PHYSICAL CONSIDERATION 



353 



illustrate, as the cross-section of all the capillaries put together is larger 

 than that of either the arteries or veins when combined into single 

 tubes, the lateral pressure as well as the velocity of flow must be much 

 less in these tubules than in the latter channels. Besides, as the fric- 

 tion in these exceedingly fine tubules is considerable, they really serve 

 the purpose of a resistance which is interposed at this point of the vas- 

 cular system to retard the flow of the blood. On account of this hin- 

 drance, the arterial blood is held back, thereby establishing a much 

 higher degree of pressure on the arterial side of the capillaries than 

 could possibly be produced if the offlow were not restricted at all. 

 Furthermore, as the arterioles are capable of actively varying their 

 calibre, this resistance may be augmented or diminished at any time, so 

 that smaller or larger quantities of arterial blood may be allowed to 

 escape into the capillaries and veins. 



These changes in the peripheral resistance may be imitated with the help of the 

 accompanying schema (Fig. 184) by equipping the horizontal tube with a stopcock 

 possessing the same diameter as the main tube. If the latter is widely open, the 

 pressure shows a gradual decline in the direction from the reservoir to the outlet. 



Fio. 184. A STOPCOCK is INSERTED AT THE MIDDLE OF THE OUTFLOW TUBE IN ILLUSTRA- 

 TION OF THE RESISTANCE FURNISHED BY THE CAPILLARIES. 



Its partial closure, however, interposes a high resistance, in consequence of which 

 the fluid accumulates between this point and the reservoir, while it declines on 

 the side toward the outlet (Fig. 184). Concurrently, the lateral pressure exhibits 

 a decided increase in the central section of this tube, and a fall in its distal portion. 

 In our circulatory system changes of this kind are brought about by the con- 

 traction of the smooth muscle cells situated in the walls of the arterioles. The 

 constriction of the lumen of these tubules increases the resistance placed in 

 the path of the arterial blood, and prevents its free escape into the capillaries 

 and veins. 



The influence which the peripheral resistance is capable of exerting upon 

 the flow of the blood, may be illustrated in a very convincing manner by con- 

 necting a piece of elastic band-tubing with an ordinary valved syringe. The 

 outlet of this elastic tube should be diminished somewhat by equipping it with a 

 narrow piece of glass tubing. If the syringe is now dipped in water and is com- 

 pressed at frequent intervals, the band-tubing is distended by each influx of 

 water, but collapses as soon as this central force ceases and allows all the water to 

 escape through the outlet. The flow is then intermittent. If the syringe is now 

 compressed at shorter intervals, the tubing remains more fully distended and the 

 flow becomes remittent and finally constant. At this time the entire stretch of 



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