The canine circulatory system is much like that of humans. The heart has four chambers. The two upper chambers are the left and right atria, while the stronger, lower chambers are the right and left ventricles.
Blood travels from the tissues and enters the right atrium.
From there it moves into the right ventricle. The right ventricle pumps blood from the body into the lungs to exchange carbon dioxide produced for oxygen. Blood rich in oxygen leaves the lungs through the pulmonary vein and enters the left atrium. The left atrium pumps blood to the left ventricle. The left ventricle is the most heavily muscled and therefore, strongest of the chambers.