The inferior vena cava is a vein. Structures such as the aorta, inferior vena cava and esophagus pass through the diaphragm. Invasive tumors can produce compression effects such as superior vena cava syndrome. Pressure in the portal vein and inferior vena cava are often measured. Sick sinus syndrome is also associated with azygos continuation of interrupted inferior vena cava. The right phrenic nerve may be crushed by the vena cava clamp during liver transplantation. It branches from the abdominal aorta and returns blood to the ascending vena cava. Rupture can also create a connection between the aorta and intestine or inferior vena cava. Blood drains from the kidneys, ultimately into the inferior vena cava. The brachiocephalic veins are the major veins returning blood to the superior vena cava. Other helpful measurements include cardiac output and superior vena cava oxygen saturation. They connect the kidney to the inferior vena cava. Sometimes the veins end directly in the superior vena cava. The tip of the catheter is positioned in the superior vena cava. Behind rests the inferior vena cava, and the common bile duct.