The right heart receives systemic venous return from the cranial and caudal venae cavae; on the dorsal aspect, the junction of the venae cavae with the right atrium is marked by a groove (the sulcus terminalis). Blood is diverted from the cava into the right atrium due to a crest of tissue (intervenous tubercle). The fossa ovalis, a remnant of the foramen ovale, forms an oval-shaped depression within the dorsal aspect of the interatrial septum. This structure will be probe-patent, but functionally closed in situ, until several weeks after birth. The right atrial appendage is roughly triangular with an undulating inner muscular surface (pectinate muscles). The coronary sinus, receiving blood from the coronary veins, is present in the dorsal aspect of the right atrium at the atrioventricular junction proximal to the septal leaflet of the tricuspid valve. The tricuspid/right atrioventricular valve separates the right atrium and ventricle. As the name suggests, this valve typically has three cusps (septal, parietal, and angular, see Figure 6.3); however, in dogs, the septal (dorsal) and parietal (ventral) leaflets can be fused, thus appearing as the valve only has two cusps. In some cases, small secondary cusps are located at the extremities of these two large cusps. The free edges of valve leaflets are supported by fibrous chordae tendineae extending to broad trabecular papillary muscles or to the septum. The lumen of the right ventricle is crescent shaped in cross-section with a muscular strut, the trabecula septomarginalis (moderator band), passing through the lumen from the free wall to the interventricular septum and carrying the right bundle branch of the conduction system. The apical ventricular lumen is lined by muscular bundles or trabeculae (trabeculae carneae). The pulmonary artery is rather prominently positioned on the ventral heart base to the left of the aorta; the gateway to the pulmonary circulation is the pulmonic semilunar valve, which has three cusps: right, left, and intermediate. The crista supraventricularis, an ultrasound landmark for the RV inflow and outflow tracts, is a muscular shelf along the septal curvature separating the right AV and pulmonic valves.

The number of pulmonary veins varies by species (see Table 6.1). The mitral valve/left AV valve is composed of two leaflets: a larger, saddle-shaped septal cusp and a smaller parietal cusp. Both are attached via chordae to prominent anterior and posterior papillary muscles (Figure 6.4). The septal cusp is also connected to the aortic valve and contributes to the left ventricular outflow tract. The interventricular (IV) septum separates the left and right ventricle but is functionally part of the left ventricle. The aorta emerges from the center of the heart. The ostia of coronary arteries are located in dilations of the proximal aorta (sinuses of Valsalva) adjacent to the cusps of the aortic semilunar valve. It is expected to have two (right and left) coronary and one noncoronary cusps. Nodular thickenings are present in the center of the free edge of the aortic semilunar valve leaflets (Nodules of Arantius), which may enhance valve coaptation.

The vascular or circulatory system conducts blood from the heart (arterial system), systemically through the tissues, and then back to the heart (venous system); it encompasses the arteries, arterioles, capillaries, postcapillary venules, and veins. With few exceptions, the arterial system carries oxygenated blood (exceptions: pulmonary arteries, umbilical arteries), while veins carry deoxygenated blood (exceptions: pulmonary vein, umbilical vein). In general, blood vessels have a basic three-layered structure: the inner most tunica intima, which supports the endothelium, the tunica media composed of variable smooth muscle cells and connective tissue fibers, and the tunica adventitia that blends into adjacent connective tissue. The thickness or existence of individual layers depends on pressure within the vessel lumen.

The trilaminar architecture is most apparent in arteries. There are several types of arteries: elastic arteries, muscular arteries (which branch into successively smaller diameter), and the arterioles. The tunica media of elastic arteries contains amounts of elastin and collagen that allow for elastic recoil and the conduction of blood. Elastic arteries are the largest in the body and have already been mentioned during the discussion of the heart: the pulmonary artery and the aorta. Muscular arteries are also known as distributing arteries with a tunica media that includes a more prominent spiraled layer of smooth muscle. Arterioles are the smallest arteries and give rise to microscopic, thin-walled capillaries where nutrient, waste, and gas exchanges occur.

Venules collect blood from capillary beds and merge to form larger veins that drain into the right atrium. Veins have valves which prevent the back flow of blood in this low-pressure system. On cross-section, veins have a thinner wall to luminal diameter versus a companion artery.

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