8y6 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



fig. 9. Schematic presentation of the 

 behavior of the different tissues in the 

 wall of elastic-type vessels at different 

 degrees of extension. Description in the 

 text. [Bader & Kapal (7).] The stretch 

 is in the vertical direction. The S- 

 shaped line is the pressure-volume 

 curve of a young human aorta. 



(40)], but at 100 mm Hg this ratio is age-dependent 

 (fig. 86). It increases from the first decade of life until 

 the end of the third decade and decreases from then 

 until the end of life. The shift of this ratio, like the 

 shift of the inflexion point, is caused by the increase 

 of the collagen tissue. The arteries become more 

 and more rigid with age. A rise of the blood pres- 

 sure in older people does not change the tension 

 on the elastica, and thus prevents stimulation of the 

 pressoreceptors. This indicates that the regulation of 

 the blood pressure of old people would become more 

 and more unstable [Bader (4)]. Since the decrease 

 of the ratio of radius to wall thickness in the aging 

 aorta simultaneously decreases the wall tension (equa- 

 tion 1), the tension of the pressoreceptors must also 

 become lower and lower. But this process may re- 

 sult in an increase of the mean blood pressure to 

 get the pressoreceptor again on the normal tension 

 level. A similar change has already been mentioned 

 with the degeneration of the tension muscles. But 

 since the changes in smooth muscles, and in the ratio 

 of radius to wall thickness, with aging, are greater in 

 proportion than the usual increase of the mean blood 

 pressure, one may assume also a change in receptor 

 sensitivity. 



The interaction of the three wall elements in the 

 elastic arteries, elastic tissue, collagen tissue, and 

 smooth muscle, may be illustrated by the scheme of 

 figure 9. Sections 1 through 4 represent different 

 stretch phases. The stretch takes place in vertical 

 direction. Both halves of each phase must be regarded 

 together. The element a represents two elastic fibers 

 which are connected by smooth muscles as in figure 7. 

 Both elastic fibers are already straight at zero pres- 



sure. The element b is an elastic fiber which is still 

 wavy (unstressed). Both a and b, in the upper and the 

 lower half, are under minimal stress. Element c is a 

 collagen fiber. In the upper half it is less wavy than 

 in the lower half. Phase 2 will be reached after the 

 stress has begun. Element a is already stressed, whereas 

 element b is just straightened. By this means the 

 recruitment of the elastic fibers is represented. The 

 stretch proceeds in phase 3, so that collagen fibers 

 are partly straight and included in the stress (upper 

 half). The collagen fiber in the lower half is still wavy. 

 Elastic fibers only are stressed in phases 1 and 2, 

 whereas elastic and collagen fibers are functional in 

 series in phase 3. Now the length available for further 

 stretching of the elastic fibers is only half as much as 

 in phases 1 and 2, since a further stretch of the elastic 

 fibers in the upper half is prevented by the collagen 

 fibers. This means that the increase in extension, per 

 unit rise in pressure, becomes less and the pressure 

 volume diagram, which until now was concave to the 

 abscissa, becomes convex. At last a point is reached 

 where all collagen fibers are straight: phase 4. Elastic 

 and collagen fibers are straight and parallel. Since 

 collagen tissue is much less distensible than elastic 

 tissue, the wall distensibility at this point depends only 

 on collagen tissue. This model illustrates the wall 

 architecture of a proximal elastic artery. The more 

 peripheral the vessel, the more the ring muscles 

 participate and the more the model of figure g will be 

 combined with the model of figure 13 (see below). 

 The effect of changes with age in the arterial wall 

 can be illustrated by having the stretch start in phase 

 2, 3, or 4. Thus unextensible elements are put in 

 action at lower pressures than in young arteries 



