A lymphangion is the functional unit of a

lymph vessel The lymphatic vessels (or lymph vessels or lymphatics) are thin-walled vessels (tubes), structured like blood vessels, that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the cardiovascular system. Lymph vessel ...

that lies between two semilunar (half moon-shaped)

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or Slurry, slurries) by opening, closing, or partially obstructing various passageways. Valves are technically Pip ...

s. Lymph vessels are channels larger than the lymph capillaries that have thicker walls, valves in their lumen and smooth muscles in their walls, thus lymph vessel lymphangion is muscular and capable of contracting on its own. Additionally the lymph in it is propelled forward only because of force exerted on its walls from the exterior. Such forces include

skeletal muscle Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...

contractions and

arterial An artery () is a blood vessel in humans and most other animals that takes oxygenated blood away from the heart in the systemic circulation to one or more parts of the body. Exceptions that carry deoxygenated blood are the pulmonary arteries in ...

pulsations. Also, the inspiration during

respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellul ...

provides a suction pressure within the lumen. The semilunar valves are directed towards the flow of the lymph and open when the pressure in the first lymphangion is greater than the pressure in the next lymphangion. Pressure in the first lymphangion may increase because of smooth muscle contraction (in lymph vessel) or because of pressure on the walls from outside (in a capillary) result because of. Alternatively, pressure within the next lymphangion may decrease because of negative pressure as a result of inspiration. Once the lymph flows into the next lymphangion, it cannot return to the previous lymphangion, as the semilunar valves close tightly. In conditions when the pressure in a lymphatic is sufficiently great, the valves may fail, and there can indeed be backward flow of lymph resulting in

edema Edema (American English), also spelled oedema (British English), and also known as fluid retention, swelling, dropsy and hydropsy, is the build-up of fluid in the body's tissue (biology), tissue. Most commonly, the legs or arms are affected. S ...

of the drained region. This may happen with blockage of lymph flow because of pathology in the draining

lymph node A lymph node, or lymph gland, is a kidney-shaped organ of the lymphatic system and the adaptive immune system. A large number of lymph nodes are linked throughout the body by the lymphatic vessels. They are major sites of lymphocytes that includ ...

or at some point in the vessel.

References

{{Lymphatic flow Lymphatic system