Which structure is the arrow pointing to in this video?

The arrow points to the coronary sinus, which is a venous structure located posteriorly in the atrioventricular groove, emptying into the right atrium. It appears as a circular anechoic structure near the left atrium in echocardiographic images.

Left lower pulmonary vein enters the left atrium more superiorly. Descending aorta is posterior to the heart but not in this location. Left atrial appendage is an anterior finger-like projection of the left atrium, separate from the coronary sinus.

This anatomy is described in the "Textbook of Clinical Echocardiography, 6e", Chapter on Cardiac Venous Anatomy【20:140-145†Textbook of Clinical Echocardiography】.