THE CONTROL OF THE FUNCTION OF THE HEART 



527 



area of increased energy requirement are rendered 

 gradably immune to instructions from the circulatory 

 centers when the latter request an increased re- 

 sistance to flow. 



The combination of elements outlined above can 

 be considered the command system of the circulation. 

 By the relatively simple expedient of maintaining an 

 adequate pressure head from which the various 

 tissues of the organism can each select a flow appro- 

 priate to its activity and energy requirement (44, 

 78, loi), this command system has the capability of 

 sustaining the competence of the total organism in 

 the face of widely varying conditions and challenges. 

 The extent to which it will be successful in so doing 

 will, however, depend upon the availability and 

 appropriateness of the information fed into the inte- 

 grating network to which the command system is 

 subservient. 



Before proceeding to this aspect of the matter, 

 however, it seems appropriate to evaluate here the 

 role of Starling's law of the heart in circulatory regu- 

 lation and the manner in which it participates as part 

 of the command system. This can be most readily 

 appreciated by posing two separate questions. /) "Is 

 the heart of the normal, unanesthetized, active 

 organism operating in accordance with Starling's 

 law of the heart?" The answer to this is clearly in the 

 affirmative. The continuing beat-to-beat operation of 

 Starling's law of the heart, with both the right and 

 left ventricles operating on the curves then obtaining, 

 provides a convincing explanation of how the ratio 

 between systemic and pulmonary blood volumes is 

 constrained within such narrow limits for the lifetime 

 of the organism. Unlike homeometric autoregulation, 

 which requires at least several beats to fully develop, 

 the beat which follows immediately after an increase 

 of end diastolic pressure and fiber length produces 

 more external work than the one immediately before 

 it. As shown most convincingly by Brecher (15) there 

 are substantial variations in venous return throughout 

 the respiratory cycle. The variations of venous return 

 to the right and left ventricle cannot be the same with 

 respect to both amount and time, and they are, in 

 fact, often reciprocating with each other to varving 

 degrees. Further, the importance of changes in left 

 ventricular volume relative to its stroke work through- 

 out the respiratory cycle, and also the fact that this 

 relationship still obtains after changing the circula- 

 tory state by inducing exercise, has been firmly 

 established (25). Hamilton (45) cites the early work 

 of Henderson & Prince (48), and supports the position 

 established by Berglund (9) in a statement which, in 



many ways, is difficult to improve upon. He said, 

 'T believe that the Starling law does play a very 

 important role in everyday life and that that role is 

 to preserve the balance between the pumping of the 

 right and the left ventricle. This balance must be 

 exact and, since the two ventricles are subject to the 

 same hormonal and nervous influences, they each 

 act as a control for the other. They have to follow 

 each other from curve to curve and, so far as I can 

 think at present, only a delicate adjustment of 

 strength of contraction to degree of filling serves as an 

 hypothesis to explain their maintained balanced 

 output ..." 



But the importance of the continuing beat-to-beat 

 operation, even of the simple Starling law, in the 

 intact organism cannot be restricted to the single 

 function of maintaining the pulmonary blood volume 

 within endurable limits, as vital as that may be to the 

 continuing existence of the organism. For if it is to be 

 assumed that Starling's law is not otherwise operative 

 to an important extent in the normal organism, to 

 what end is there a venous pumping action in muscles, 

 what significance does reflexly induced venoconstric- 

 tion have, to what end is it possible for the atria to 

 vary so markedly the amount of blood they propel 

 into the ventricles as the result of their reflexly con- 

 trolled contractility? On the basis of known mecha- 

 nisms, these phenomena achieve physiologic signifi- 

 cance only to the extent that they influence ventricular 

 filling, stroke work, and stroke volume. Those who 

 come to the conclusion that the generalit y elaborated 

 by Starling is of importance mostly because it is 

 informative about the heart failure syndrome but 

 "has only limited application to the normal human 

 heart," (75) have the unequivocal responsibility of 

 assigning some other biologically meaningful signifi- 

 cance to these phenomena. 



2) The second question is "Does the simple opera- 

 tion of Starling's law of the heart of itself account for 

 the observed changes in cardiac output with varying 

 states, for example, exercise?" The answer to this is 

 clearly no, nor have we ever put forward the contrary 

 view. It is difficult, therefore, to understand those 

 quixotic attempts to abolish a position which doesn't 

 exist by showing that the ventricle may not enlarge or 

 even may become smaller during exercise or with a 

 change in posture (86, 90-92). The positive position 

 that is maintained is that the central nervous system 

 has available to it both direct and indirect pathways 

 by means of which it can continuously provide varying 

 degrees of gain for the fundamental Frank-Starling 

 relation, and that the exploitation of these available 



