288 MANUAL OF PHYSIOLOGY. 



left out of the question. This occurs some seventy times a min- 

 ute, so that an enormous quantity of blood is removed from the 

 veins and forced into the arteries. The ventricles in filling the 

 arteries have to work against considerable pressure, and may be 

 said to pump the blood from the low-pressure veins up into the 

 high-pressure arteries, and this work is the cause of the pressure 

 difference between the two sets of vessels. During the contrac- 

 tion of the heart the ventricular pressure must exceed that of the 

 aorta, while during the diastole it falls to that of the auricle or 

 even of the great veins. The heart then is the first and most 

 important agent by which the arteries are kept stretched and 

 over-filled, and the veins are emptied. 



A second important factor in enabling the high blood-pressure 

 to be kept up, is the resiliency of the middle coat of the arteries. 

 It is only on account of the great elasticity of the arterial walls, 

 that these vessels are capable of being so over-filled, and only on 

 account of the perfect resiliency of the elastic coat, that they are 

 able to exert such powerful pressure on the blood for such an un- 

 limited time. If the arteries were rigid tubes, overfilling them 

 with a fluid itself inelastic would be out of the question ; the out- 

 flow from the distal extremity would take place exactly when the 

 additional charge of blood was injected by the heart. 



With each contraction the ventricle overcomes arterial pressure, 

 and further stretches the elastic artery. But the act of injecting 

 the blood into the aorta only occupies about one-quarter of each 

 heart beat. The semi-lunar valves bear the pressure of the blood 

 in the aorta for the rest of the time. The whole force of the ven- 

 tricle is therefore used up in causing arterial distension. During 

 the greater part of the time (about three-quarters of the heart's 

 cycle) the arteries are in the condition of over-filled elastic tubes, 

 with their cardiac end firmly closed by the aortic valves, and 

 their distal ends open. 



It follows that the blood flowing constantly out of the distended 

 arteries through the capillaries into the veins tends to equalize 

 the pressure in the veins and arteries. But why is not this con- 

 stant outflow sufficient to allow the pressure in the arteries to fall 

 to the level of that in the veins? Or in other words, what is the 



