362 THE HUMAN BODY 



A, and only indirectly upon the pump which serves simply to 

 keep the pressure high in B and low in A. At each stroke of the 

 pump it is true there will be a slight increase of pressure in B due 

 to the fresh 180 cubic centimeters (6 ounces) forced into it, but 

 this increase will be but a small fraction of the total pressure and 

 so have but an insignificant influence upon the rate of flow through 

 the small connecting tubes. 



Arterial Pressure. The condition of things just described repre- 

 sents very closely the phenomena presented in the blood-vascular 

 system, in which the ventricles of the heart, with their auriculo- 

 ventricular and semilunar valves, represent the pump, the small- 

 est arteries and the capillaries the resistance at D', the large 

 arteries the elastic tube B, and the veins the tube A. The ventri- 

 cles constantly receiving blood through the auricles from the 

 veins, send it into the arteries, which find a difficulty in emptying 

 themselves through the capillaries, and so blood accumulates in 

 them until the elastic reaction of the stretched arteries is able to 

 squeeze in a minute through the capillaries just so much blood as 

 the left ventricle pumps into the aorta, and the right into the 

 pulmonary artery, in the same time. Accordingly in a living 

 animal a pressure-gauge connected with an artery shows a much 

 higher pressure than one connected with a vein, and this persist- 

 ing difference of pressure, only increased by a small fraction of 

 the whole at each heart-beat, brings about a steady flow from the 

 arteries to the veins. The heart keeps the arteries stretched and 

 the stretched arteries maintain the flow through the capillaries, 

 and the constancy of the current in them depends on two factors : 

 (1) the resistance experienced by the blood in its flow from the 

 ventricles to the veins,. and (2) the elasticity of the larger arteries 

 which allows the blood to accumulate in them under a high pres- 

 sure, in consequence of this resistance. 



Since the blood flows from the aorta to its branches and from 

 these to the capillaries and thence to the veins, and liquids in a 

 set of continuous tubes flow from points of greater to those of 

 less pressure, it is clear that the blood-pressure must constantly 

 diminish from the aorta to the right auricle; and similarly from 

 the pulmonary artery to the left auricle. At any point, in fact, 

 the pressure is proportionate to the resistance in front, and since 

 the farther the blood has gone the less of this, due to impediments 



