172 



Mechanical and Electrical Character of the Heartbeat /9 : 6 



not include differential equations are advised to omit the remainder of 

 this section and to accept certain statements in the next section as a 



matter of faith. 



The heart behaves as a group of current sources in a finite conducting 

 medium. A current source is an emf whose internal resistance is much 



Terminal 



Source 



R Equivalent 

 to External 

 Load 



R Equivalent 

 to External 

 Load 



Source 



R«r 



R«r 



(a) 



R Equivalent 

 to External 

 Load 



Source 



R»r 

 (b) 



Figure 8. (a) Current source. Two equivalent forms are 

 shown. In either case, if r > R, the current source approxi- 

 mations can be made, namely, 



I = I = E /r 



V=RI 

 Thus V and / are determined by I and the load, (b) Voltage 

 source. If r <^ R, the voltage source approximations can be 

 made, namely, 



I=E /R 

 V=E 

 Thus V and I are determined by E and the load. 



greater than the external load. Thus, the external current will remain 

 constant no matter how the external load is varied. A current source is 

 illustrated in Figure 8. (A voltage source is one in which the internal 

 resistance is so low that the terminal voltage will remain constant as the 

 external load resistance is varied.) The tissues surrounding the heart 

 are electrically similar and comparatively low in impedance. Because 

 the heart muscle may be regarded as a group of current sources, the 



