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 



Arterial BP 



A[ f Arteries 



Veins Splanchnic area 

 nc^rf 



H 



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. 



B 



D 



