358 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



confined to particular divisions of the vascular system. These local 

 hyperemias and anemias may be neutralized by vasomotor changes and 

 a transfer of the plasma, insuring a continuance of the normal circu- 

 latory conditions. ^\ 



The Elasticity of the Blood-vessels. — This factor betrays itself by a 

 distention of the walls of the blood-vessels whenever the pressure 

 within them rises. This elastic play is most clearly in evidence on the 

 arterial side and particularly in the central arteries, where we find the 

 largest number of elastic fibers. In the more distal channels, the 

 elastic tissue is gradually displaced by smooth muscle cells, which ap- 

 pear here chiefly in the form of a thick layer arranged circularly 

 around the lumen of the vessel. Some of these cells are also arranged 

 in a longitudinal direction and in such a way that they form a thin 

 coat externally to the circular. The peripheral arteries and arterioles, 

 therefore, contain practically no elastic fibers, but are made up of a 

 heavy layer of smooth muscle tissue. This difference in the structural 

 character of the arterial system leads us to infer that the elastic 

 forces have full sway centrally, while peripherally the prevailing factor 

 is muscular contraction. Hence, the aorta may be regarded as an 

 elastic pouch, the walls of which are forced outward with every ven- 

 tricular output. Directly thereafter a recoil sets in at a moment when 

 the elastic power of the arterial wall is capable of overcoming the 

 internal pressure. This means that they accomplish their work 

 during the diastolic interim, and constitute therefore a most important 

 aid to the heart, because the power generated by this organ during 

 each systole, is immediately stored in their walls as elastic tension to 

 be made use of during the period of cardiac rest. Inasmuch as the 

 blood is thus held under a constant pressure, the arteries serve the 

 same purpose as the air-bladder of a bag-pipe from which the air 

 may be drawn in a continuous stream. 



The energy of the heart, the quantity of the blood and the periph- 

 eral resistance are adjusted in such a way that the arterial system 

 is constantly retained in a state of hyperfilling. This implies that the 

 escape of the blood into the capillaries is regulated in such a way 

 that it is always exceeded by the ventricular output. In this way, a 

 definite head of pressure is established which cannot be nullified 

 during the diastolic period of the heart. It is true, however, that the 

 pressure is somewhat greater during the systolic inrush of blood, than 

 during the diastolic phase of gradual emptying. The offlow must 

 necessarily be limited, because the peripheral resistance and the 

 frequency of the heart are so accurately balanced that more than 

 a moderate recoil of the arterial walls cannot result. Only in case 

 the heart ceases to beat altogether do we obtain a complete collapse 

 of these channels, the blood then accumulating on the venous side 

 and principally in the central veins and right side of the heart. 

 This is the condition prevailing after death. 



The preceding statement leads us to infer that the diastolic fall 



