This is located between the atria and the ventricles.
The atrioventricular node (AV node) is another node in the cardiac electrical conduction system. The cardiac action potential then spreads across both atria causing them to contract, forcing the blood they hold into their corresponding ventricles. This is a group of pacemaker cells which spontaneously depolarize to create an action potential. The sinoatrial (SA) node is located in the posterior aspect of the right atrium, next to the superior vena cava. With certain conditions, the left atrial appendage may be associated with risks of stroke from blood clot formation, because of which surgeons may choose to close it during open-heart surgery. The left pulmonary artery passes posterosuperiorly and is separated from the atrial appendage by the transverse sinus. The body of the left atrial appendage is anterior to the left atrium and parallel to the left pulmonary veins. The left atrial appendage can serve as an approach for mitral valve surgery. The left atrial appendage can be seen on a standard posteroanterior X-ray, where the lower level of the left hilum becomes concave. This appears to "function as a decompression chamber during left ventricular systole and during other periods when left atrial pressure is high". High in the upper part of the left atrium is a muscular ear-shaped pouch – the left atrial appendage. Left atrial appendage shown at upper right The interatrial septum separates the right atrium from the left atrium this is marked by a depression in the right atrium – the fossa ovalis. The right atrial appendage is a pouch-like extension of the right atrium and is covered by a trabecula network of pectinate muscles. Its base surrounds the superior vena cava. Looking from the front, the right atrial appendage appears wedge-shaped or triangular. The right atrial appendage is located at the front upper surface of the right atrium. Attached to each atrium is an atrial appendage. The sinus venarum is the adult remnant of the sinus venosus and it surrounds the openings of the venae cavae and the coronary sinus. Internally, there are the rough pectinate muscles and crista terminalis of His, which act as a boundary inside the atrium and the smooth-walled part of the right atrium, the sinus venarum, which are derived from the sinus venosus. The atria do not have valves at their inlets, and as a result, a venous pulsation is normal and can be detected in the jugular vein as the jugular venous pressure. The right atrium and right ventricle are often referred to as the right heart similarly, the left atrium and left ventricle are often referred to as the left heart. The left atrium receives the oxygenated blood from the left and right pulmonary veins, which it pumps to the left ventricle (through the mitral valve (left atrioventricular valve) for pumping out through the aorta for systemic circulation. The right atrium receives and holds deoxygenated blood from the superior vena cava, inferior vena cava, anterior cardiac veins, smallest cardiac veins and the coronary sinus, which it then sends down to the right ventricle (through the tricuspid valve), which in turn sends it to the pulmonary artery for pulmonary circulation. Humans have a four-chambered heart consisting of the right atrium, left atrium, right ventricle, and left ventricle.
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