n BLOOD-PRESSURE 59 



the artery is always under considerable, though variable, 

 pressure. This pressure is called blood-pressure. We 

 have already explained how this pressure comes' to be 

 estabUshed ; but its impoi-tance is so great as a factor in 

 the circulation that we may with advantage refer to this 

 point once more. 



The smallest arteries and capillaries offer a considerable 

 frictional resistance to the flow of blood through them 

 into the veins, called as we have already said, " peripheral 

 resistance." Owing to this resistance, of the total amount 

 of blood forced into the arteries at each beat of the heart, 

 only a portion can during the actual beat, apart from the 

 pause between it and the next beat, pass on into the 

 veins. The remainder is lodged in the arteries whose 

 walls, being distensible, are pxi on the stretch by the 

 pressure of the blood thrust into thein at each stroke of the 

 heart, and this pressure of the blood on the arterial waU 

 is what we mean by " blood-pressure." As soon as the 

 arterial walls are stretched their elastic properties come into 

 play ; they recoil and press on the blood with a force 

 equal to that which puts them on the stretch. This 

 elastic recoil squeezes the blood on in the intervals 

 between the successive beats of the heart, and thus 

 renders the circulation continuous. In short the whole 

 arterial system is always in a state of distension ; the 

 work of the heart consists in keeping up this distended 

 condition by thrusting fx-esh blood into the arteries under 

 pressure ; and the pressure thus established forces the 

 blood through the capillaries, on through the veins, and 

 so back to the heart. 



Blood-pressure is greatest in tlie large arteries near the 

 heart and diminishes gradually along the arterial system 

 until we come to the smallest arteries and capillaries ; 

 here the pressure falls suddenly. The sudden fall of 

 pressure is due to the existence of what we have already 

 referred to as "peripheral resistance." This resistance 

 must be overcome in order to drive the blood on into the 



