Lymphatic system

The lymphatic system (also known as the lymphoid system or systema lymphoideum in Terminologia Anatomica) 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 tissue 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.

The lymphatics start as lymphatic capillaries, which coalesce to form larger precollectors and then collector lymphatic vessels. These collecting vessels merge to form lymphatic trunks, before draining into the two largest trunks a.k.a. the lymphatic ducts: thoracic duct and right lymphatic duct, on the left and right sides of the body respectively ¹⁰.

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

Lymphatic capillaries

The lymphatic capillaries (a.k.a. initial lymphatics or, less commonly, terminal 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.

Precollecting and collecting lymphatics

The lymphatic capillaries converge to form the collecting lymphatics (or collectors) which have both valves and smooth muscle, with walls similar to veins, although unlike the venous wall, the various tunica layers are harder to distinguish. The number of valves is much greater than in veins.

In some lymphatic networks, the capillaries merge to form precollector lymphatics first, which are like the capillaries in that they have a simple single-layer endothelial wall, but are distinguished from them by the presence of valves, but no smooth muscle layer. 

Smooth muscle in the walls of the collectors creates peristaltic waves ensuring unidirectional flow of the lymph ^3,10. A segment of lymphatic vessel between two valves is termed a lymphangion(plural: lymphangia), a contractile unit that ensures the lymph moves to the next lymphangion through their shared interposing valve.

This one-way flow is also supported by skeletal muscle action, the inspiratory decrease in intrathoracic pressure and a pressure gradient towards the veins into which the largest lymphatics drain (due to the suction effect formed as the veins continuously drain towards the heart) ³.

The collecting lymphatics then pass into the cortex of local lymph nodes as the prenodal collecting or afferent lymphatics(prenodal collecting lymphatics). Thepostnodal collecting or efferent lymphatics (postnodal collecting lymphatics) leave the nodes, either passing to further nodes in the chain or merging with other efferent lymphatic vessels to form the lymphatic trunks.

Lymphatic trunks

There are nine named lymphatic trunks in total, comprising four paired sets (one trunk for each side of the body) and the unpaired central intestinal trunk:

• jugular trunks: drainsdrain the neck
• subclavian trunks: drainsdrain the upper limbs
• bronchomediastinal trunks: drainsdrain the chest
• lumbar trunks
• intestinal trunk: drains the gut

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 microvilli of the gut wall are specially named - due to their unique function in absorbing and transporting fats - and are called lacteals.

Alymphatic organs

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.

** recent work demonstrates that the brain has a lymphatic system, the glymphatic pathway, however, no true lymphatic vessels have been found.The meninges do have a lymphatic system ⁸.

For many years there was controversy surrounding the presence of a lymphatic drainage system in the eye. More recently the presence of lymphatics in the eye seems to have become accepted, although, many of the details remain unknown ⁷.

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)

History and etymology

The written historical record likely first describes lymphatics in Classical Greece, when both Hippocrates (c.460-c.370 BC) and Aristotle (c.384-c.322 BC) wrote about channels that were likely lymphatic in nature ¹. In ancient Egypt, Erasistratus (c.304- c.250 BC) detailed "milky arteries" in the mesenteric fat ¹. 

In 1622, the Italian anatomist Gaspare Aselli (1581–1625) was the first to describe lymphatics in the modern era. He was actually researching the recurrent laryngeal nerves in canines when he noted some new white cords in the mesentery that filled after dogs had been fed ^10,11. Thomas Bartholin (1616–1680), the Danish anatomist, coined the term "vasae lymphaticae", translated as lymphatics, for these vessels, and was the first to distinguish between the chyle-carrying lymphatics draining the GI tract and the systemic lymphatics draining everything else ¹¹. By the end of the 18th century the general arrangement of the lymphatics had been elucidated ^10,11.

Related pathology

• lymphoedema