x M Kl< I VKK OF BLOOD-VESSELS 97 



distended ; their elastic walls are stretched, so that they are 

 holding more blood than is required just to fill them. The 

 pirssure of the extra quantity of blood on the walls is called 

 the blood pressure. The distension of the elastic walls 

 exerts an equal pressure on the blood in the arteries, and this 

 presses some of the extra blood out of them into the capillaries. 

 The arteries arc constantly trying to get rid of the extra amount 

 of blood by the recoil of their elastic walls on the blood, the 

 elastic walls tending to return to their undistended state. During 

 life just as much blood as is being pressed on through the capil- 

 laries is being thrown into the aorta by the beat of the heart. So 

 that, during life a distension is always kept up, and the pressure 

 which the walls of the arteries exert on the blood in them always 

 exists. The blood constantly flows out of the arteries into the 

 capillaries, because it is constantly being pressed out of them 

 by the blood pressure. With each beat of the heart a fresh 

 quantity of blood is thrown into the aorta, and the distension 

 is increased a little, that is, the blood pressure is raised a 

 little, and so the blood flows, for the moment, a little faster on 

 through the capillaries, flowing on more slowly again when the 

 beat is over. Hence, although the blood is thrown into the 

 aorta at intervals J?y the beat of the heart, it flows out of the 

 arteries continuously, being quickened only in its flow by each 

 beat. The blood pressure is greatest at the beginning of the 

 aorta, where the distension is being kept up by the beats of 

 the heart, and gradually diminishes along the whole vascular 

 system round to the heart again. It presses the blood on 

 along the arteries, through the capillaries, and along the veins. 

 In the capillaries there is, owing to the great friction of the 

 minute passages, a strong resistance to the flow of blood, so 

 that a great pressure in the arteries is needed to send the 

 blood on through the capillaries, and a great deal of this is 

 used up in doing so, so that the pressure in the veins on the 

 far side of the capillaries is much less than the pressure in the 

 arteries on the near side of the capillaries, though enough is 

 left to force the blood on through the veins. In the veins, 

 the channels being wider, the resistance is less, and gets less 

 and less as the veins get wider and wider towards the heart. 

 Hence the pressure is greatest in the arteries, less in the 

 capillaries, and least of all in the veins! IFTs the muscular 



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