124 HUMAN PHYSIOLOGY. 



tent 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, regu- 

 lates the amount going to each capillary area, 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, capillaries 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, capil- 

 laries 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 corresponding 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 termin- 

 ations of the venae cavae. 



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 accumu- 

 lated 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 pres- 

 sure in the arteries. 



The arterial pressure is increased or decreased by influences which act 

 upon the heart or upon this peripheral resistance. 



If while the force of the heart remains the same, the rate increases, thus 

 increasing the volume of blood in the arteries, the pressure rises. If the 

 rate remains the same, but the volume of blood discharged increases, the 

 pressure will also rise. If the peripheral resistance is increased by contrac- 

 tion of the arterioles the pressure rapidly rises. On the contrary, a diminu- 

 tion in the rate and force of the heart or a diminution in peripheral resistance 

 by a dilatation of the arteries cause a fall in pressure. 



