872 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



FIG. 5. Average stress-relaxation 

 curves of carotid and umbilical arteries. 

 Vertical coordinate is given in per- 

 centage of total pressure rise, following 

 injection. Upper curve, common carotid 

 artery of the dog. Lower curve: umbilical 

 artery of the human. [Zatzman el al. 

 (103).] 



OO - 



80 



6O- 



40-x 



20- 



Secorid t 



others by another, since, for example, the uterus and 

 the bladder are very different in properties and action 

 [Bader (3)]. 



If a smooth muscle is stretched quickly to a certain 

 length, it will show a tension increase. If this length 

 is held for a longer time, the tension will decrease, at 

 first quickly, later more slowly. This typical stress 

 relaxation is a result of the visco-elasticity of the 

 smooth muscle, which may be due to breaking of 

 the linkages within the myofilaments. A typical 

 stress-relaxation curve of smooth muscles is like 

 that of the lower curve of figure 5. For further 

 details of the mechanical properties of muscles, see 

 Reichel (71). 



If these mechanical properties of smooth muscles 

 are to be compared with those of the vascular wall, 

 one must keep in mind the modifying effects of col- 

 lagen and elastic tissue [Remington (73)]. Another 

 point is that most of the experiments with smooth 

 muscles are made on organs other than blood vessels, 

 in vitro, and without innervation. Smooth muscles 

 in vivo are under a continuous stimulation, and they 

 are also under constant contraction and tension in the 

 vascular wall. It is therefore very likely that the 

 smooth muscles of the vessel wall in vivo would show 

 different visco-elastic and plastic behavior from those 

 found during in vitro experiments. 



Zatzman et al. (103) have shown that there is a 

 great difference in the stress-relaxation behavior of 

 the elastic carotid artery and the muscular umbilical 

 artery. After 10 sec of stress, relaxation of the carotid 

 artery amounts to about 20 per cent of the original 

 tension, whereas in the umbilical artery it is about 95 

 per cent (fig. 5). This indicates that in the umbilical 

 artery the tension is applied mostly to smooth muscle 

 with its large visco-elasticity and plasticity. It is not 

 easy to say to what degree smooth muscle is respon- 



sible for the relaxation of the carotid artery, since 

 elastic and collagen tissues may each have both a 

 visco-elastic component (hysteresis loop during stretch 

 cycle) and a plastic component [irreversible elonga- 

 tion (table 1)]. This passive behavior is responsible 

 in part for the stress relaxation of elastic arteries 

 [see Kapal (42)]. 



Smooth muscle tissue degenerates and decreases in 

 amount with age, and is replaced by collagen fibers 

 [Meyer (58), Kobayashi (46)]. This tends to render 

 the arterial tree more rigid and to explain the systolic 

 hypertension of old age. The high systolic pressure in 

 this condition puts an extra burden on the heart. 

 [See Bader & Kapal (5) and the paragraphs on elastic 

 arteries below.] 



Smooth muscles have the ability to regenerate. 

 For example, Petry & Heberer (67) have described 

 cell formations which are found on the inside of aortic 

 grafts some weeks after implantation. These cells seem 

 to be muscle cells, and are assumed to originate from 

 fibroblasts. 



DIFFERENT TYPES OF VESSELS 



Vessels differ in their architectural structure and 

 their behavior according to their varied tasks. There 

 are, in general, four different vessel types: on the 

 arterial side are elastic arteries and muscular arteries, 

 but it is hard to say where the one ends and the other 

 begins, since the structural changes are gradual. 

 Usually the aorta, the pulmonary artery, the common 

 carotid artery, the subclavian artery, and the common 

 iliac artery are regarded as elastic arteries. Arteries 

 more peripheral than the above, down to the ar- 

 terioles, are classed as muscular arteries. After these 

 are the capillaries, which consist mostly of endothe- 



