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HWUBOOK OF PHYSIOLOGY 



CIRCULATION II 



of external forces on the heart [Hosier (81), Stephen- 

 son (150)]. In closed-chest cardiac massage, vigorous 

 pressure on the lower part of the sternum causes 

 ejection of the ventricular content into the large 

 arteries. Conversely, when pressure is released, the 

 recoil is sufficient to permit the venous pressure to fill 

 the ventricles again [Kouwenhoven et al. (92)]. In 

 this manner, a sufficient, though subnormal, cardiac 

 output can be maintained in the absence of any 

 myocardial activity. This points again to the fact 

 that, in principle, it does not matter whether the 

 propulsion of blood through the body is brought 

 about by the contraction of cardiac fibers or by any 

 other suitable forces applied to the blood contained 

 in the ventricles. 



MACROSCOPIC STRUCTURES 



A great deal of commonly accepted knowledge 

 about cardiac pumping is derived from purely 

 morphological considerations. Although conclusions 

 reached in this manner have occasionally proved to 

 be correct, morphological reasoning often leads to 

 fallacious lunctional interpretations of structural 

 findings. In the case of the heart, physical vector 

 analvsis of all the mechanical forces involved is 

 especially difficult because of the great complexity of 

 the anatomical structures and of the perplexing 

 geometry of cardiac filling and emptying. We have 

 only a limited knowledge of the sequence of events 

 as they occur during muscular contraction and 

 relaxation within various parts of the myocardium. 

 In this particular section an attempt is made to 

 describe the macroscopic structures of the heart with 

 reference to their probable function as deduced from 

 the anatomical observations. A topographic anatomi- 

 cal description of the heart is available in standard 

 texts (51, 95, 98, 101). 



Composition nj Cardiac Tissues 



The myocardium is the most important structure 

 of the heart because its contraction causes the blood 

 to flow. However, it should be realized that only 

 part of the cardiac walls consists of muscle fibers, and 

 that within the muscle fibers, the contractile substance 

 is limited to the fibrils. Indeed about half of the 

 heart's weight is made of noncontractile material 

 such as the sarcolemma in the muscle fibers, con- 

 nective tissue in the heart skeleton, tendons and 

 valves, and finally blood vessels, lymphatics, and 

 nerve fibers. All these elements are interwoven with 



the muscle fibers or closely connected to them (45, 

 59). During cardiac contraction or relaxation, they 

 are deformed and resist to some degree the shortening 

 or lengthening of the myofibrils. 



Little is known about the mechanical effects of the 

 coronary vessels upon the function of the ventricles. 

 Though relatively inconspicuous in a "dead" heart, 

 they appear heavily engorged with blood in the live 

 organ. In fact, since 5 to 10 per cent of the cardiac 

 output passes through the coronary system, a signifi- 

 cant mass of the beating heart consists of circulating 

 blood contained within the anatomical bounds of the 

 epicardium. During heavy exercise the coronary- 

 blood supply is probably so great that one might look 

 upon the myocardium as a spongy structure of muscle 

 fibers suspended like chains of islands in a lake of 

 blood. In the past it has been postulated frequently 

 that the degree of filling of the coronary vascular bed 

 affects in some form the ventricular contraction. 



AV + 



fig. 2. Left ventricular pressure-volumes curves of a dog 

 heart illustrating the changes resulting from coronary perfu- 

 sion. The freshly excised heart of a 13.5-kg dog was submerged 

 in Locke's solution and assumed its elastic equilibrium state 

 (zero transmural pressure, origin of the coordinates) upon 

 cessation of spontaneous contraction. The curves were obtained 

 by addition or reduction of the intraventricular volume 

 [Brecher & Kissen (22)]. The origins of the coordinates for the 

 perfused and unperfused heart were arbitrarily superimposed. 

 At negative (and up to +5 mm Hg) intraventricular pressures 

 the ventricle accommodated a greater volume with coronary 

 perfusion than without. At pressures above +5 mm He, the 

 ventricle accommodated less fluid with coronary perfusion 

 than without (Horres et al., unpublished data). 



