The lymphatic system has neither a heart nor arteries. Its microscopic dead-end capillaries extend into most tissues, paralleling the blood capillaries.

The lymphatic circulation is a drainage system. Its job in maintaining fluid balance is to:

	collect excess interstitial fluid and return it to the blood (approximately 3 litres daily).
	return plasma proteins to the blood.

Once interstitial fluid enters a lymph capillary, it is referred to as lymph.

The three main types of lymphatic vessels are lymph capillaries, lymphatics, and lymph ducts.

Lymph capillaries are microscopic tubes located between cells. Lymph capillaries resemble blood capillaries somewhat, but differ in important ways. Whereas a blood capillary has an arterial and a venous end, a lymph capillary has no arterial end. Instead, each lymph capillary originates as a closed tube. Lymph capillaries also have a larger and more irregular lumen (inner space) than blood capillaries and are more permeable.

The wall of a lymph capillary is constructed of endothelial cells that overlap one another.

When fluid outside the capillary pushes against the overlapping cells, they swing slightly inward--like a swinging door that moves in only one direction. Fluid inside the capillary cannot flow out through these openings.

This animation of a section through a lymph capillary shows how pressure in the interstitial fluid surrounding the capillary pushes open the overlapping cells.

The arrows represent the direction of flow of the lymph.

Note the internal valve which allows the lymph to flow in one direction only.

Lymph capillaries branch and interconnect freely and extend into almost all tissues of the body except the CNS (Central Nervous System) and the avascular tissues such as the epidermis and the cartilage.

Lymph capillaries join to form larger vessels called lymphatics or lymph veins. These resemble blood-conducting veins but have thinner walls and relatively larger lumen, and they have more valves. In the skin, lymphatics are located in subcutaneous tissue and follow same paths as veins. In the viscera, lymphatics generally follow arteries and form plexuses (networks) around them.

At certain locations lymphatics enter lymph nodes. These are structures that consist of lymphatic tissue.

As the lymph flows slowly through the lymph sinuses within the tissue of the lymph node, it is filtered. Macrophages remove bacteria and other foreign matter as well as debris.

Lymphocytes are added to the lymph as it flows through the sinuses of a lymph node. Thus the lymph leaving the node is both cleaner of debris and richer in lymphocytes. Lymphatics leaving lymph nodes are called efferent lymph vessels and conduct lymph toward the shoulder region. Large lymphatics that drain groups of lymph nodes are often called lymph trunks.

Lymphatics from the lower portion of the body converge to form a dilated lymph vessel, the cisterna chyli, in the lumbar region of the abdominal cavity. The cisterna chyli extends for about 6 centimetres just to the right of the abdominal aorta. At the level of the twelfth thoracic vertebra, the cisterna chyli narrows and becomes the thoracic duct.

Lymphatic vessels from all over the body, except the upper right quadrant, drain into the thoracic duct. This vessel delivers the lymph into the base of the left subcIavian vein at the junction of the left subcIavian and internal jugular veins. In this way lymph is continuously emptied into the blood where it mixes with the plasma. At the junction of the thoracic duct and the venous system, a valve prevents blood from flowing backward into the duct.

Only about 1 centimetre in length, the right lymphatic duct receives lymph from the lymphatic vessels in the upper right quadrant of the body. The right lymphatic duct empties lymph into the base of the right subclavian vein (at the point where it unites with the internal jugular vein to form the brachiocephalic)

An example of the pattern of lymph circulation is:

Lymph capillaries lymphatic  lymph node  lymphatic cisterna chyli thoracic duct

Lymphatic vessels and lymph nodes can be visualised by the process of lymphangiography. A radiopaque (not transparent to x-rays) contrast material is injected into the a lymphatic vessel. This will show up the vessel and it’s connections to other lymph vessels. The fluid is left in the system for 24 hours and the lymph nodes can then be observed by X-rays. This technique is quite important in the treatment of neoplasms and other disorders of the lymphatic system. The technique is also used to locate lymph nodes for radiation therapy or for surgical removal.