Thoracic duct

The thoracic duct is the larger of the two tubes that carry lymph back to the blood. Lymph is the fluid that leaks out of the vessels into the tissues and has to be returned to the circulation. This duct handles almost the whole body; the right side of the head, the neck, and the chest, along with the right arm, drain through the other one.

It starts low, at the cisterna chyli, a lymphatic sac that sits at the level of the second lumbar vertebra, runs up through the chest, and ends high up, right where the left subclavian vein and the internal jugular vein come together.

To keep lymph moving, the duct does its part. Its middle layer is smooth muscle, the kind of muscle you cannot move at will, and it contracts regularly to push the fluid forward. And right where it empties into the vein there is a bicuspid valve, meaning one with two leaflets, that keeps venous blood from backing up into the lymphatic system: it works like the valve on a bicycle pump.