I IOO 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



Though the venous return is normally exactly 

 equal to ventricular output, this may not be true for 

 short periods of time. However, when the venous 

 return is greater than the ventricular output, blood 

 will accumulate in the heart. During the ensuing few 

 heartbeats a new state of equilibrium will develop, 

 and venous return will again become equal to the 

 output. Yet, since there are times when venous return 

 and cardiac output are not equal, it is justified to use 

 the term ""venous return" separately from the term 

 "cardiac output." 



Normally, in speaking of cardiac output, one thinks 

 principally of cardiac activity, whereas in speaking of 

 venous return, he thinks of all the functions of the 

 peripheral circulation that have to do with blood 

 flow into the heart. For this reason many circulatory 

 physiologists consider cardiac output to be regulated 

 principally by the heart and venous return to be regu- 

 lated principally by peripheral factors. By all means, 

 the reader must be cautioned at the outset against 

 this viewpoint, because except for instantaneous 

 periods of time, any factor that affects cardiac output 

 also affects venous return, and any factor that affects 

 venous return also affects cardiac output. This princi- 

 ple can be expressed in another way: The circulatory 

 system is a circuit, and the total flow of blood through 

 any one cross section of the circuit is exactly the same 

 as the total flow through any other cross section. 



Principles of Circuit Analysis <v\ They 

 Apply To I 'enous Return 



( )ften, physiologists have attempted to analyze the 

 regulation of venous return entirely on the basis of 

 local factors in the veins and right heart. However, 

 this is a completely fallacious approach, for any 

 factor that affects blood flow in any single unit of the 

 entire circulation will likely at the same time affect 

 venous return. For this reason, a complete circuit 

 analysis of the entire circulation is required for any 

 degree of accuracy in determining the factors that 

 regulate venous return (46, 47, 78, 81, 126-128, 192). 

 This means that the same factors that regulate cardiac 

 output are those which also regulate venous return. 

 In general, these can be divided into two major 

 groups: those concerned with the ability of each side 

 of the heart to pump blood, and, second, those con- 

 cerned with the ability of blood to flow through the 

 two divisions of the peripheral circulation, the sys- 

 temic circulation, and the pulmonary circulation. In 

 this chapter we will be concerned with both of these 

 groups of factors and with the manner in which thev 



operate together to regulate blood flow in the circula- 

 tion. However, the contribution of the heart to the 

 regulation of blood flow in the circulation is discussed 

 in great detail in several other chapters, including 

 especially those by Hamilton, Sarnoff & Mitchell, 

 Brecher, and Rushmer (Chapters 17, 15, 23, and 16, 

 respectively). Therefore, in this chapter we will discuss 

 the cardiac factors only briefly while discussing the 

 peripheral factors in far greater detail. 



the problem. Figure 1 depicts a highly simplified 

 diagram of many of the important factors that must 

 be considered in analyzing blood flow around the 

 circulatory circuit. First of all, we note in the diagram 

 the term blood flow (F) appearing at four different 

 points: /) at the output of the right heart, 2) at the 

 output of the pulmonary circulation, j) at the output 

 of the left heart, and ./) at the output of the systemic 

 circulation. Under steady-state conditions, the blood 

 flow at all of these four points will be exactly equal. 

 Yet, in a circulatory analysis we must consider them 

 separately, because they are not always equal, and 

 during the periods of time when they are unequal, 

 blood will be actively translocated from one part of 

 the circulation to another, thus causing new equilib- 

 rium conditions to develop. Now to describe the 



fig. 1. Circulatory schema, showing the effects of indi- 

 vidual factors in different parts of the circulation on other 

 functions of the circulation. Each arrow represents a separate 

 effect, and a composite analysis of the circulation requires 

 that all these effects be considered simultaneously as explained 

 in the text. 



