Thoracic duct, thaw RAS ihk, thoh RAS ihk, or thuh RAS ihk, is the largest vessel of the lymphatic system, a network of tubes that returns fluid from body tissues to the bloodstream. The thoracic duct passes through the thoracic cavity, which lies in front of the spinal column. The duct empties a clear fluid called lymph and a milky-white fluid called chyle into the bloodstream through the left subclavian vein, a large vein lying under the collarbone. Eventually lymph from all but the upper right quarter of the body reaches the thoracic duct.