FUNCTIONAL ANATOMY OF CARDIAC PUMPING 



7 8. 



flow of blood toward the atria which catches the 

 valves like a pair of sails and flings them into apposi- 

 tion. As pointed out by Rushmer ( 1 39) this mechanism 

 inevitably involves a leak before the orifice is closed. 

 The occurrence of such regurgitation is widely 

 acknowledged when the atrioventricular valves are 

 closed by a ventricular systole which is not preceded 

 by atrial contraction, i.e., "premature ventricular 

 contraction" [see also Paul et al. (128)]. Since the 

 normal wave of excitation propagated by the Purkinje 

 fibers enters the ventricular myocardium over the 

 endocardial surface, at the roots of the papillary 

 muscles, the early contraction of papillary muscles 

 draws the valve edges toward the apex and thereby 

 produces some shortening of the ventricular chamber 

 with a resulting passive lateral displacement of the 

 ventricular walls. During ventricular ejection, the 

 decrease in ventricular volume is accompanied by a 

 further shortening of the papillary muscles, taking 

 up any slack in the valves which might develop. 



It has been suggested that under normal condi- 

 tions, the valves approximate, i.e., begin to close 

 before the onset of ventricular contraction, although 

 it is difficult to substantiate this view by chrono- 

 logically precise measurements. Many explanations 

 have been advanced for this: passive, upward move- 

 ment of the valves at the end of diastole caused by 

 retrograde flow of blood along the ventricular wall, 

 or by elastic recoil of the ventricular wall to the 

 strain of atrial systole (8); eddy formation beyond 

 the valves during atrial systole (102); or develop- 

 ment of a wave of negative pressure as atrial inflow 

 abruptly ceases [Henderson & Johnson (70)]. 

 Through these mechanisms, the atrioventricular 

 valves are approximated or almost closed just before 

 ventricular systole. Then, when the ventricular 

 myocardium contracts, the valves are completely 

 closed to prevent backflow into the atria. 



The concept of a presystolic approximation of the 

 atrioventricular valves received considerable impetus 

 when Dean (37) succeeded in obtaining direct re- 

 cording of the valve movement in an isolated, sur- 

 viving heart. Dean demonstrated that when the 

 interval between atrial and ventricular systole is 

 sufficiently long, there is indeed a rapid movement 

 of the valves toward the atrium at the end of atrial 

 systole, followed by a second period of separation of 

 the cusps just before the onset of ventricular systole. 

 When the period of ventricular filling is shorter (at 

 faster heart rates) there is no time to observe sepa- 

 rately the effects of atrial systole and of ventricular 

 systole. Then there is '"only a single closure move- 



ment beginning before ventricular systole, a single 

 movement due in part to auricular contraction and 

 in part to ventricular contraction" (37). 



The extent to which the valves and their attach- 

 ments move in the intact organism has been recently 

 questioned by Rushmer (139). Having observed that 

 exposed or excised hearts tend to shrink, he and his 

 associates surmised that the valves might have much 

 less slack under their normal operating conditions 

 than reported by previous investigators. They used 

 cinefluorography to observe the movements of the 

 mitral valves which had been marked with tiny metal 

 clips at a previous operation. These studies demon- 

 strated that the excursion of the valves, at least where 

 the metal clips were placed, toward the atrium is 

 remarkably small, and pointed to a more or less con- 

 tinuous restraint by the chordae tendineae. 



Arterial Valves 



The aortic and pulmonary valves consist of three 

 symmetrical cusps attached, similarly to suspension 

 bridges, around the circumference of the valve orifice 

 (see fig. 3). When the cusps are approximated they 

 form a starlike figure; when open, they delineate a 

 nearly rounded but still somewhat triangular orifice 

 of an area slightly smaller than that of the artery. At 

 the tip of each triangular valve leaflet, where the 

 three valve leaflets come in contact, there is a dis- 

 crete thickening called the nodulus Arantii. There 

 are also thin, membrane-like structures (lunulae) 

 on the free edges of the valves on either side of the 

 noduli. Normally the free edges come into contact 

 surface against surface rather than border to border. 

 Toward the valvular orifice, the ventricular muscula- 

 ture assumes the shape of a funnel (conus arteriosus), 

 whereas bevond the valves at the origin of the aorta 

 and pulmonary artery there are three outpouchings 

 which provide some free space behind the valve 

 cusps even when they are maximally opened. 



The mechanism of action of the arterial valves can 

 be described as follows. During ventricular ejection, 

 the blood stream opens the valves from below and a 

 rapid flow is established along the axis of the valvular 

 orifices. However, the valves are not pushed flush 

 against the arterial wall. On the contrary, eddy cur- 

 rents, generated by the axial jet of blood, swirl in the 

 spaces behind the cusps. Indeed, the action of turbu- 

 lent eddies is such that the faster the ventricular 

 ejection, the closer to the center of the axial stream 

 the valve edges are brought [Hochrein (76)]. Thereby 

 the valves are prepared to close almost instantane- 



