994 



PHYSIOLOGY 



coats of the vessels, or by pressure on the vessels exerted by the sur- 

 rounding muscular and elastic structures. 



It will simplify the discussion of the main factors of the circulation 



FIG. 380. Artificial schema to demonstrate the main features of the 



circulation. 



The heart is an enema syringe with valves at v and u. The artery is a 

 thick-walled rubber tube. On the venous side is placed a length of wide 

 thin-walled tubing, to represent the large thin-walled distensible veins. The 

 arterioles and capillaries (peripheral resistance) are represented by wide glass 

 tubes packed with sponges. By opening the clamp on the tube D (' splanchnic 

 area arterioles ') the peripheral resistance can be removed, and a free passage 

 of fluid allowed from arterial to venous side. 



in a closed system if, for the present, we neglect the variable factors 

 and see what would take place in such a system of elastic tubes all 

 situated on one horizontal plane. Such a system is represented in the 

 diagram (Fig. 381), and a working model of 

 it in Fig. 380. 



The heart H is interpolated at one part 

 of the circuit, while the free outflow of the 

 fluid from B to D is impeded by the presence 

 of a peripheral resistance at c. Such a 

 system would have a definite capacity at 

 zero internal pressure, but a very much 

 greater amount of fluid might be forced into 

 it under a positive pressure. We will assume 

 that the pressure throughout the system is 

 equal to 10 mm. Hg, i.e. the elastic tubes are all slightly distended. 

 If the heart H now begins to contract it will pump fluid from E into A. 

 The pressure in E will fall from 10 mm. to mm., while that in A will 

 rise to a corresponding extent, the resistance at c preventing the free 

 escape of fluid from B to D and so causing the heart to pile up the 

 fluid which it has taken from E into A. 



