OUTPUT OF HEART AND VENOUS INFLOW 219 



to the maximal extent just before systole supervenes, so that more blood is 

 discharged than in the first case ; but, as in the first case, acceleration does 

 not improve the output ; when, 'on the other hand, the inflow is still more 

 rapid, the ventricle is practically filled to its limit some time before di- 

 astole ends and the output is also increased by acceleration. In other 

 words, when there is no so-called period of diastasis following active di- 

 astole acceleration of the beat will not increase the output in a unit of 

 time. 



The condition in which increased heart rate occurs with greatest cer- 

 tainty is muscular exercise. The initial quickening is due to impulses 

 traveling to the cardiac centers in the medulla from centers in the cere- 

 brum (see page 229), and it is clear that should no change occur in the 

 rate of venous filling of the heart, the acceleration would be of no value in 

 increasing the cardiac output. But the venous inflow increases because 

 of the muscular activity, the diastolic filling is more complete, and the 

 systolic discharge greater. As the muscular activity continues, the heart 

 rate continues to accelerate, and although increase in the temperature and 

 in the C H of the blood (see pages 162 and 168) may be partly responsible 

 for the change, another, if not more probable, cause is increase in venous 

 pressure (Hooker 56 and Bainbridge). This increased pressure, probably 

 by creating a slight tension on the walls of the ventricle (right) during 

 diastole, sets up afferent impulses which act on the vagus and sympathetic 

 cardiac centers. 



These most important principles governing the cardiac output have been 

 admirably summed up by Bainbridge as follows : ' ' The minute volume of 

 the heart is the product of its output per beat and of the pulse rate; its 

 output per beat is the resultant of the rate of venous inflow, of the con- 

 tractile power of the heart and of the duration of diastole. It is clear that 

 an intimate relationship must exist between the pulse rate, the venous 

 inflow and the contractile power of the heart, if the optimal efficiency of 

 the heart as regards its minute volume is to be maintained." 



Finally it should be pointed out that the maximal pulse rate that can 

 ultimately be attained by muscular exercise is very much the same in dif- 

 ferent individuals; it rarely exceeds 160 beats per minute. The maximal 

 minute volume, on the other hand, varies considerably for different indi- 

 viduals, because of variability in the capacity of the heart to increase its 

 output per beat. This adaptation depends, of course, on the nutritional 

 condition and the degree of development of the cardiac musculature. In 

 a trained athlete, for example, the heart does not become accelerated nearly 

 so rapidly as in an untrained person, because a slight increase in the 

 venous pressure calls forth a systolic discharge which is adequate to empty 

 the ventricle completely. 



