CIRCULATION. 



the arterial wall expands before the entering blood, the pressure rises, for 

 more blood is entering the arterial system than is leaving it; when, at each 

 cardiac diastole, the arterial wall recoils, the pressure falls, for blood is leaving 

 the arterial system, and none is entering it. But before the fall has had time 

 to become pronounced, while the arterial pressure is still high, the cardiac sys- 

 tole recurs, and the pressure rises again, as at the preceding fluctuation. 



The Arterial Pulse. The increased arterial pressure and amplitude at 

 the cardiac systole, followed by diminished pressure and amplitude at the 

 cardiac diastole, constitute the main phenomena of the arterial pulse. They 

 are marked in the manometric trace by those lesser rhythmic fluctuations of 

 the mercury which correspond with the heart-beats. The causes of the arte- 

 rial pulse have just been indicated in dealing with the causes of the arterial 

 pressure. The pulse, in some of its details, will be studied further for itself 

 in a later chapter. For the sake of simplicity, the respiratory fluctuations of 

 the arterial pressure have not been dealt with in the discussion just con- 

 cluded. The causes of these important fluctuations are very complex and are 

 treated of under the head of Respiration. 



The arterial pressure, then, results from the volume and frequency of the 

 injections of blood made by the heart's contraction ; from the friction in the 

 vessels ; and from the elasticity of the arterial wall. 



The CapiUary Pressure and its Causes. When we studied the move- 

 ment of the blood in the capillaries, we found the pressure in them to be low 

 and free from rhythmic fluctuations. In both of these qualities the capillary 

 pressure is in sharp contrast with the arterial. What is the reason of the differ- 

 ence ? The work of driving the blood through as well as into the capillaries is 

 done during the contraction of the heart's wall by its kinetic energy. During 

 the repose of the heart's wall and the arterial recoil this work is continued by 

 kinetic energy derived, as we have seen, from the preceding cardiac contraction. 

 The work of producing the capillary flow is done in overcoming the resistance 

 of friction. The capillary walls are elastic. The same three factors, then 

 the power of the heart, the resistance of friction, the elasticity of the wall 

 which produce the arterial pressure produce the capillary pressure also. Why 

 is the capillary pressure normally low and pulseless? The answer is not 

 difficult. The friction which must be overcome in order to propel the blood 

 out of the capillaries into the wider venous branches is only a part of the total 

 friction which opposes the admission of the blood to the minuter vessels. The 

 resistance is therefore diminished which the blood has yet to encounter after 

 it has actually entered the capillaries. The force which propels the blood 

 through the capillaries, although amply sufficient, is greatly less than the 

 force which propels it into and through the larger arteries. In both 

 cases alike the force is that of the heart's beat. But, in overcoming the 

 friction which resists the entrance of the blood into the capillaries, a large 

 amount of the kinetic energy derived from the heart has become converted 

 into heat. The power is therefore diminished. As, in producing the high 

 arterial pressure, much power is met by much resistance, and the elastic wall 



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