242 THE HUMAN BODY. 



indicate a much higher pressure than that on A. As B is 

 more stretched, however, it squeezes harder upon its con- 

 tents, until at last a time comes when this squeeze is power- 

 ful enough to force through the small tubes just 180 cubic 

 centimeters (6 oz.) in a second. Then further accumulation 

 in B ceases. The pump sends into it 10,800 cubic centi- 

 meters (360 ounces) in a minute at one end and it squeezes 

 out exactly that amount in the same time from its other end ; 

 and so long as the pump works steadily the pressure in B 

 will not rise, nor that in A fall, any more. But under such 

 circumstances the flow through the small tubes will be nearly 

 constant since it depends upon the difference in pressure pre- 

 vailing between B and 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 cub. 

 cent. (6 oz.) forced into it, but this increase will be but a 

 small fraction of the total pressure and so have but an in- 

 significant influence upon the rate of flow through the small 

 connecting tubes. 



Arterial Pressure. The condition of things just de- 

 scribed represents 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 smallest arteries and the capillaries 

 the resistance at D', the large arteries the elastic tube B, and 

 the veins the tube A. The ventricles 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 persisting 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 experi- 



