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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



must not be traumatized. The valves must open and 

 close an orifice which continuously changes in size 

 and provides scant room for support between pul- 

 satile and irregular chambers. The motion of the 

 valves depends upon cardiac action and must be 

 coordinated with those structures which impart it. 

 And, finally, the valvular apparatus must withstand 

 the stress of beating more than forty million times a 

 year during the life span of the subject." 



In a multichambered pump such as the heart, 

 valves or valve-like mechanisms are expected at each 

 boundary between functionally separate chambers. 

 We shall therefore discuss the function of the struc- 

 tures which prevent backflow: a) at the veno-a trial 

 junction; b) at the atrioventricular junction; c) at 

 the ventricle-arterial junction. 



I < no- A/rial Junction 



In adult mammals the sinus venosus is completely 

 incorporated in the atrium and there are only 

 remnants of "valves" which are incapable of prevent- 

 ing backflow into the superior caval vein (valvula 

 Eustachii) or in the coronary sinus (valvula Thebesii). 

 In the left atrium there is not even the most rudi- 

 mentary valve to prevent backflow in the pulmonary 

 veins. Nevertheless, there are some structures which 

 may contribute to prevent backflow from the atria, 

 such as circular muscle fibers around the pulmonary 

 veins and the coronary sinus, and a complex system 

 of more or less discrete muscular bundles around the 

 orifices of the caval veins. It has been speculated that 

 these fibers contract very early in atrial systole and 

 that by a narrowing of the orifices a valve-like or 

 sphincter-like action occurs [see also Kjellberg & 

 Olsson (91), Burch & Romney (28), Campeti et al. 

 (30)]. This would prevent or at least diminish back- 

 flow of blood from the atria into the venous trees at 

 the beginning of atrial systole. Although backflow 

 from the atria into the caval veins can be frequently 

 observed and quantitatively registered with flow- 

 meters (18) and contrast media (14), the amount of 

 backflow is surprisingly small as compared to the 

 amount of blood simultaneously pushed by the atria 

 into the ventricles. The precise mechanism by which 

 backflow is kept so small is still enigmatic (144). An 

 interesting light has been cast upon this problem 

 recently by Little (101). He determined pressure- 

 volume curves in the left atrium of dogs during tem- 

 porary ventricular asystole. His findings suggest that 

 upon a slight rise in atrial pressure above the pul- 

 monary venous pressure there is a closure of the 



pulmonary veins near their atrial junction. This 

 closure, apparently brought about by collapse of the 

 vein in a critical region, prevents regurgitation of 

 blood from the atrium into the pulmonary bed. 

 However, at high atrial pressures the closed segment 

 opens and blood flows into the pulmonary veins. 



Atrioventricular Valves 



The atrioventricular valves are funnel-shaped 

 structures inserted on a fibrous ring. They are de- 

 veloped as an ellipsoidal diaphragm and separated 

 by commissures into somewhat independent cusps, 

 the edges of which delineate the valvular orifice 

 (37, 38). The commissures do not extend all the 

 way to the valve ring (see fig. 3). Traditionally, one 

 distinguishes two cusps on the mitral valve and three 

 on the tricuspid valve, although both valves es- 

 sentially consist of two large opposite cusps and a 

 variable number of small intermediate cusps at 

 each end of the ellipse [see also Rusted et al. (140)]. 

 The strands of collagenous fibers known as chordae 

 tendineae extend from the papillary muscles either 

 to the free edge of the cusps (first order chords) or to 

 a few millimeters beyond the edge (second order 

 chords), or even quite far back into the substance of 

 the valve through a kind of ''goose foot" forked 

 insertion (third order chords). The anatomy of 

 papillary muscles is quite variable. One usually 

 recognizes in the right ventricle three groups of 

 papillary muscles to which the tricuspid valve is 

 fastened, whereas there are usually two such groups 

 to fulfill the corresponding function in the left ven- 

 tricle. The chords are of unequal length, so that 

 probably the same tension is exerted on each at the 

 time the valve closes. The chords from adjacent 

 regions of opposite cusps are inserted on the same 

 or adjacent papillary muscles, in order to insure 

 leakproof closure [see also Brandt (15), Hubacher 

 (82)]. 



The exact mechanism of closure of the atrioven- 

 tricular valves has been the subject of much debate 

 [Kantrowitz et al. (87)]. The old theory of closure 

 mainly by active contraction of the papillary muscles 

 has been abandoned, and the role of active contraction 

 of muscular fibers at the base of the valves just after 

 atrial systole is taken as either minor, or nonexistent 

 [see also Little (100)]. The decisive factor is prob- 

 ably the onset of ventricular contraction, which 

 establishes a higher pressure in the ventricle than 

 in the atrium. It can be shown that whenever the 

 ventricles begin to contract, there is a retrograde 



