166 Mechanical and Electrical Character of the Heartbeat /9 : 4 



2-5 msec. By contrast, some cardiac muscle fibers take as long as 200 

 msec to recover their resting potential. This period of time is com- 

 parable to the period of contraction of the ventricle. 



A closely related property is the recovery of the normal low net 

 permeability to potassium ions. When the resting potential of a voltage- 

 clamped squid axon is suddently decreased, the net permeability to 

 potassium ions rises rapidly and then falls. The cardiac muscle cells, 

 in contrast, do not recover their original impermeability to potassium 

 until after the membrane potential returns to its original value. 



Like nerve and skeletal muscle, cardiac muscle exhibits a so-called 

 "positive after potential," during which time the resting potential is 

 greater in magnitude, around 100 mv instead of 90 mv, the outside being 

 positive relative to the inside. The after potential may last close to 

 500 msec before it is completely abolished. (The U-wave of the electro- 

 cardiogram appears about at the height of the positive after potential. 

 The U-wave is very small; it barely shows on the diagram in Figure 5.) 



The exact roles played by potassium and sodium ions in the resting 

 and action potentials of cardiac muscle are not known. Nonetheless, all 

 experiments indicate that, except for time constants, and perhaps some 

 absolute values, the electrical behavior of cardiac muscle is very similar 

 to that of squid axons discussed in Chapters 4 and 24. 



C. Energy 



Each time the heart beats, it converts chemical energy into hydro- 

 dynamic energy. The rate of work, that is, power, expended by the 

 heart varies with the activity of the organism. At rest, both the heart 

 output per beat and the number of beats per minute are comparatively 

 low. During strenuous exertion, both increase. The work done at 

 each beat is of two types, kinetic and potential (compare Equation 1 , 

 p. 159). Because the aorta is on the same level as the heart, the potential 

 energy is purely hydrostatic. Thus, from Equation 1, the work per 

 milliliter is 



H = \ P v* + p (2) 



If q is the volume per stroke, then the work w per stroke is 



w = qH = ± P qv 2 + pq (3) 



where the bar indicates average values. 



Of even greater interest is the power II developed by the heart. To 

 find this, one must replace the stroke volume q by the volume rate of 

 flow Q (also called the heart output). Including the contribution of 

 both halves of the heart leads to the expression 



n = p R Q + p L Q + Ip%Q + h P 4Q W 



