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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



ment of the atrioventricular junction and the drop 

 of pericardial pressure during early ventricular 

 ejection are caused by the same force, i.e., the ven- 

 tricular myocardial contraction. Which of the two 

 factors is predominant in facilitating atrial filling 

 during ventricular systole will not be easily decided. 

 From the presently available evidence one can 

 conclude that the pericardium aids the pumping 

 function of the heart. In contradiction to the wide- 

 spread opinion that the absence of the pericardium 

 does not have a noticeable effect upon the circulation, 

 there is some experimental evidence that a large 

 percentage of pericardiectomized animals develop 

 heart hypertrophy and perform poorly on the tread- 

 mill [see Nelemans (118)]. It is quite possible that 

 under conditions of rest or mild exercise the heart 

 without pericardium can satisfy the metabolic 

 demands of the tissues. However, under conditions of 

 strenuous exercise the reserve power of the heart 

 without pericardium is probably diminished. 



CLOSING REMARKS 



In recent years the opinion has been often voiced 

 that hemodynamics is a dead science, in which no 

 more essential work needs to be done. This view refers 

 primarily to the mechanical features of the cardio- 

 vascular system which are supposedly well known. 

 It applies less to the regulatory aspects which ad- 

 mittedly require further clarification. However, the 

 analysis of such a presumably simple function as the 

 heart's normal pumping, even without consideration 

 of any neural or hormonal regulatory processes, 

 reveals wide gaps in our knowledge. 



There are numerous reasons for these shortcomings. 



Many of the measurements on which present con- 

 cepts are based were obtained under highly artificial 

 conditions, such as excised heart preparations and 

 open-chest animals, which limit the applicability of 

 the results to intact normal organisms. Findings in 

 one animal species often cannot be transferred or 

 extrapolated to other species. For instance, the me- 

 chanics of cardiac pumping in man differ from those 

 in other animals because of his upright position, 

 minimal splanchnic pooling, and several other fac- 

 tors. Finally, measurements are often performed with 

 inadequate instrumentation. For example, errors 

 caused by insufficient sensitivity and time resolution 

 make it difficult to correlate simultaneous events in 

 the cardiac cycle. 



Only rather guarded conclusions can therefore be 

 drawn from the available experimental evidence 

 as to the exact nature of the heart's pumping func- 

 tion. Wide discrepancies of information obtained 

 with different methods need to be reconciled. For 

 example, measurements of the ventricular volumes 

 by various methods differ so greatly in their order of 

 magnitude that it is today still impossible to state 

 how large is the normal functional residual capacity 

 of the heart. Obviously, views based on insufficient 

 data must remain in the realms of speculations and 

 postulates. This is the state of knowledge about many 

 age-old problems such as cardiac filling and the 

 precise moment of valve closure. How much more 

 complex these problems become under various patho- 

 logical conditions does not need to be elaborated 

 upon. In view of the recent progress in the bio- 

 medical sciences, it is shocking to observe how a 

 seemingly simple mechanical process such as cardiac 

 pumping still remains so enigmatic. 



The help of Dr. Robert C. Schlant in reviewing the manu- 

 script is gratefully acknowledged. 



REFERENCES 



i. Adcock, J. D., R. H. Lyons, and J. B. Barnwell. 

 The circulatory effects produced in a patient with pneu- 

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2. Agress, C. M., L. G. Fields, S. Wegner, M. Wilburne, 

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3. Akman, L. C, A.J. Miller, E. N. Silber, J. A. Schack, 

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4. Anzola, J. Right ventricular contraction. Am. J. Physiol. 

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5. Bakos, A. C. P. The question of the function of the right 

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6. Bauereisen, E., H. Bohme, H. Krug, U. Peiper, and 

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7. Bauereisen, E., U. Peiper, and K. H. Weigand. The 

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8. Baumgarten (1843). Quoted by Tigerstedt, R. In: A 

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