Often called lymphatics, lymphatic vessels include lymphatic capillaries, passageways, nodes, valves, and others.
Like the blood vessels, lymphatic vessels form an extensive network with a distribution similar to that of veins. Every organ in the body has an ample supply of lymphatic vessels, which pick up tissue fluid and transport it to the heart.
Because of the lymphatic valves, lymphatic vessels form a one-way path which prevents any backflow of lymph. Therefore, lymph can only move toward the heart.
Flow of lymph through the lymphatic vessels
The walls of the lymphatic capillaries are made up of a single layer of epithelium or endothelium. Both have large holes or pores which allow the lymphatic capillaries to drain tissue fluid and proteins in order to form lymph.
Once absorbed by the lymphatic capillaries, the lymph flows toward the heart through a series of ever-increasing lymph vessels until it reaches the two lymphatic ducts.
The thoracic duct collects lymph from the lower abdomen, pelvis, and lower limbs, as well as from the left half of the head, neck, and chest. It empties its collected lymph into the venous system near the junction between the left internal jugular vein and the left subclavian vein.
The smaller right lymphatic duct ends on the right side and delivers lymph from the right side of the body above the diaphragm.
Movement of lymph is dependent on the movement of lymphatic vessels. It is not pumped through as blood is from the heart. Lymph moves in response to the following actions:
- milking of skeletal muscles: As the muscles contract, they squeeze the surrounding lymphatic vessels, pushing lymph toward the heart.
- movement of the chest during respiration: The contraction and relaxation of the chest muscles causes changes in pressure within the thorax, which affects the flow of lymph through the vessels.
- rhythmic contraction of lymphatic vessels smooth muscles: The alternating contraction and relaxation of the smooth muscles cause lymph to flow.
