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 comparison between the output of the heart and such conditions 

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

 resistance offered to its systolic discharge. 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. According to these workers 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. When the venous inflow is rapid, the heart becomes dilated 

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

 fectly dilated and when contraction supervenes the beat is feeble. This is 

 the law of the heart, and in the case of the cold-blooded heart, it is rigidly 

 obeyed as is demonstrated by the fact that when the perfusion fluid flow- 

 ing into the venous end is suddenly increased by a certain amount, the 

 ventricular output with the next beat is correspondingly augmented, and 

 the volume of the heart remains 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 experi- 

 mentation (heart-lung preparation), there is some lag in the adaptation 

 of the strength of systole to diastolic filling, although the law of the 

 heart 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 



According to these views the factor that primarily determines the sys- 

 tolic discharge is the extent to which the cardiac muscle fiber becomes 

 stretched by the venous filling of the heart, i.e., the initial length of the 

 fibers. If the muscle be depressed on account of malnutrition, or fatigue, 

 or disease, its response for an equal degree of stretching will be lessened 

 so that to bring about an equal discharge during systole the heart will 



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