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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



serve as a "jacket" just as they do in the elastic vessels. 

 They provide a safety factor to prevent overstretching 

 the smooth muscles. 



The model of figure i 3 gives only the arrangement 

 of the ring muscles in relation to the other elements. 

 The more centrally the arteries of the muscular type 

 are located, the greater is their amount of tension 

 muscles [Benninghoff (10, 11)]. We must then assume 

 a mixture of models shown in figures 9 and 13. The 

 change over from the pure elastic-type model (fig. 

 9) to the pure muscular-type model (fig. 13) is grad- 

 ual. The different behavior of the arteries may be 

 the reason why different authors have different opin- 

 ions about the architecture of the same wall. For 

 instance Burton (20) suggests an arrangement similar 

 to figure 9, whereas Bergel (12, 13) speaks of an 

 arrangement similar to figure 13. Both may be right. 



The smooth muscles in the muscular arteries are, 

 during life, under a continuous stress since they are 

 in parallel. Therefore they must have a certain basic 

 tone to withstand the stress of the blood pressure. 

 There is strong evidence that this basic tone may 

 derive from myogenic activity. Bayliss (9) suggested, 

 in 1902, that the blood pressure might act as a me- 

 chanical stimulus to the vascular wall. Lately it has 

 become more and more evident that the smooth 

 muscle possesses the capability of spontaneous activity 

 (see above). For instance, denervated intestinal 

 smooth muscles respond to a stretch with a contraction 

 (fig. 4). The tonus of the vascular smooth muscles 

 may be assumed to depend on the tension of the wall 

 and consequently on the pressure within the vessel. 

 Folkow (26), Thurau & Kramer (91), and earlier 

 workers have found that the blood flow becomes 

 constant above a certain pressure which may mean 

 that the pressure or, rather, the wall tension serves 

 as a stimulus for contraction of the smooth muscles, 

 and so causes an increase in the peripheral resistance 

 (see above). This autoregulation of flow results in a 

 homeostasis of wall tension for, in contracting, the 

 smooth muscles increase the thickness of the wall and 

 reduce the radius of the lumen. Both of these changes 

 reduce tension on individual muscle fibers. So the 

 vascular smooth muscles may keep their tension near 

 a constant level by contraction, when the pressure 

 rises. 



The suggestion has been made that basal tone may 

 derive from locally released constrictor agents or 

 regional reflex arcs of independent nerve plexuses in 

 the vascular wall. However, Folkow & Oberg (29) 

 have recently published experiments which eliminate 

 these possible mechanisms. These experiments show 



that the basic tone of precapillary resistance vessels 

 and autoregulation of flow is not due to nerve plexuses 

 or vasoconstrictor agents, but to myogenic activity. 

 The task of the autonomic nervous system, which 

 innervates the vascular muscles, would then be to 

 control the myogenic activity and adjust it to the 

 appropriate situation of the circulatory system (see 

 above). In the same way tone may be controlled by 

 chemical agents. This matter is also discussed in 

 Chapter 37. 



As the pressure perfusing a vascular bed is gradually 

 reduced, the flow becomes less in proportion, the 

 exact nature of this relation changing under different 

 circumstances and with various vascular beds, as 

 discussed in Chapter 28. The flow stops before the 

 arteriovenous pressure difference reaches zero. The 

 pressure at which this stoppage occurs has been 

 called the "critical closing pressure" by Burton (19). 

 The physical and physiological factors which deter- 

 mine the height of this pressure are discussed in 

 Chapter 6. 



Capillaries and Arteriovenous Anastomoses 



The arterial side of the circulatory system is con- 

 nected with the venous side by two types of vessels: 

 capillaries and arteriovenous anastomoses (see also 

 Chapter 27). 



Capillaries are the tiny vessels through the walls of 

 which materials are exchanged between blood and 

 the tissues. They consist of a thin layer of endothelial 

 cells, which sit on a basal membrane. On the outside 

 of the capillaries are found the pericytes, which are 

 cells with many irregular branches. Capillaries have 

 no smooth muscles and, in spite of earlier contentions 

 to the contrary, it is the current consensus that the 

 pericytes cannot constrict mammalian capillaries 

 (see Chapter 27); nor can the swelling of endothelial 

 cells cause stoppage of flow [for additional references 

 see Illig (41)]. 



The arteriovenous anastomoses are vessels the 

 walls of which consist almost entirely of smooth 

 muscle. They serve as a direct connection between 

 the arteries and veins, bypassing the capillaries. The 

 large amount of smooth muscle enables the arterio- 

 venous anastomoses to keep their lumens closed over 

 long periods of time. It is not impossible that these 

 anastomoses regulate the capillary blood flow- through 

 the several organs, according to their activity. If an 

 organ is active the anastomoses close and the blood 

 may flow through the capillaries, whereas in a resting 



