462 



VETERINARY PHYSIOLOGY 



heart's action, inasmuch as the arterial pressure, by which 

 blood is driven into the capillaries, depends upon this. In 

 cardiac inhibition not only is arterial pressure lowered, but 

 capillary pressure may also fall. In augmented heart action 

 both arterial and capillary pressure are raised (iig. 194, B.). 



2nd. Variations in Resistance to Outflow. — Normally the 

 flow from capillaries to veins is free and unobstructed ; but, 



if the veins get blocked, 

 or if the flow in them is 

 retarded by gravity, the 

 capillaries get engorged 

 with blood. This in- 

 creased pressure in the 

 capillaries is very diiier- 

 ent from that caused by 

 increased inflow. The 

 flow through the vessels 

 is slowed or may be 

 stopped instead of being 

 accelerated, and the blootl 

 gets deprived of its oxygen 

 and of its nourishing 

 constituents, loaded with 

 waste products, and tends 

 to exude into the lymph spaces, causing dropsy (fig. 

 194, C). A ver}^ similar condition results if the arterioles 

 are contracted and the capillaries dilated ; the same stagna- 

 tion of blood may occur. 



It is therefore most important to distinguish between 

 high capillary pressure from dilated arterioles or an active 

 heart, and high pressure due to venous obstruction. 



A condition very similar to that described, but producing 

 a capillary pressure high relatively to the pressure in the 

 arteries — though not absolutely high — is seen in cases of 

 failure of the heart, when that organ is not acting sufficiently 

 strongly to pass the blood on from the venous into the 

 arterial system. Here the arterial pressure becomes lower and 

 lower, the venous pressure higher and higher, and, along with 

 this, the capillary pressure becomes high in relationship to 



Fig. 194. —The Changes in Blood 

 Pressure in the Capillaries produced 

 bj' increasing the arterial pressure 



, and by obstructing the venous 



flow . A., arteries ; G. , 



capillaries ; V. , veins. 



J 



