The hearts of large mammals have four cardiac valves with similar structures and locations. Left and right atrioventricular valves sit between the atria and ventricle on both sides of the heart while two semilunar valves separate the right and left ventricles from the pulmonary artery and aorta respectively. While the structures of each valve are largely similar, variations in the valves exist that distinguish the valves of different species.

(terms in parenthesis describe attitudinally correct terms for describing the human heart.)

Tricuspid Valve
Named for its three major cusps, this valve separates the right atrium from the right ventricle. Papillary muscles attach to the cusps via chordae tendinae and prevent the prolapsed of leaflets into the atria. Interspecies and interindividual differences exist in the total number of papillary muscles supporting the valve. Canine tricuspid valves are noticeably different from those of humans, swine and sheep in that the commissures between the anteriosuperior leaflet and the inferior leaflets can be fused, giving the appearance of having only two cusps (9). While both the anterior (superior) to posterior (inferior) diameter and the septal (posterior) to lateral (anterior) diameter are largely similar between humans and these model animals, the anterior to posterior diameter is noticeably larger in humans compared to swine.

Mitral (Bicuspid) Valve
Named for its two major cusps, this valve separates the left atrium from the left ventricle. Aside from the two major cusps that are always present, a variable number of small commissural cusps, also called scallops, exist in large mammals giving the impression of extra leaflets (3). Differences also exist between species in the composition of the leaflets and in the fibrous ring that supports the mitral valve. For example, a segment of the fibrous ring is visible in humans at the base of the mural cusp however; this structure is difficult to distinguish in certain breeds of dogs and is inconspicuous in sheep (25). Regarding the size of the Mitral valve, the anterior (superior) to posterior (inferior) diameter of the mitral valve is similar in length between humans, canines, swine and sheep. The septal (posterior) to lateral (anterior) diameter is significantly longer in humans compared to the animal models, which have similar measurements.

Aortic Valve
The aortic valve separates the left ventricle from the aorta, preventing the backflow of blood into the heart during ventricular relaxation. The valve is formed by three cusps, each one evenly spaced around the valve's circumference. Differences in leaflet shape and structure exist between species. For example, the noncoronary cusp of swine is noticeably smaller than its other two, while the three cusps of human, canine and sheep aortic valves are largely uniform in size (26). Another significant anatomical difference is in the aortic valves of sheep where the cusps are substantially thinner compared to those of humans, swine or canine (26). Differences also exist in the structural support around the valve. Notably, humans have much less muscular attachment surrounding the valve compared to animal hearts (26). Human aortic valves also tend be have a wider diameter compared to the animal hearts here and the canine aortic valve is notably smaller in diameter than swine or sheep, which are similar in size (1).

Intervalvar septum (membranous septum)
The intervalvar septum is a fibrous continuity between the mitral and aortic valves that extends from the central fibrous body to the left fibrous trigone (3). The length of the septum varies between species but is absent in sheep (25).