20 - thoracic duct

The thoracic duct

This main lymphatic duct conveys all the lymph from the lower limbs, pelvis, abdomen, left side of the thorax and head and neck and left upper limb to the venous system; that is, all but the right superior quadrant of the body.

It drains the cisterna chyli, which is an elongated sac that lies on the right side of the bodies of the 1^st and 2^nd lumbar vertebrae below the diaphragm.

The thoracic duct passes superiorly from the cisterna chyli through the aortic hiatus in the diaphragm on the right side of the aorta. In the posterior mediastinum, the thoracic duct ascends among the thoracic aorta on its left, the azygos vein on its right, the esophagus anteriorly, and the vertebral bodies and the origins of the right posterior intercostal arteries posteriorly.

At about the level of the sternal angle the thoracic duct deviates to the left, posterior to the esophagus and ascends through the superior mediastinum into the root of the neck to empty into the venous system at the union of the left internal jugular and subclavian veins.

The duct has a number of valves, which prevent lymph flowing downwards and give it a beaded appearance.

The cysterna chyli receives lymph from the lower limbs, pelvis and abdomen. The thoracic duct receives the left bronchomediastinal trunk in the thorax, and in the neck the left jugular and left subclavian lymph trunks. All these trunks can open independently into the left internal jugular or subclavian vein.

Because the thoracic duct is thin walled and may be colorless, it may be difficult to identify during surgical procedures. If the thoracic duct is cut or torn during an accident, chyle (L. chylus, juice) escapes from it. This milky fluid contains a considerable amount of fine fat droplets. Leakage of chyle may be prevented by tying off the thoracic duct. The lymph then returns to the venous system by other lymph channels, which join the duct superior to the ligature. During digestion the thoracic duct is distended with chyle, a product of the small intestine.

A brachiocephalic venous catheterization is inadvisable on the left side since the insertion of a needle into the angle formed by the confluence of the internal jugular and subclavian veins might well encounter the terminal portion of the thoracic duct. The brachiocephalic route for a central venous catheterization on the left-hand side has been known to cause a chylothorax.