THE FLOW OF BLOOD IN THE ARTERIES 205 



time. If it does, the blood pressure increases (supposing that the caliber of 

 the vessels has not changed) ; if not, the pressure remains the same. For 

 a quicker rate does not necessarily imply greater energy of the heart beat, 

 and so does not of necessity produce a greater output. Now it is evident that 

 unless the total output in a unit time is increased, the quantity of blood 

 coming back to the heart between two systoles is less with a rapid pulse than 

 with a slow one, or, in other words, the pulse volume is less. Hence, the 

 blood pressure following acceleration will depend upon the reciprocal relation 

 between the increase of pulse rate and the decrease of pulse volume. 



Direct investigations of this subject have resulted in showing that no 

 general law can be formulated. If a large quantity of blood is found in the 



FIG. 77. Blood pressure curve, showing a slight fall under feeble stimulation of the vagus. 

 To be read from right to left. The time of stimulation is indicated by the two vertical 

 lines, j 1 = ten seconds. 



great veins, and is only waiting an opportunity to get into the heart, and if 

 the resistance in the arterial system is sufficiently high, acceleration may 

 produce a considerable increase in blood pressure. If these conditions are 

 not fulfilled, the increased frequency will occasion no rise in pressure worth 

 mentioning. 



2. Resistance in the Arteries. It is evident that with a given heart energy, 

 if the resistance in the vessels decreases, the pressure also must decrease. If 

 the resistance increases the pressure also must increase. 



A fall in pressure in consequence of diminished resistance occurs, if the 

 vessels in a large vascular region lose their tonus. Not only the arteries, but 

 the veins as well are to be considered as taking a part in this; for the lat- 

 ter possess a certain tonus with the disappearance of which they are consid- 

 erably dilated and so can contain more blood than usual. Hence the blood 



