102 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



TABLE 2. Adult Heart {Normal) 



Avg. .36 



The thickness of the ventricular wall, which is 

 much greater in the "flatter" portions of the wall 

 than at the highly curved apices, then is in ac- 

 cordance with the law of Laplace. Woods drew an 

 important conclusion as to the dilated heart. If 

 the heart were to dilate to twice its original linear 

 dimensions (remaining geometrically similar), the 

 radii of curvature would be doubled. The \'alue of 

 (i/Ri + i/R-i) would be halved, at every point on 

 the ventricular wall. Thus, to produce the same 

 systolic pressure, the tension T, per unit length of a 

 hypothetical slit in the ventricle, would have to be 

 doubled. Since the total length of such a cut (fig. 

 19) would also be doubled, the conclusion is ines- 

 capable that in a heart dilated to twice its normal 

 size the force of contraction per ventricular muscle 

 fiber would be four times as great (8). 



This factor of geometry of the heart, and mechani- 

 cal advantage, or "disadvantage," of the ventricular 

 muscle in producing pressure within the cavity 

 cannot be ignored in the explanation of the decom- 

 pensation of congestive heart failure. While an 

 "overstretching" of the fibers, which has been usually 

 cited as explanation, may be a real factor, the con- 

 sequences of the law of Laplace are equally, pcrliaps 

 more, important. The influence of this upon the 

 "load" of the heart, related to the C).. consumption 

 and total energy turnover of the heart muscle, has 



X2 

 X2 



X2 

 X4 



P=T(I/R|f l/Rj) 



RADII OF CURVATURE 

 TENSION DYNES/CM 

 CIRCUMFERENCE'. CM 



FORCE PER FIBER DYNES 



TO PRODUCE THE SAME P 



FIG. 19. Effect of doubling the size of the heart on the tension 

 required in ventricular muscle to produce a given systolic 

 pressure. [From Burton (8).] 



been pointed out (9). The major factor in this load 

 is not the mechanical work of pumping, but the 

 steady energy consumption to maintain tension in 

 the muscle for a given length of time, i.e., the "ten- 

 sion-time integral" of the ventricular muscle. The 

 practical implication is that increasing the mechanical 

 work of the heart, as in exercise, is not per se as 

 important in increasing the load of the heart as are 

 increases in sytolic pressure, in heart rate, and in 

 the size of the heart. 



24. MEASUREMENT OF ACTIVE TENSION IN 

 VASCULAR SMOOTH MUSCLE 



The final development of the body of theory con- 

 cerned with the physical equilibrium has been the 

 attempt to devise methods for measuring the actual 

 tension of vascular smooth mu.scle under vasomotor 

 tone or vasoactive drugs. The basic difficulty is 

 that when a blood vessel constricts, the total tension 

 in the wall, if the transmural pressure is unchanged, 

 has decreased, according to the law of Laplace. 

 However, as has been shown, this is because though 

 the active tension, T,i, has increased, the elastic 

 tension, T p,, has more than nullified this, in the total 

 tension, by its automatic decrease (fig. 15) because 

 of less stretch. Thus the relation between the radius 

 of the vessel, of which the resistance to flow can 

 serve as an index, and the magnitude of 7^,i is a 

 \ery complicated and nonlinear one. Only if the 

 details of the elastic behavior of the vessel were 

 known could the changes in acti\-e tension be deduced. 



An attempt has been made to devise a "null 



