THE CIRCULATION OF THE BLOOD. 339 



point of the apparatus represents the resistance yet to be overcome. 8. The 

 high pressure in the arterial system and its marked fall at its periphery is 

 more especially the result of the very great resistance at this point, known 

 as the peripheral resistance, the result of a rapid diminution in the diameter 

 of the arterioles and the capillary vessels, modified by the tonic contraction 

 of the arteriole muscles. 9. The pressure in the arterial system undergoes 

 considerable variation both above and below the mean pressure during the 

 e and diastole of the heart. 



The Heart. The primary factor in the production of the pressure is the 

 pumping action of the heart. Should there be any cessation in its activity, 

 the elastic walls of the arteries would recoil and force the blood into the 

 veins. There would be coincidently a fall of the pressure to that of the 

 atmosphere. Even under normal circumstances this condition is approxi- 

 mated during the diastole. The recoil of the arterial wall by which the for- 

 ward movement of the blood is maintained is attended by a fall in pressure. 

 But before this reaches any considerable extent, the heart again contracts 

 and forces its contained volume of blood into the arteries. 



That this may be accomplished it is essential that the cardiac energy 

 be sufficient not only to drive a portion of the blood through the capillaries 

 into the veins, but to oppose the recoiling arteries, and to distend them to 

 their previous extent, so that the incoming volume of blood may be ac- 

 commodated. This at once reestablishes the pressure at its former level. 



During the contraction of the heart the kinetic energy is transformed 

 into potential energy, represented by the tense distended walls of the arteries. 

 With the relaxation of the heart and the closure of the semilunar valves the 

 potential energy of the arteries is again transformed into kinetic energy, 

 represented by the moving blood. The artery thus continues the work 

 of the heart during its period of inactivity. The rapidity with which the 

 cardiac contractions succeed each other prevents the pressure from sinking 

 below a certain average level. 



The Resistance. The secondary factor is the resistance to the flow 



of blood through the vessels, the nature of which has been previously stated. 



So long as the resistance, and especially that variable element of it at the 



periphery of the arterial system, maintains a certain average value, so long will 



the pressure in each division of the vascular apparatus maintain an average or 



a physiologic value. Should the resistance at the periphery of the arterial 



.rv in either direction, the result of an increase or a decrease in the 



^e of the contraction of the arteriole muscle, there will arise a change 



in the relative degree of pressure in each of the three divisions of the vascular 



The Elasticity of the Vessel Walls. A tertiary factor is the elasticity 

 of the arterial wall. While it can hardly be said that the elasticity is a cause 

 of the pressure, there can be attributed to it the capability of modifying 

 and assisting in the maintenance of the pressure at a more or less constant 

 were it not for this property of the vessel wall the variations in 

 pressure during and after the systole would be far more ex: :han they 



are, and would approximate the variations observed in tubes with rigid 

 walls. The elasticity, moreover, assists in the equalization of the blood- 

 stream, convening the intermittent and remittent flow charac of the 



