394 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



CHAPTER XXXIII 

 THE BLOOD FLOW 



The Volume of the Blood Stream. — If the arterial system were com- 

 posed of a number of rigid tubes, each ventricular output would be 

 forced through this system in the form of a uniform column which 

 would come to a standstill at some distance from the heart. But as the 

 vascular system is elastic, and is kept in a state of hyperfilling by an 

 appropriate peripheral resistance, the different ventricular discharges 

 must be retained temporarily near the outlet of the heart, their 

 retention being made possible by an enlargement of the main distribut- 

 ing tube, the aorta. The elastic recoil immediately following this 

 distention, then forces a portion of this blood into the more peripheral 

 segment and from here into the adjoining one, and so on until the periph- 

 ery has been reached. In this way, the conditions incited by the 

 ventricular discharge are repeated again and again and are thus 

 propagated throughout the arterial system. Moreover, as the blood- 

 bed of the aorta is larger than that of its branches put together, 

 this blood-vessel, and especially its ascending and transverse portions, 

 serve the purpose of an elastic reservoir from which all the peripheral 

 blood-vessels are supplied. 



Soon after its emergence from the heart, th% blood enters the differ- 

 ent branches of the aortic system and is distributed to the various 

 tissues and organs in amounts commensurate with their activity. In 

 close proximity to the heart, the flow very nearly equals the ventricular 

 output, only that amount of blood having been removed from it which 

 is destined to nourish the cardiac musculature. Farther distally, how- 

 ever, the reduction becomes more apparent, because a considerable 

 quantity of blood is now diverted into the blood-vessels of the head 

 and anterior extremities. In endeavoring to obtain an idea regarding ] 

 the volume of the blood stream in any particular artery, it is not 

 sufficient to collect the blood escaping from the opened blood-vessel in 

 a graduated cylinder, because the removal of the peripheral resistance 

 seriously disturbs normal dynamical conditions. With a closed 

 vascular system, two procedures are practicable which may be desig- 

 nated respectively as the direct and the indirect. 



The direct method consists in connecting the artery with an instrument known 

 as a current-measurer or stromuhr. The one described by Ludwig^ is composed 

 of two glass bulbs (A and B) which are placed upon a metal disc (P) and may be 

 rotated around a common vertical axis (Fig. 207). In this way, it is possible to 



^ Stolnikow, Archiv fiir Anat. und Physiol., 1886. This instrument has been 

 modified by Tigerstedt, Skand. Archiv fiir Physiol., iii, 1891. 



