CHAPTER XXV 



THE OUTPUT OF THE HEART IN RELATION TO THE VENOUS 

 INFLOW, CHANGE OF RATE, ETC. 



The Output of the Heart per Beat 



In the heart-lung preparation described on page 163 it is possible to 

 make accurate comparison between the output of the heart and such con- 

 ditions as its rate of filling during diastole, the frequency of its beat and 

 the nutritional condition of its musculature. Starling and his pupils have 

 in this way thrown much light on the methods by which the cardiac out- 

 put is adjusted so as to meet the ever-varying demands of the body for 

 blood. The fundamental principle which determines cardiac output may 

 be stated thus: the force with which the heart contracts during systole 

 varies directly with its volume at the end of diastole. Now, since the 

 heart does not exhibit any tone in diastole (see page 220), it is plain that 

 the rate of venous inflow must be the main factor determining the diastolic 

 volume, a secondary one being the arterial pressure. When the venous 

 inflow is rapid, the heart becomes dilated as far as the pericardium will 

 permit, and it contracts to its full force ; when the inflow is slow, it is im- 

 perfectly dilated when contraction supervenes, and the beat is feeble. 

 This is tlie law of the ~keart, and it is rigidly obeyed in the case of the 

 cold-blooded heart, which is demonstrated by the fact that when the per- 

 fusion fluid flowing into the venous end is suddenly increased by a cer- 

 tain amount, the ventricular output with the next beat is proportionately 

 augmented. If a record be taken of the volume of the heart, this will be 

 found to be unchanged at the end of systole, though it has of course 

 become greater during diastole because of the increased filling. 



In the warm-blooded heart, at least under the conditions of experimen- 

 tation (heart-lung preparation) there is some lag in the adaptation of the 

 strength of systole to diastolic filling; the law however, is ultimately 

 obeyed. This is well shown in the tracings in Fig. 63, in which C is a 

 tracing of the volume of the isolated heart (obtained by using a cardiac 

 plethysmograph), B.P. the arterial blood pressure, and V.P., the venous 

 blood pressure. 50 



It will be observed that when V.P. is suddenly increased the cardiac 

 volume immediately becomes greater (indicated by a general fall in the 

 level of C), and that, although the contractions of the ventricle also bc- 



216 



