82 THE CIRCULATION OF THE BLOOD 



pheral resistance, for the capillaries are beyond the site at 

 which this resistance principally occurs — the arterioles. 

 If two reservoirs at different levels are connected together 

 with a narrow pipe, the resistance which the water meets in 

 passing through the pipe is unaffected by the size of the 

 lower reservoir. The variations in the capacity of the 

 circulation other than the arterial part will affect the blood- 

 pressure only by altering the proportion of the blood which 

 is in the arteries at any given moment. We shall see that 

 under certain circumstances a low blood-pressure may even 

 be associated with constriction of the arterioles, when the 

 capillaries are greatly distended. Under these conditions 

 the blood is nearly all in the capillaries. 



Such being the effect upon blood-pressure of changes in 

 any one of the factors upon which it depends, the position 

 becomes more comphcated when more than one factor 

 varies at a time. If the output of the heart and the 

 peripheral resistance increase simultaneously, it is to be 

 expected that the resulting rise in pressure will be greater 

 than if either of these factors were to act alone. But if 

 an increase in the cardiac output takes place concurrently 

 with a decrease in the peripheral resistance, the two changes 

 may so antagonise one another as to leave the blood- 

 pressure unaltered. The net effect upon the circulation 

 is an increase in the velocity of the blood. 



The above effects can readily be imitated on an artificial 

 schema of the circulation. But in the hving body the 

 effects may be very different owing to the close inter- 

 relation between the several factors. This interrelation 

 is partly direct, partly indirect through the interven- 

 tion of the central nervous system. If, for instance, the 

 arterioles be constricted all over the body, the pressure in 

 the arteries is raised, that in the capillaries and veins 

 lowered. The raised arterial pressure causes, reflexly, 

 slowing of the heart {Mareys Law). But this is not the 

 only way in which the heart is affected. The lowering of 

 the venous pressure, as we have seen, causes by a direct 



