The lymphatic system is a network of vessels, lymph, nodes and organs.

The lymphatic system is the unsung hero of the human body. It is one of our primary systems of detoxification; it also helps protect the body from infection

The lymphatic system consists of: 

  1. a complex network of lymphatic vessels; 

  2. lymph, the fluid contained in those vessels; 

  3. lymph nodes that cleanse the lymph as it passes through them; and 

  4. lymph-related organs - the tonsils, the spleen, the thymus gland, the vermiform appendix, and Peyer’s patches.

Inasmuch as the lymph nodes form part of the lymphoid organs and tissues, the structures and functions of the lymphatic system overlap with those of the lymphoid organs and tissues. In addition to lymph nodes, the lymphoid organs and tissues include the spleen, thymus, tonsils, and other lymphoid tissues scattered throughout the body.
 

The lymphoid organs house phagocytic cells and lymphocytes, which play essential roles in the body’s defense mechanisms and its resistance to disease. Together, the lymphatic system and the lymphoid organs and tissues provide the structural basis of the immune system. 

As blood circulates through the body, nutrients, wastes, and gases are exchanged between the blood and the interstitial fluid. The hydrostatic and colloid osmotic pressures operating at capillary beds force fluid out of the blood at the arterial ends of the beds (“upstream”) and cause most of it to be reabsorbed at the venous ends (“downstream”). The fluid that remains behind in the tissue spaces, as much as 3 L daily, becomes part of the interstitial fluid. 

This leaked fluid, plus any plasma proteins that escape from the bloodstream, must be carried back to the blood to ensure that the cardiovascular system has sufficient blood volume to operate properly. This problem of circulatory dynamics is resolved by the lymphatic vessels, or lymphatics, an elaborate system of drainage vessels that collect the excess protein-containing interstitial fluid and return it to the bloodstream. Once interstitial fluid enters the lymphatics, it is called lymph.

Lymphatic Vessels

Lymphatic Vessels - by Sky-David

Watercolor by Sky-David

The lymphatic vessels form a one-way system in which lymph flows only toward the heart. This transport system begins in microscopic blind-ended lymphatic capillaries. These capillaries weave between the tissue cells and blood capillaries in the loose connective tissues of the body. Lymphatic capillaries are widespread, but they are absent from bones and teeth, bone marrow, and the entire central nervous system (where the excess tissue fluid drains into the cerebrospinal fluid). 

Although like blood capillaries, lymphatic capillaries are so remarkably permeable that they were once thought to be open at one end like a straw. We now know that they owe their permeability to two unique structural modifications: 

 

1. The endothelial cells forming the walls of lymphatic capillaries are not tightly joined. Instead, the edges of adjacent cells overlap each other loosely, forming easily opened, flaplike mini-valves. 

 

2. Collagen filaments anchor the endothelial cells to surrounding structures so that any increase in interstitial fluid volume opens the mini-valves, rather than causing the lymphatic capillaries to collapse. 

 

This system is analogous to one-way swinging doors in the lymphatic capillary wall. When fluid pressure in the interstitial space is greater than the pressure in the lymphatic capillary, the mini-valve flaps gape open, allowing fluid to enter the lymphatic capillary. However, when the pressure is greater inside the lymphatic capillary, the endothelial mini-valve flaps are forced closed, preventing lymph from leaking back out as the pressure moves it along the vessel. 

 

Proteins in the interstitial space are unable to enter blood capillaries, but they enter lymphatic capillaries easily. In addition, when tissues are inflamed, lymphatic capillaries develop openings that permit uptake of even larger particles such as cell debris, pathogens (microorganisms such as bacteria and viruses), and degenerative and cancer cells. The pathogens can then use the lymphatics to travel throughout the body. This threat to the body is partly resolved by the fact that the lymph takes “detours” through the lymph nodes, where it is cleansed of debris and “examined” by cells of the immune system. 

Highly specialized lymphatic capillaries called "lacteals" are present in the fingerlike villi of the intestinal mucosa. The lymph draining from the digestive viscera is milky white rather than clear because the lacteals play a major role in absorbing digested fats from the intestine. This fatty lymph, called "chyle", is also delivered to the blood via the lymphatic stream.

