354 TEXT-BOOK OF PHYSIOLOGY 



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 

 in the arteriole resistance, it is evident that a rise in arterial pressure alone would 

 increase the capillary pressure. If both arterial and venous pressures rise, 

 the capillary pressure increases; if both fall, it decreases. 



C. In Venous Pressure. Independent of any change in the venous 

 pressure in a given area from local or temporarily acting causes e.g. , aspira- 

 tion of the thorax or heart, muscle contractions, change of position, etc. the 

 general venous pressure will be increased by a decrease in the value of those 

 factors which produce the difference of pressure between the arteries and 

 veins. An increase in the value of these factors would necessarily decrease 

 the pressure. 



Variations in the Relation of the Arterial and Venous Pressures. 

 So long as the heart maintains a given rate and force and the resistance at 

 the periphery of the arterial system (due to the contraction of the arteriole 

 muscle) a given value, will the usual physiologic difference between the pres- 

 sure in the arteries and veins remain unchanged. If, however, either 

 factor changes in one direction or another, there will arise a change in the 

 relative degree of pressure in the different divisions of the vascular apparatus. 

 Thus if the heart force increases and a larger volume of blood is discharged 

 into the arteries in a unit of time, the amount of blood in the venous system 

 diminishes, and the result is a rise of the arterial and a fall of the venous 

 pressures. If, on the contrary, the heart force decreases or the mitral valve 

 permits of a regurgitation, a smaller volume of blood is ejected into the 

 arteries in a unit of time, the amount of blood in the venous system increases, 

 and the result is a fall of the arterial and a rise of the venous pressure. 



Again if the arteriole muscle relaxes and a larger volume of blood flows 

 from the arteries into the veins in a unit of time, the result will be a fall of 

 arterial and a rise of venous pressure. If, on the contrary, the arterial muscle 



