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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



rately; then the function of the pulmonary circulation 

 can be analyzed; then that of the left heart, and that 

 of the systemic circulation. Once these four analyses 

 have been made they can be put together either 

 algebraically or graphically to derive a composite 

 analysis of the entire circulation. In a still further 

 simplified analysis one can even analyze the function 

 of the entire heart and pulmonary circulation as a 

 single segment of the circulation, such as is usually 

 done in studying the heart-lung preparation. Then 

 the svstemic circulation can be analyzed as a separate 

 segment and the two analyses put together to provide 

 a composite analysis for the entire circulation. 



Obviously, the more extensively the different parts 

 of the circulation are grouped into large circulatory 

 segments the less accurate becomes the over-all 

 analysis. Furthermore, the type of analysis and the 

 tvpe of grouping of circulatory segments that will be 

 used will depend to some extent on the factors of the 

 circulation and the type of circulatory stress one wishes 

 to study. Fortunately, it is usuallv possible to find 

 some specific grouping of circulatory segments that 

 is both simple and yet accurate enough for the desired 

 purpose. 



The Classical Analysis of Venous Return — 

 Vis a Tergo and Vis a Fronte 



The classical method for analyzing the factors that 

 affect venous return has been to consider that two 

 forces affect blood flow from the systemic veins into 

 the right atrium (31, 70, 132, 133): /) the force from 

 behind, called "vis a tergo," which pushes blood along 

 the veins toward the right atrium, and 2) the force 

 from in front, that is, from the right heart itself, called 

 "vis a fronte," which either impedes blood flow into 

 the heart or perhaps at times aids the flow. Actually, 

 the vis a tergo is the force that remains after dissipation 

 of the arterial pressure during the course of blood flow 

 through the systemic vessels. Conversely, the vis a 

 fronte is a back pressure from the heart that depends 

 principally on the over-all pumping activity of the 

 heart. If the heart is very active as a pump, the back 

 pressure will be low and consequently the vis a tergo 

 can force blood into the heart with ease. Conversely, 

 if the pumping activity of the heart is weakened, then 

 the back pressure from the heart will be greatly en- 

 hanced, this in turn impeding the flow of blood into 

 the heart. 



A specific factor that must be considered in relation 

 to the vis a fronte is the possibility of suction of blood 

 into the heart caused by relaxation of the cardiac 



chambers or caused by respiratory movements, and a 

 vast segment of the literature on venous return has 

 concerned itself with these considerations. They will 

 be discussed in detail in the latter part of the chapter. 



History of More Complete Circulatory Analyses 



Unfortunately, the classical analysis of venous re- 

 turn has never been quantitative because such a 

 simple presentation does not provide an adequate 

 basis for delineating either the many different periph- 

 eral or cardiac factors that affect vis a tergo or those 

 that affect vis a fronte. For this reason, progressive 

 attempts have been made — very slowly, to be sure, 

 but beginning about one hundred years ago — to 

 provide more complete analyses of the circulation, 

 with consequently higher degrees of quantitation of 

 the interrelationships between the individual circula- 

 tory factors. The first real attempt in this direction 

 seems to have been made by Weber (194) in 1850, at 

 which time he expressed in general the complexity 

 of the relationships illustrated in figure 1, though not 

 expressing these in precisely the same manner as 

 shown here. From the literature it is equally as 

 evident that these general principles have been well 

 understood by Starling (179), Bolton (23), Starr (180, 

 182), Katz et al. (126-128), Daly et al. (46, 47), Rose 

 et al. (168), and Rashkind et al. (158). Yet, it has 

 been only in the last half decade that serious attempts 

 have been made to provide composite analyses of the 

 circulation. From our laboratory has come a graphical 

 approach to this problem (81 ), both in a simplified 

 version and in a more complex version, portions of 

 each of which will be presented in this chapter. In 

 addition to this, two different and closely similar 

 algebraic analyses have been presented by Grodins 

 (78) and by Warner (192). All these analyses are 

 based on the general principles illustrated in figure 1. 

 However, each of the analyses groups the components 

 of the circulation somewhat differently, they go to 

 greater or lesser degree into the details of either 

 cardiac or peripheral circulatory factors, and they 

 make slightly different basic assumptions. Yet, the 

 results of these three entirely independent and differ- 

 ent analyses are almost exactly identical. For this 

 reason they all deserve the highest degree of considera- 

 tion. Since each of these analyses is relatively complex 

 and long, it would be impossible to present them all in 

 this chapter. Therefore, this chapter will be limited 

 to a presentation of the graphical analysis, and the 

 serious student of this subject will be referred to the 

 very complete and interesting algebraic analysis by 



