The atrioventricular valve in the animal kingdom

All animals with a vertebral column have a heart with an atrium and a ventricle, and the atrioventricular junction is guarded by a valve of leaflets of connective tissue. The atrioventricular valve, however, exhibits pronounced variation in morphology and in size relative to the ventricular base. When we compare a broad range of vertebrates, from jawless fishes to human, mapped onto a state-of-the-art phylogeny, we can infer plausible anatomical and physiological causes of this variation. Thus, the evolution of breathing with lungs introduced a circulation in parallel with the systemic circulation, and the atrioventricular valve increased concomitantly in size. The setting of full atrial septation is always associated with the merger of the dorsal and ventral atrioventricular valvar leaflets, yielding a left valve that resembles the human aortic leaflet. Full ventricular septation always associates with the formation of large parietal leaflets, and a pronounced expansion of the atrioventricular junction, particularly rightwards. In the left atrioventricular junction, this results in a left valve that closes in bifoliate fashion. The evolution of warm-bloodedness, and a much elevated cardiac output, in mammals and birds associates with the formation of compact ventricular walls and large papillary muscles with multiple tendinous cords. The mitral valve, furthermore, retains its likeness to the human arrangement both in animals with extremely high left ventricle blood pressure, as in the giraffe, and extremely large stroke volume, as in the elephant. This suggests its common bifoliate configuration is efficient across a broad range of physiological settings.