SECTION V 

 THE FLOW OF BLOOD THROUGH THE ARTERIES 



THE PULSE. Owing to the elasticity and extensibility 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 wa^ve 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 differ- 

 ence 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 contraction 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. 418) provided with a piston at the 

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

 1 he 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 applied 

 to the piston will raise the pressure simultaneously at all points in the 

 tube AB. The increased pressure applied at A is therefore transmitted 

 with practically no loss of time to all parts of the tube AB This immediate 

 spread of the wave of pressure applies only to an incompressible fluid 

 within a rigid tube. If the fluid were compressible, if it consisted, e. g. of 



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