BLOOD-PRESSURE AND BLOOD-VELOCITY. 



515 



factors namely, a great resistance placed in the middle of the 

 course may be illustrated by the model shown in Fig. 209, which 

 differs from that in Fig. 208 in having a stopcock in the outflow 

 tube, which, when partly turned off, makes a narrow opening and a 

 relatively great resistance. When the stopcock is open the pressure 

 falls equally throughout the tube, provided the bore of the stopcock 

 is equal to that of the tube. If, however, it is partially turned the 

 side pressure is much increased between it and the reservoir on what 

 we may term the arterial side of the schema, and it is correspond- 

 ingly diminished between the stopcock and the exit, on the venous 

 side of the schema. Substantially this condition prevails in the body. 

 The capillary region, including the smallest arterioles and veins, 

 offers a great resistance to the flow of blood, and this resistance is 

 spoken of in physiology as the peripheral resistance. Its effect is to 



Fig. 209. Schema like the preceding except that a stopcock is inserted at the middle 

 of the outflow to imitate the peripheral resistance of the capillary area. The relations of 

 the internal pressure on the arterial and venous sides of this special resistance is shown by 

 the height of the water in the gauges. 



raise the pressure on the arterial side and lower it on the venous side. 

 When other conditions in the circulation remain constant it is found 

 that an increase in peripheral resistance, caused usually by a con- 

 striction of the arterioles, is followed by a rise of arterial pressures 

 and a fall of venous pressures. On the contrary, a dilatation of the 

 arterioles in any organ is followed by a fall of pressure in its artery or 

 arteries and a rise of pressure in its veins. The effect of the elastic- 

 ity of the arteries is of importance in connection with the fact that 

 in reality the circulation is charged with blood not from a constant 

 reservoir as in the models, Figs. 208 and 209, but by the rhythmical 

 beats of the heart. If the vascular system were perfectly rigid each 

 rhythmical charge into the aorta would be followed by an equal dis- 

 charge from the venae cavse, the pressure throughout the system 

 would rise to a high point during systole and fall to a low point dur- 



