THE CIRCULATION OF THE BLOOD. 309 



changes, approximating that in the arteries. As the veins pass from 

 the periphery toward the heart and their individual sectional areas 

 increase there is again a diminution of the friction. 



As a consequence of the friction throughout the entire vascular 

 apparatus the blood experiences a resistance to its onward move- 

 ment which, working backward, causes the blood to exert a lateral 

 or radial pressure against the walls of the vessels. The high pres- 

 sure in the aorta is the result of the total resistance of the vascular 

 system, and the pressure at any point of the system represents the 

 resistance yet to be overcome.^ The larger part of the pressure in 

 the arterial system, however, is due to the resistance offered by the 



FIG. 144. DIAGRAM SHOWING THE RELATIVE HEIGHT OP THE BLOOD-PRESSURE IN 

 THE DIFFERENT REGIONS OF THE VESSELS. H. Heart. A. Arteries. C. Capil- 

 laries. V. Large veins. 0, 0, being the zero line (= atmospheric pressure), the 

 pressure is indicated by the height of the curve. The numbers on the left give 

 the pressure (approximately) in millimeters of mercury, h. Pressure in heart. 

 a. Arteriole region showing sudden fall of pressure, c. The fall of pressure in 

 the capillaries, v. The negative pressure in the large veins. (Yeo.} 



smallest arteries and capillaries, and when peripheral resistance is 

 alluded to as a factor in the production or in the variation of blood- W 

 pressure, the resistance of these vessels is mainly understood. 



The primary factor in the production of the pressure is the pump- 

 ing action of the heart. Should there be any cessation in its activity, 

 the elastic walls of the arteries would recoil and force the blood into the 

 veins. There would be coincidently a fall to a pressure equal to 

 that of the atmosphere. Even under normal circumstances this 

 condition is approximated during the diastole. The recoil of the 

 arterial wall by which the forward movement of the blood is main- 



