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 venae 

 cavse 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 really happens, as it takes less pressure to distend 

 the aorta than to move forward the entire mass of blood under the 

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

 lunar 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 out of 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. At the end of the outflow from the heart 

 all the arteries are beginning to enlarge, the maximum extension 

 being in the aorta, and room is thus made for the new quantity of 

 blood. The new blood that is actually discharged from the heart 



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