The<unk>  (or<unk>  is the large vein that carries de-oxygenated blood from the lower half of the body into the heart.
It is posterior to the abdominal cavity and runs along side of the vertebral column on its right side (i.e. it is a retroperitoneal structure). It enters the right atrium at the lower right, back side of the heart.
Inferior vena cava
Superior vena cava
Thoracic aorta
Coronary sinus
Descending aorta
Aortic sinus
Ascending aorta
Subclavian vein