THE CIRCULATION OF THE BLOOD 349 



resistance, the result of a rapid diminution in the diameter of the arterioles 

 and the capillary vessels, together with the tonic contraction of the arteriole 

 muscle. 9. The pressure in the arterial system undergoes considerable 

 variation both above and below the mean pressure during the systole 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 

 system vary in either direction, the result of an increase or a decrease in the 

 degree 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 

 apparatus. 



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 modii 

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

 level; for were it not for this property of the vessel wall the variations i 

 pressure during and after the systole would be far more extensive than they 

 are, and would approximate the variations observed in tubes with i nj 

 walls. The elasticity, moreover, assists in the equalization c 

 stream, converting the intermittent and remittent flow characteristic of the 

 large arteries into the continuous equable stream characteristic : of the capil 

 laries. It also permits of wide variations in the amount of blood the art 

 can contain between their minimum and maximum distention. 



