352 THE MECHANICS OF THE CIKCTJLATION, HEMODYNAMICS 



Analogous Features of the Circulation of the Blood. Essentially 

 the same conditions prevail in the vascular system. The heart which 

 here assumes the function of the rhythmically discharging reservoir 

 or piston pump, contracts and forces a certain quantity of blood into 

 the vascular channels. The frequency of this organ, as well as the 

 peripheral resistance, is adjusted in such a way that the blood-vessels 

 are constantly retained in a condition of hyperfilling, made possible 

 by the elastic tonicity of their walls. In this way, the intermittent 

 ventricular discharge is converted into a continuous flow. The power 

 of the heart is transferred each time into elastic tension. The latter 

 acts while the heart is at rest. 



Veins 



FIG. 183. PRESSURE VESSEL WITH PROGRESSIVELY BRANCHING TUBES WHICH ARE AGAIN 



UNITED INTO ONE COLLECTING CHANNEL. 



This arrangement illustrates the conditions prevailing in the vascular system. 

 (Brubaker.) 



If it were not for the fact that the diameters of the different blood- 

 vessels vary considerably, the pressure prevailing in the vascular 

 system would be practically identical with that existing in a system of 

 tubes such as has been represented in the preceding schema. In 

 reality, however, the central arterial trunk or aorta, divides again and 

 again into much smaller branches which eventually give rise to the 

 capillaries. Beyond this point, these fine tubules constantly unite 

 into larger ones until the venae cavse and right side of the heart 

 have been reached. This multiple division brings it about that the 

 total cross-area of the vascular system is steadily increased from the 

 arteries to the capillaries, while beyond these tubules, it is again gradu- 

 ally diminished. For this reason, these conditions of pressure and 

 flow must closely resemble those described in one of the earlier para- 

 graphs dealing with the dynamics in tubes of varying diameter. To 



