Lymphatic system

The lymphatic system (also known as the lymphoid system or systema lymphoideum in TA) is the collective term given to the lymphatic vessels and lymphoid tissues in the body ^1,4.

Terminology

Occasionally the lymphatic system is considered with the reticuloendothelial system (RES), with the combined whole termed the lymphoreticular system.

Lymphatic vessels

A portion of the blood entering any capillary bed does not return via the veins, but instead forms interstitial fluid in the extracellular space, which is returned to the circulation via the lymphatic vessels (a.k.a. lymphatics), which form the lymph vascular system. When the fluid is in the lymphatic system it is known as lymph. It is known as chyle when the lymph draining the gut also contains chylomicrons from a meal.

Lymphatic capillaries

The lymphatic capillaries (or initial lymphatics) are the smallest vessels in the system and arise in the tissue capillary beds as blind-ending single layer endothelial tubes. Compared to capillaries, the lymphatic capillaries are relatively dilated with greater variability in calibre. Dynamic collapse of the capillaries is prevented by anchoring filaments binding their walls to the surrounding structural tissues ⁵.Ultrastructurally, the lymphatic endothelium of the capillaries lacks fenestrations with a rather sparse/absent basal lamina. Tight junctions are absent between individual cells of the endothelium ^3,4. These cytological features allow larger macromolecules to pass with ease from the extracellular space to the lymphatics, which includes proteins, cellular debris and microorganisms.

Collecting lymphatics

As the lymphatics enlarge, their walls become more like veins, although unlike the venous wall, the various tunica layers are harder to distinguish. Like veins, these collecting lymphatics also have valves, but in a much larger number. Smooth muscle in their walls creates peristaltic waves ensuring unidirectional flow of the lymph ³. This flow is also supported by skeletal muscle action, inspiratory decrease in intrathoracic pressure and a pressure gradient towards the veins into which they drain ³.

Finally, the lymph returns to the main systemic circulation via the thoracic duct on the left, and on the other side of the body, the right lymphatic duct.

For the most part, subcutaneous lymphatic vessels accompany veins, whilst deeper lymphatics accompany arteries ^2-4. The initial lymphatics in the villi of the gut wall are known as lacteals.

Lymphatic vessels are present throughout the body with the following exceptions:

• cartilage
• bone marrow
• bone*
• placenta
• cornea
• brain
• teeth

*the presence of lymphatic vessels in the bone is contentious, and if present is much sparser than other body tissues ^2,6.

Until recently this list would Recent work demonstrates that the brain has a lymphatic system, the glymphatic pathway, however no true lymphatic vessels have been found. Meninges also have included the CNS, but recent work shows that both the brain (glymphatic pathway) and meninges have their own lymphaticsa lymphatic system ⁸.

For many years there was controversy surrounding the presence of a lymphatic drainage system in the eye, although this now seems to have been settled although many of the details remain unknown ⁷.

Overall lymph flow is estimated to be 2-4 litres per day ³.

Functions

The primary function of the lymphatic system is to return exuded capillary fluid and protein to the systemic circulation. Approximately 25-50% of the plasma proteins re-enter the central veins every day from the lymph ³. In the GI tract, the draining lymphatics have a key role in transporting cholesterol and long chain fatty acids (as chylomicrons) to the liver.

Radiographic features

Visualisation of the lymphatic system by medical imaging has lagged behind radiology of many other body systems due to the small calibre of many of the lymphatics ⁹. Methods employed:

• direct radiographic lymphangiography
  • extremity lymphangiography
• lymphoscintigraphy
  • extremity lymphoscintigraphy
• indocyanine green lymphangiography
• MR lymphangiography (MRL)

See also

• glymphatic system
• lymph nodes

See also

• glymphatic system
• lymph nodes