# Translational Echocardiography: The Dog as a Clinical Research Model of Cardiac Dysfunction
## 3 Cardiac Anatomical Similarities between the Human and the Dog
### Auricular Anatomical Coincidences  
Similar to humans, the right and left atria of the adult dog heart are separated by the interatrial septum a muscular wall that extends from the base of the heart to the atrioventricular valves. The interatrial septum is composed of two layers: the muscular layer, which is continuous with the myocardium of the atria, and the membranous layer, which is a thin fibrous membrane that covers the muscular layer [7; 11]. The cardiac base receives all the great vessels. It is generally cranially oriented and in mammals, including dogs, the orientation of the cardiac base is primarily determined by the animal's posture [9; 13]. Recent studies suggest that cardiac orientation also varies with age and other factors, such as body size and physical activity [18]. Additionally, advances in imaging technology have allowed for more detailed visualization and understanding of the complex anatomical structures of the heart [19]. Symmetrically to humans, during fetal development, the human heart is structured to allow blood to flow directly from the right to left atrium, bypassing the lungs. This occurs via the foramen ovale, which is a small opening in the interatrial septum. The foramen ovale has a valve-like flap on the left atrial side, which prevents blood from flowing back into the right atrium when the left atrium contracts. Once the newborn begins to breathe air, the pulmonary circulation is established, and the foramen ovale usually closes, becoming a small depression in the interatrial septum known as the fossa ovals [11; 19]. In adults, the fossa ovals is a slight depression on the right atrial side of the interatrial wall [13; 19]. Dogs have more posterior fossa ovals than humans [7].  
In large mammals, the venous sinus is integrated into the right atrium and marked by the sinoatrial node. Dogs have the same atrial architecture, which includes the venous sinus, crista terminalis, fossa ovals, and Eustachian valves (inferior vena cava and coronary sinus valves) [9; 13]. Normally, there is an anterior (cranial or superior) and a posterior (caudal or inferior) vena cava. The location of the vena cava ovstia entering the atrium may vary in some mammals [7; 11; 19]. Specifically, the inferior and superior vena cava ovstia enter almost at right angles in animal models of large mammals, whereas in humans, they enter the atrium almost in line [9; 11]. The inferior vena cava in domestic animals is usually long (>5 cm) in contrast to its short length in humans (1-3 cm) [7; 11]. In most species, the coronary sinus ostium is located in the posterior wall of the right atrium, but its location may vary slightly. There are also considerable species differences in the number of pulmonary veins entering the left atrium. In human hearts, there are four [9; 20; 21] or occasionally five pulmonary veins, whereas in dog hearts, there are five or six [11]. In large mammals, the atria are separated from the ventricles by a fibrous tissue called the cardiac skeleton. The cardiac or fibrous skeleton is composed of dense connective tissue and consists of four fibrous rings that encircle the four heart valves - the mitral valve, tricuspid valve, aortic valve, and pulmonary valve. These rings provide support for the heart valves and help to maintain their proper position. In addition to the fibrous rings, the cardiac skeleton also contains fibrous tissue that forms a barrier between the atria and ventricles. This barrier prevents the electrical signals that control the heart's contraction from spreading between the atria and ventricles, ensuring that the heart beats in a coordinated manner. The fibrous skeleton also serves as an attachment site for the heart's muscle fibers and for the connective tissue that surrounds the heart [11; 22].