FUNCTIONAL ANATOMY OF CARDIAC PUMPING 



7 6 9 



0ORIENTATION OF MYOCARDIAL FIBERS IN VENTRICULAR WALLS 



RIGHT VENTRICLE — • 



LEFT VENTRICLE 



(|)FUNCTIONAL COMPONENTS OF VENTRICULAR MUSCULATURE 



fig. 13. Muscular structures of the ventricles diagram" 

 matically arranged so as to reveal their functional components. 

 A: blocks of tissue removed from the walls of the ventricles are 

 composed of three layers of muscle. The myocardial fibers in 

 these layers are oriented roughly in the three general directions 

 indicated by the arrows. B: from a functional point of view, 

 the ventricles are formed of two sets of myocardial bundles: 

 a, the internal and external layers of spiral muscle, which en- 

 close b, the ventricular constrictor muscles. The internal and 

 external investments of the ventricular chambers are composed 

 of the same muscle bundles, which are strongly twisted at the 

 vortex and spiral in opposite directions from the apex toward 

 the base. [From Rushmer (139).] 



impeding, though not completely blocking, the 

 backflow of blood into the veins during atrial systole. 

 Looped fibers are also found which run from the 

 anterior to the posterior segments of the atrioventric- 

 ular junction, directly beneath the endocardium. At 

 many places these fibers bulge into the atrial cavities 

 forming various ridges which are most conspicuous 

 at the inner walls of the atrial appendages, where they 

 are named musculi pectinati from their resemblance 

 to a comb. 



Group 2: The fibers common to both atria are less 

 numerous and lie superficially with respect to the 

 proper fibers of each individual atrium. They consist 

 of two thin muscle sheets which extend in a transverse 

 direction from one atrium to the other. They can be 

 subdivided into anterior and posterior fascicles. The 

 muscle fibers of the atria and ventricles are separated 



by connective tissue except at one place, known as the 

 atrioventricular bundle or bundle of His. 



The atrial cavity is surrounded by the thin myo- 

 cardial fibers of both groups arranged in layers which 

 are partly parallel and partly crisscrossed. The con- 

 certed action of all fibers is that, upon their con- 

 traction, they diminish the size of the atrial cavity 

 and push blood into the region of least resistance, i.e., 

 primarily into the ventricles, secondarily into the 

 venous orifices. In addition to the main atrial cavity, 

 there is an adjoining cavity formed by the lumen of 

 the atrial appendage, also called "auricle" because 

 of its resemblance to a little ear. The function of the 

 auricles is unknown. Excision of the auricles in various 

 operative procedures does not influence the circulation 

 noticeably. Yet one cannot state bluntly that the 

 atrial appendages have no function at all, since in a 

 complex system, such as the heart, the function of a 

 missing part may often be taken over or substituted 

 by increased activity of other components. The 

 mere presence of the atrial appendages results in an 

 increase in the cardiac reserve. According to Benning- 

 hoff (10) and Bohme (14), the atrial appendages fill 

 the space which is created within the pericardial sac 

 during ventricular systole, as the ventricles eject 

 blood into the large arteries and decrease in size. 

 During this period the atrial appendages accommo- 

 date a considerable amount of blood. This blood is 

 immediately available at the beginning of the rapid 

 ventricular filling phase to be transferred into the 

 ventricular cavities. 



PRESSURE AND FLOW EVENTS DURINO 

 THE CARDIAC CYCLE 



Historically the cardiac cycle was first divided into 

 "systole," or period of contraction, and "diastole," 

 or period of relaxation of the ventricles. It was soon 

 recognized that the terms systole and diastole should 

 refer equally to the atrial contraction and relaxation, 

 although the ventricular events were most con- 

 spicuous in the gross observation of cardiac activity. 

 Since the atrial contraction precedes that of the 

 ventricle, terminological difficulties arose as to which 

 systole was meant in describing the time sequence of 

 cardiac events. As knowledge about the heart's 

 action increased, it was also deemed necessary to 

 subdivide the cardiac cycle in greater detail [see also 

 Mackenzie (no)]. With the advent of methods for 

 precise pressure recording from the cardiac chambers 

 and great vessels, the ventricular pressure tracings 



