CHAPTER XXVII. 

 THE PULSE. 



General Statement. When the ventricular systole discharges 

 a new quantity of blood into the arteries the pressure within these 

 vessels is increased temporarily. If the arteries, capillaries, and 

 veins were perfectly rigid tubes it is evident that this pressure would 

 be transmitted practically instantaneously throughout the system, 

 and that a quantity of blood would be displaced from the vense 

 cavae into the auricles equal to the quantity forced into the aorta by 

 the ventricle. The flow of blood throughout the vascular system 

 would take place in a series of spurts or pulses, the pressure rising 

 suddenly during systole and falling rapidly during diastole. Since 

 the blood is incompressible and the walls of the vessels if rigid 

 would be inextensible, the rise of pressure, the pulse, would be 

 simultaneous in all parts of the system. The fact, however, that 

 the walls of the vessels are extensible and elastic modifies the trans- 

 mission of the pulse wave in several important particulars: It 

 explains why it is that the pulse dies out in or at the beginning of 

 the capillaries and why it occurs at different times in different 

 arteries that is, why the wave of pressure takes a perceptible 

 time to travel over the arteries. The result that follows from the 

 elasticity of the arteries may be pictured as follows: Under the 

 normal conditions of the circulation when the heart contracts and 

 forces a new quantity of blood into the aorta room must be made 

 for this blood either by moving the whole mass of the blood forward 

 that is, by discharging an equal amount at the other end into the 

 auricle or by the enlargement of the arteries. This latter alter- 

 native is what substantially happens, as it takes less pressure to dis- 

 tend the arteries than to move forward the entire mass of blood 

 under the conditions that exist in the body. So soon, therefore, as 

 the semilunar valves open and the new column of blood begins to 

 enter the aorta, the walls of that vessel begin to expand, and during 

 the time that the blood is flowing oufrof the heart that is, in round 

 numbers, about 0.3 sec. the extension of the walls passes from 

 point to point along the arterial system, since the increased pressure 

 is transmitted rapidly over the arterial system, and at every point 

 in its progress it causes an expansion of the arterial wall as well as an 

 acceleration in the onward movement of the blood. At the end of 

 the outflow from the heart all the arteries are beginning to enlarge, 



523 



