138 HUMAN PHYSIOLOGY. 



The arteries possess both elasticity and contractility. 



The property of elasticity allows the arteries already full to 

 accommodate themselves to the incoming amount of blood, and to 

 convert the intermittent acceleration of blood in the large vessels 

 into a steady and continuous stream in the capillaries. 



The contractility of the smaller vessels equalizes the current of 

 blood, regulates the amount going to each part, and promotes the 

 onward flow of blood. 



Blood Pressure. The immediate cause of the movement of the 

 blood from the beginning of the aorta, through the arteries, capil- 

 laries and veins, to the right side of the heart, is a difference of 

 pressure between these two points. A corresponding difference of 

 pressure exists between the beginning of the pulmonary artery and 

 the left side of the heart. To this pressure the term blood pressure 

 is given and may be denned as the pressure exerted laterally by 

 the moving blood stream against the walls of the arteries, capillaries 

 and veins. That there is such a pressure different in amount in 

 each of these three divisions of the vascular apparatus is evident from 

 the results which follow division of an artery or a vein of cor- 

 responding size. When an artery is divided the blood spurts from 

 the opening for a considerable distance and with considerable 

 velocity. When a vein is divided the blood as a rule merely wells 

 out of the opening and- with but slight momentum. These results 

 indicate that the blood exerts a greater pressure in the arteries 

 than in the veins. Experimentally it has been shown that the 

 pressure is greatest in the aorta, less in the capillaries, and least in 

 the veins. The pressure in the aorta expressed in millimeters of 

 mercury is about 160, in the capillaries 35 to 20, and in the veins 

 from 20 to o or less at the terminations of the venae cavse. 



The causes of the blood pressure are first, the driving power of 

 the heart, and second, the resistance offered by the walls of the 

 blood-vessels to the flow of blood through them. Owing to this 

 resistance, the blood has accumulated and in consequence the whole 

 system has become distended by the lateral pressure. The largest 

 part of the resistance, however, is found at the periphery of the 

 arterial system and is partly the cause of the high pressure in the 

 arteries. 



The arterial pressure is increased or decreased by influences which 

 act upon the heart or upon this peripheral resistance. 



