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The tricuspid and bicuspid valves are atrioventricular valves. Both are valves that prevent backflow of blood pumped from the atria to the ventricles. However, the former has 3 "flaps" while the latter has 2.

Despite searching the internet, I could not find a biological explanation for this difference.

Might this be just a relic of evolution or some sort?

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During embryogenesis at the level of an atrioventricular canal there is so called endocardial "cushion" tissue. This tissue bulges to the left side more than it does to the right and contribute to the fusion of the valvular ring to a one leaflet, while from the remaining part of the ring develops the second leaflet:

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AVEC = atrioventricular endocardial cushions

As has been stated, the cushion tissue bulges more to the left, thus on the right side where the tricuspid valve is forming the leaflets does not "merge" and develop into three-leaflet valve.

Interestingly, this left side "suture" might sometimes fail to develop with resulting three-leaflet mitral valve.

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