used. 

 Unfortunately this requirement has often been over- 

 looked, and a large bulk of published data must be 

 questioned because of failure to impose exactly 

 defined and standardized procedures for obtaining 

 distensibility determinations. 



A second important facet to this problem is the 

 effect of a repeated stretch. As is to be expected of a 

 tissue showing a significant stress relaxation, a second 

 stretch curve differs greatly from an initial stretch. 

 This characteristic is most prominent in the con- 

 stricted vein, in which the sigmoid pattern of dis- 

 tensibility characteristic of an initial stretch is ob- 

 literated in succeeding stretches (5). Studies have 



shown that 20 to 30 min are required for a complete 

 restoration of the initial distensibility pattern follow- 

 ing an initial stretch of the tissue. Related to this is 

 the fact that the distensibility pattern observed on 

 release of stretch of a constricted vessel is identical 

 to the pattern observed with the release of stretch 

 of a dilated vessel (fig. 7). Stretch of the tissue appears 

 to "pull out" the muscle so that it no longer con- 

 tributes to the distensibility characteristics of the tissue 

 until considerable time has elapsed for this muscular 

 influence to become restored. This dictates that not 

 only must precise distensibility data be gathered with 

 rigorously defined rates of stretch, but also the in- 

 tervals between stretches must be rigidly controlled. 

 A further problem should be mentioned to which 

 there is at present no satisfactory answer. Many have 

 argued that distensibility data have no absolute mean- 

 ing until they are converted into terms of a quanti- 

 tative modulus of elasticity which relates the tension- 

 length or pressure-volume increments to the initial 

 length or volume parameters of the tissue. There are 

 certain practical difficulties which interfere with ac- 

 complishing this objective. With in vivo measure- 

 ments, accurate estimates of initial dimensions are 

 extremely difficult to obtain even when pressure and 

 volume increments are readily measured. Further- 

 more, inherent to the calculation of an elasticity 

 modulus is the assumption of homogeneity of the 

 tissue, and some investigators have questioned whether 

 a meaningful modulus can be calculated from tissue 



fig. 7. Injection and withdrawal of blood at the rate of 50 

 cm 3 /min into mesenteric veins of a dog constricted by neo- 

 synephrine (NEOj and subsequently dilated by adenylic acid 

 (A.A.). The injection curves demonstrate the characteristic 

 convexity and concavity patterns portrayed in fig. 5, but the 

 withdrawal curves were perfectly superimposed. 



