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 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 before draining into the two lymphatic ducts: thoracic duct and right lymphatic duct, on the left and right sides of the body respectively ¹⁰.

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 ^3,10. 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 abutting 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 merge to form the lymphatic trunks.

Lymphatic trunks

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

• jugular trunks: drains the neck

• subclavian trunks: drains the upper limbs

• bronchomediastinal trunks

• lower intercostal trunks

• lumbar trunks

• intestinal trunk

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 specially named - due to their unique function in absorbing and transporting fats - and are called 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. Recent work demonstrates that the brain has a lymphatic system, the glymphatic pathway, however no true lymphatic vessels have been found. Meninges also have a 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.

See also

• glymphatic system
• lymph nodes

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 ^3,10. 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 abutting 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.

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 specially named - due to their unique function in absorbing and transporting fats - and are called 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. Recent work demonstrates that the brain has a lymphatic system, the glymphatic pathway, however no true lymphatic vessels have been found. Meninges also have a 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.

See also

• glymphatic system
• lymph nodes