THE CIRCULATION OF THE BLOOD 357 



sure thus established may be raised by all agents which increase the 

 peripheral resistance by (i) directly stimulating the arteriole muscle or, 

 indirectly, by stimulation of the vaso-motor center, and (2) by stimulating 

 the cardio-inhibitor center and thus restoring its inhibitor control over the 

 heart. With the diminution in the rate the length of the diastole is increased , 

 and hence a larger volume of blood accumulates in the ventricle and is 

 subsequently discharged into the arteries. If at the same time the heart 

 muscle can be simultaneously stimulated, as it may be by certain drugs, 

 there will be an increased force imparted to the systole. By the coopera- 

 tion of these various factors the blood-pressure is raised to the normal 

 level. Here again it may be observed that a fall of pressure leads to 

 an increase in the rate of the heart and a rise in pressure to a retardation 

 in the rate, though the mechanism in this instance is somewhat different 

 from that stated in the foregoing paragraph. 



Local Variations in the Arterial Blood-supply. The variations in 

 pressure and velocity from variations either in the activity of the heart or in 

 the peripheral resistance recorded in preceding paragraphs, have reference to 

 the arterial system in its entirety; but it is evident from many facts that 

 similar variations take place in special regions or organs of the body. Thus, 

 it is a well-known fact that for the exhibition of the functional activity 'of 

 every organ there must be an increase in the volume of blood supplied to it 

 in each unit of time. This is accomplished by an active dilatation of the 

 arterioles of the artery of supply, and unless the area or organ supplied is 

 large, as the splanchnic area for example, there will be no necessary diminu- 

 tion in either the general blood- pressure or the average velocity. With the 

 cessation of functional activity, there is no longer any need for so large a 

 blood-supply and hence the arterioles contract, diminish the outflow, and raise 

 the pressure. If, on the other hand, the area to be supplied be large, as the 

 splanchnic area, the dilatation of the intestinal arteries will be attended by 

 such a large inflow of blood that not only will there be a fall of pressure in 

 these vessels, but a fall of pressure in other arteries as well, combined with a 

 diminution in velocity through them. With the contraction of the intestinal 

 arteries the reverse conditions at once arise. By constant variations in 

 the peripheral resistance of individual arteries in each and every region of the 

 body, and in association with variations in the rate or force of the heart, the 

 blood is shunted now into this, now into that organ in accordance with its 

 functional needs. All variations in peripheral resistance are largely brought 

 about reflexly by the vaso-motor nerves, the origin, distribution, and mode of 

 action of which will be considered in subsequent paragraphs. 



B. In Capillary Pressure. The pressure in the capillaries, though 

 for the most part possessing a permanent value, is subject to variations in 

 accordance with variations in the pressure in either the arterial or venous 

 systems or both. The marked difference in the pressure in the large arteries 

 and the capillaries is partly due to the resistance offered by the narrow arteri- 

 oles. If the latter dilate in any given area, the capillary pressure increases 

 because of the propagation into them of the arterial pressure. The reverse 

 condition would decrease the pressure. On the other hand, any interference 

 with the outflow from any given area, due to venous compression, would 

 likewise increase the pressure; any factor which would, on the contrary, 

 favor the outflow would decrease the pressure. Independent of any change 



