CII. XXI.] VENOUS PRESSUKE 275 



It is easy to understand how this is ; when the rate of the heart 

 increases, the total volume of blood discharged into the aorta per 

 second is increased ; similarly, an increase in the force of the beat 

 also results in an increase in the cardiac output, and in both cases 

 a more rapid and complete emptying of the auricle is produced. This 

 is felt throughout the whole of the pulmonary circulation, and the 

 accelerated flow therefore causes a fall in the venous pressure. If, 

 however, the rise of pressure is due to a contraction of the arterioles, 

 a stage may be reached in which the heart is no longer able to over- 

 come the high pressure produced. It then fails to empty itself, and 



FIG. 277. Effect of weak stimulation of the peripheral end of vagus on arterial blood-pressure (carotid 

 of rabbit). BP, Blood-pressure ; A, abscissa or base line ; T, time in seconds. Note fall of blood- 

 pressure and slow heart-beats. 



the blood is dammed up on the venous side, i.e. the venous pressure 

 rises. 



With regard to the arterioles, contraction means a rise in arterial 

 pressure, because while the amount sent into the arteries remains the 

 same the outflow is cut down. More blood is therefore retained in 

 them ; they become more distended and the pressure rises. The first 

 effect of this upon the venous pressure will be to diminish it, because 

 if more blood is retained in the arteries there is less for the veins 

 and capillaries. Also the rate of flow into the veins is at first 

 decreased, and the venous pressure therefore falls. Moreover, the 

 heart usually responds to a rise in pressure by increasing its force 

 and rate, and consequently more blood is taken from the veins near 



