276 VETEKINARY PHYSIOLOGY 



heart's action is diminished, and no rise in the pressure 

 occurs. Under certain conditions, however, this compen- 

 satory action is not complete, and changes in the arterial 

 pressure are thus brought about. 



The volume of blood has a comparatively small influence 

 on the arterial pressure, because the veins are so large that 

 they accommodate very varying amounts of fluid. 



Factors controlling Arterial Pressure 



(a) Heart's Action. The influence of this may be readily 

 demonstrated by stimulating the vagus nerve while taking a 

 tracing of the arterial pressure. The heart is inhibited, less 

 blood is forced into the arteries, and the pressure falls (fig. 127). 



If, on the other hand, the accelerator nerve is stimulated, 

 the increased heart's action drives more blood into the 

 arteries, and the pressure rises. 



(&) Peripheral Resistance. The resistance to outflow from 

 the arteries depends upon the resistance offered in the small 

 arteries, the walls of which are chiefly composed of visceral 

 muscle fibres. When these fibres are contracted, the lumen 

 of the vessels is small and the resistance is great. When 

 they are relaxed, the lumen of the vessels dilates, and the 

 resistance to outflow is diminished. This muscular tissue of 

 the arterioles acts as a stop-cock to the flow of blood from 

 the arteries to the capillaries. It is of great importance 



1st. In maintaining the uniform pressure in the arteries. 



2nd. In regulating the flow of blood into the capillaries. 



During the functional activity of a part, a free supply of 

 blood in its capillaries is required. This is brought about 

 by a relaxation of the muscular coats of the arterioles leading 

 to the part. When the part returns to rest, the free flow of 

 blood is checked by the contraction of the muscular walls of 

 the arterioles. 



The action of the arterioles is well seen under the influence 

 of certain drugs (vaso-dilators and vaso-constrictors). If, 

 while a tracing of the arterial pressure is being taken, nitrite 

 of amyl is administered to the animal, it will be seen that the 

 skin and mucous membranes become red and engorged with 

 blood, while at the same time the arterial pressure falls. 



