OUTPUT OF HEART AND VENOUS INFLOW 



217 



come more vigorous, they do not do so with sufficient promptitude to main- 

 tain the systolic volume constant. In other words the output. of the heart 

 does not at first keep pace with the inflow, so that the mean volume of 

 the heart becomes progressively greater, and it is only ofter some mo- 

 ments that the contractile power increases sufficiently so that the output 

 equals the inflow and the mean volume recovers somewhat and then be- 

 comes steady. It is evident that precisely the same adjustments will oc- 

 cur when the arterial pressure is suddenly raised; the first beat following 

 the rise in pressure will be insufficient to lift all the extra load of blood 



VP 



Fig. 63. Kffcct of alteration in venous supply on volume of heart. Read from left to right. 

 (Patterson, Piper and Starling.) 



opposed to it in the aorta, so that some will remain in the ventricle when 

 diastole sets in; this will entail greater diastolic filling, the ventricle will 

 become dilated and iir obedience to the law of the heart the systolic dis- 

 charge will be increased. Dilation of the heart is therefore the first step 

 in the adjustment which it undergoes in response to an increase in venous 

 inflow or to higher arterial pressure, and the extent to which it may oc- 

 cur is considerable, being limited only by the pericardium. Indeed the 

 heart may entirely fill the pericardium, without there being any decided 

 increase in the output of blood with each beat, namely in cases where the 



