356 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



in the blood pressure. The cause of this discrepancy most frequently 

 lies in a lessened rate of the heart. For very similar reasons it cannot 

 be taken for granted that a rapid heart always gives rise to a higher 

 blood pressure, because the filling power of this organ may be decreased 

 in a measure to compensate for the increase in the frequency. More- 

 over, as a diminution in the power of contraction of the cardiac muscle 

 must be followed by a reduction in the force of ejection, the blood pres- 

 sure must fall even when the frequency and the filling power of the 

 heart remain practically unaltered. And again, while an increase in 

 the power of contraction of the cardiac musculature generally raises 

 the pressure, this result cannot be attained if the frequency or the 

 filling power of this organ is diminished. 



In further illustration of these complex interactions between the 

 factors giving rise to the energy of the heart, it might be mentioned that 

 the stimulation of the vagus nerve leads to a fall in the general blood 

 pressure, because the ventricular outputs are either diminished in 

 number or are stopped altogether. ^ But if a strength of current 

 is employed which is just sufficient to cause a moderate reduction in 

 the cardiac rate, the 'filling power of the organ may thereby be aug- 

 mented in such a measure that the blood pressure is enabled to retain 

 its former level. Quite similarly, the cutting of the vagi nerves most 

 generally produces a rise in blood pressure, because the removal of 

 the inhibitory impulses permits the heart to increase its frequency, 

 so that the number of ventricular outputs in a unit of time becomes 

 greater. But it also happens at times that this procedure produces no 

 augmentation at all, because a proper relaxation of the cardiac muscle 

 cannot be effected, owing to the high frequency of contraction. Under 

 this condition, the heart is quite unable to eject a greater quantity 

 of blood. Similar compensations occur at times during the stimulation 

 of the acceleratory nerves so that the rises in pressure ordinarily 

 resulting from this procedure, cannot attain their full development. 

 These variations are not mere theoretical possibilities, but are fre- 

 quently observed under pathological conditions. They have been 

 cited here somewhat at length, in order that they may be made use of 

 in explaining some of the peculiar changes in the blood-pressure occur- 

 ring in the course of cardiac diseases. 



It has previously been stated that the height of the blood pressure 

 most commonly bears a direct relationship to the cardiac energy as 

 expressed in terms of the ventricular output. This means that an in- 

 crease in the latter, is followed by a rise in the blood pressure, and vice 

 versa. In the second place, we have seen that the blood pressure is the 

 product of four different factors, namely, the energy of the heart, the 

 total quantity of the circulating blood, the elasticity of the blood- 

 vessels and the peripheral resistance. In view of this fact, the pre- 

 ceding general rule should therefore be amplified to include the provi- 

 sion that the other three factors must remain constant. If they do not 



1 O. Frank, Zeitschr. fiir Biologie, xxiii, 1901, 1. 



