SECTION V 

 THE FLOW OF BLOOD THROUGH THE ARTERIES 



THE PULSE. Owing to the elasticity and distensibility of the arterial 

 wall, the rhythmic rise of pressure corresponding to each heart- 

 beat causes an expansion, which can be felt by the finger placed on 

 any exposed artery, such as the radial, and is spoken of as the pulse. 

 Just as the blood pressure diminishes from heart to periphery, so the 

 amplitude of the pulse decreases as we go farther away from the heart. 



If the arterial system were perfectly rigid the increased pressure due 

 to the forcing of the blood into the arterial system at each ventricular 

 systole would occur practically simultaneously at every point. The 

 arteries are, however, elastic and distensible, so that the first effect of 

 the flow of blood into the aorta is to distend the section of the aorta 

 nearest to the heart. The elastic reaction of this forces a portion of 

 the blood into the nearest section, so that the increased pressure is 

 transmitted from segment to segment of the arteries in the form of a 

 wave at the velocity of about seven metres per second. 



It is important not to confuse the velocity of the pulse- wave with 

 that of the blood-flow ; the latter is never greater than 0-5 metre per 

 second, and is very much less than this in the smaller arteries. Perhaps 

 the difference between the two quantities may be made clearer by 

 illustration : If the hindmost of a row of billiard-balls be struck 

 sharply with a cue the foremost ball flies off and the others stop still ; 

 in this case the energy imparted to the first ball by the stroke has been 

 transmitted from ball to ball, just as the effect of the ventricular con- 

 traction is .transmitted from section to section of the arterial blood- 

 stream. If the balls are struck so that the cue continues pressing on 

 the hindmost after the stroke is delivered, the front ball flies off, while the 

 others move slowly along in the direction of the stroke. In the arteries 

 this continuous pressure is furnished by the elastic reaction of the 

 arterial wall, and we see how the impact of the blood may travel 

 quickly as a wave of increased pressure while the blood itself is moving 

 slowly along, impelled by the reaction of the arterial wall. 



If we imagine a rigid tube AB (Fig. 408) provided with a piston at 

 the end A, and filled with an incompressible fluid, an inward movement 

 of the piston at A will cause a simultaneous outflow of fluid at the end B. 

 If the end B is closed the piston at A cannot be moved at all. Pressure 



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