HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



FIG. I. Diagram illustrating the translocation of successive 

 stroke volumes through the aorta and its branches. [After 

 Lequime (43).] 



either to maintain allegiance to or to revolt against 

 traditional viewpoints. 



THE ARCHITECTURE .^ND FUNCTION OF THE 

 CIRCULATORY SYSTEM 



From a functional standpoint, the systemic circuit 

 may be divided into a) a distributing system (left 

 ventricle, aorta and its branches, and their final 

 subdivisions into arterioles); b) a division for the 

 interchange of substances (capillaries and venules); 

 and c) a collecting system (small and large veins, 

 venae cavae, and right atrium). The pulmonary 

 circuit similarly begins with the right ventricle and 

 drains into the left atrium. 



The entire cardiovascular system is lined with a 

 continuous smooth layer of endothelium which 

 minimizes friction between the flowing blood and 

 vascular walls, and thus limits resistance to flow to 

 shearing forces in layers of blood near the vascular 

 walls. 



The Distributing System 



The architecture of the arterial tree is adapted in 

 many ways to the proper distribution of blood but 



also introduces a number of complexities in its propul- 

 sion. The main branches of the aorta divide dichoto- 

 mously, each time widening the stream bed and broad- 

 ening the areas of distribution. The longitudinal 

 divisions of the abdominal aorta into iliac and femoral 

 arteries and the extension of axillary arteries into the 

 brachial and radial arteries (fig. i) create physical 

 resonating systems of low natural frequencies that 

 modify the fluctuating aortic pressures and flow in 

 their transmission to the periphery. 



The greatest proportion of blood injected into the 

 aorta by the left ventricle leaves it by numerous 

 lateral branches which form parallel circuits (fig. 2). 

 This physical arrangement serves two important 

 dynamic purposes: /) it permits redistribution of blood 

 from one territory to another through regional 

 changes in resistance without necessarily altering 

 mean aortic pressure, and 2) it reduces the total 

 resistance to efflux from the arterial system The 

 latter is occasioned by the fact that, as in parallel 

 electric circuits, total peripheral resistance (TPR) 

 depends on reciprocal relations, for e.xample, 

 i/TPR = I A, 4- i/r,+ i/n ■■■ iA„. 





\ 



CORONARIES 

 TRUNK 



-TT- 



LIVER 



ABDOMINAL ORGANS 



I SPLEE N I 



isr: 



KIDNEY 



/L r 



PELVIC ORGANS 

 HINDLIMBS 



-XT' 



T' 



5r 



FIG. 2. Diagram illustrating the principle of parallel cir- 

 cuits and resistances. [Modified from Wezler & Boger (67).] 



