VARIATIONS IN OUTPUT 



67 



in H ion concentration diminishes the amplitude of the 

 beat, the rate being unaffected. 



3. Adrenalin. — Adrenahn, the substance produced by the 

 suprarenal glands, causes an acceleration and augmentation 

 of the beat. 



4, Changes in the Arterial Pressure. — The effect of chang- 

 ing the arterial pressure is shown in Fig. 15 and in this 

 Table :— 



It will be seen that the output of the heart and the rate 

 of the beat are unaffected. A gin nee at Fig. 15 will show 

 that as the arterial pressure rises the volume of the heart 

 increases (downward movement of the cardiometer curve), 

 and that this increase occurs by a slight distension at each 

 beat until the new volume is acquired. Another change is 

 the great increase in the blood-flow through the coronary 

 circulation, associated with a rise in the amount of oxygen 

 used. How these occur is as follows. Suppose the mean 

 arterial pressure is 80 (systohc pressure 100, diastohc 60). 

 Suppose 8 c.c. is the amount of blood expelled at each beat, 

 the ventricle being completely emptied. Blood begins 

 to flow from the ventricle when the pressure in this chamber 

 just exceeds 60, and in order that the ventricle may be 

 completely discharged the pressure within it must finally 

 exceed 100. Suppose that the mean arterial pressure is now 

 artificially raised to 110 (systohc 130, diastolic 90). At the 

 next beat following the change no blood leaves the ventricle 

 until the intraventricular pressure exceeds 90 ; for the 

 ventricle to be completely discharged, a pressure exceeding 

 130 is necessary. But systole terminates as before, when 



