VENOUS RETURN 



events depicted in figure i , we can start by pointing 

 out that each arrow represents an effect of one factor 

 on another factor. The different effects denoted in 

 this figure can be listed as follows : 



(i 



(a 



(3 

 (4 

 (5 

 (6 



(7 

 (8 



(9 



(10 



(n 



(.2 



('3 

 ('4 

 ('5 

 (16 



(17 

 08 

 ('9 



(20 



(a i 



(aa 



(23 

 (24 



Right heart blood flow (Frh) directly affects pulmonary 

 arterial pressure (Pa p ). 



Right heart blood flow (Frh) inversely affects systemic 

 venous pressure (Pv,). 



Pulmonary arterial pressure (Pa p ) inversely affects 

 right heart blood flow (Frh). 



Pulmonary arterial pressure (Pa„) directly affects 

 pulmonary blood volume (Vp). 



Pulmonary arterial pressure (Pa p ) directly affects 

 pulmonary blood flow (F p ). 



Pulmonary blood volume (Vp) inversely affects pul- 

 monary resistance (Rp). 



Pulmonary resistance (Rp) directly affects pulmonary 

 arterial pressure (Pa p ). 



Pulmonary resistance (Rp) inversely affects pulmonary 

 blood flow (Fp). 



Pulmonary blood flow (Fp) directly affects pulmonary 

 venous pressure (Pv p ). 



Pulmonary venous pressure (Pv p ) directly affects pul- 

 monary blood volume (Vp). 



Pulmonary venous pressure (Pv p ) inversely affects 

 pulmonary blood flow (Fp). 



Pulmonary venous pressure (Pv p ) directly affects left 

 heart flow (Flh). 



Left heart blood flow (Flh) inversely affects pulmonary 

 venous pressure (Pv p ). 



Left heart blood flow (Flh) directly affects systemic 

 arterial pressure (Pa s ). 



Systemic arterial pressure (Pa B ) inversely affects left 

 heart blood flow (Flh). 



Systemic arterial pressure (Pa J directly affects systemic 

 blood volume (Vs). 



Systemic arterial pressure (Pa^) directly affects systemic 

 blood flow (F.I. 



Systemic blood volume (Vs) inversely affects systemic 

 resistance (Rs). 



Systemic resistance (Rs) directly affects systemic arterial 

 pressure (Pa,). 



Systemic resistance (Rs) inversely affects systemic blood 

 flow (Fs). 



Systemic blood flow (Fs) directly affects systemic 

 venous pressure (Pv,). 



Systemic venous pressure (Pv,) inversely affects sys- 

 temic blood flow (Fs). 



Systemic venous pressure (Pv„) directly affects sys- 

 temic blood volume (Vs). 

 Systemic venous pressure (Pv,) directly affects right 



^ heart flow (Frh). 



Now we have completed the circuit, giving most of 

 the major factors within the circulation that affect 

 each other; obviously, a change in any one of these 

 will change all the others. However, this has been a 

 highly simplified schema, for, onto these basic me- 

 chanical factors, we must superimpose the effects of 



circulatory reflexes, hormones, special conditions such 

 as exercise, hemorrhage, myocardial infarction, or 

 any other condition that can affect the function of 

 any single segment of the circulation. 



To complicate the picture further, we must also 

 note how greatly simplified some of the individual 

 effects illustrated in figure i have been made. For 

 instance, one of the effects noted in this figure is the 

 direct effect of systemic venous pressure on right heart 

 blood flow. Actually, the systemic venous pressure 

 does not directly affect the right heart blood flow but 

 instead affects the filling of the right heart. This in 

 turn affects the degree of stretch of the cardiac 

 musculature, which then affects the force of contrac- 

 tion. Finally, it is this force of contraction that di- 

 rectly affects the right heart blood flow. Thus, it can 

 be seen that four steps have actually been compressed 

 into one in the simplified diagram of figure i . 



We can summarize the problem, therefore, by 

 pointing out that the regulatory factors of the circula- 

 tory system can be divided into literally hundreds or 

 even thousands of individual factors distributed 

 throughout the circulation. Indeed, even closure of a 

 single capillary exerts its infinitesimal effect on the 

 functions of all other parts of the circulation. This, 

 then, is the problem with which we are faced in this 

 chapter, to unravel the myriad of different factors 

 that affect blood flow in the circulation and to deter- 

 mine those that are especially important in relation 

 to venous return. 



solution to the problem. Several reasonably satis- 

 factory solutions to the above problem have now been 

 offered, and all of them have taken the same general 

 form [Guyton (8i), Grodins (78), Warner (192)]. 

 First, in each of the solutions, many or most of the 

 parallel segments of the circulation are grouped to- 

 gether. For instance, in a general solution of the 

 problem one would not attempt to analyze the effect 

 of occluding each single arteriole on the circulatory 

 dynamics, but, instead, all of the arterioles are 

 grouped together. Similarly, the capillaries are 

 grouped together, the venules, the large arteries, the 

 large veins, and so forth. This is done separately for 

 the systemic circulation and pulmonary circulation. 

 The next step in the solution is to group the individual 

 segments of the circulation into several convenient 

 larger segments and then to analyze the function of 

 each of these larger segments separately, this followed 

 by a synthesis of the separate analyses into a composite 

 analysis. Thus, as we shall show later in the chapter, 

 the function of the right heart can be analyzed sepa- 