 

From the lymphatic capillaries, lymph flows through successively larger and thicker-walled channels—first collecting vessels, then trunks, and finally the largest of all, the ducts. The lymphatic collecting vessels have the same three tunics as veins, but the collecting vessels are thinner walled, have more internal valves, and anastomose more. In general, lymphatics in the skin travel along with superficial veins, while the deep lymphatic vessels of the trunk and digestive viscera travel with the deep arteries. The exact anatomical distribution of lymphatic vessels varies greatly between individuals, even more so than it does for veins. 

The lymphatic trunks are formed by the union of the largest collecting vessels and drain large areas of the body. The major trunks, named mostly for the regions from which they collect lymph, are the paired lumbar, broncho-mediastinal, subclavian, and jugular trunks, and the single intestinal trunk. 

Lymph is eventually delivered to one of two large ducts in the thoracic region. The right lymphatic duct drains lymph from the right upper limb and the right side of the head and thorax. The much larger left thoracic duct receives lymph from the rest of the body. It arises anterior to the first two lumbar vertebrae as an enlarged sac, the cisterna chyli, that collects lymph from the two large lumbar trunks that drain the lower limbs and from the intestinal trunk that drains the digestive organs.

 

As the thoracic duct runs superiorly, it receives lymphatic drainage from the left side of the thorax, left upper limb, and the head region. Each terminal duct empties its lymph into the venous circulation at the junction of the internal jugular vein and subclavian vein on its own side of the body.

Distribution and Structure of Lymphatic Vessels

Structure of Lymphatic Vessels - by Sky-David

Watercolor by Sky-David

Lymph Flow

There's a specialist for everything: cardiologists, pulmonologists, dermatologists, etc. But there's no 'lymphologist'.

And why is that?

Because there's no pharmaceutical that supports the lymphatic system. The way you support it is through movement.

The lymphatic system lacks an organ that acts as a pump. Under normal conditions, lymphatic vessels are low-pressure conduits, and the same mechanisms that promote venous return in blood vessels act here as well - the milking action of active skeletal muscles, pressure changes in the thorax during breathing, and valves to prevent backflow.

 

Lymphatics are usually bundled together in connective tissue sheaths along with blood vessels, and pulsations of nearby arteries also promote lymph flow. In addition to these mechanisms, smooth muscle in the walls of the lymphatic trunks and thoracic duct contracts rhythmically, helping to pump the lymph along. 

Even so, lymph transport is sporadic and slow. About 3 L of lymph enters the bloodstream every 24 hours, a volume almost exactly equal to the amount of fluid lost to the tissue spaces from the bloodstream in the same time period. Movement of adjacent tissues is extremely important in propelling lymph through the lymphatics. When physical activity or passive movements increase, lymph flows much more rapidly (balancing the greater rate of fluid loss from the blood in such situations). 

The lymphatic system has no pump. To work properly,it relies on the human body to do what it was designed to do: move.

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Lymphomaniacs know:
it's all about MOVEMENT. You move; it moves.

6 Ways to Help Move your Lymph

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sauna
Dry brushing helps your lymphatic system.

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Click on the images above for a beautiful slideshow on the lymphatic system by Sky-David, M.P.T., C.L.T., inventor of some of the world's most renowned lymphatic devices - watercolors and annotations by Sky-David

Lymph Nodes

The principal lymphoid organs in the body are the lymph nodes, which cluster along the lymphatic vessels of the body. There are hundreds of these small organs, but because they are usually embedded in connective tissue, they are not ordinarily seen. Large clusters of lymph nodes occur near the body surface in the inguinal, axillary, and cervical regions, places where the lymphatic collecting vessels converge to form trunks. 

Lymph nodes have two basic functions, both concerned with body protection:

(1) As lymph is transported back to the bloodstream, the lymph nodes act as lymph “filters”. Macrophages in the nodes remove and destroy microorganisms and other debris that enter the lymph from the loose connective tissues, effectively preventing them from being delivered to the blood and spreading to other parts of the body.

 (2) They help activate the immune system. Lymph nodes and other lymphoid organs are strategically located sites where lymphocytes encounter antigens and are activated to mount an attack against them.

Lymph Nodes - by Sky-David

Watercolor by Sky-David

Circulation in the Lymph Nodes

Lymph enters the convex side of a lymph node through several afferent lymphatic vessels. It then moves through a large, baglike sinus, the subcapsular sinus, into a number of smaller sinuses that cut through the cortex and enter the medulla.

 

The lymph meanders through these sinuses and finally exits the node at its hilum, the indented region on the concave side, via efferent lymphatic vessels. There are fewer efferent vessels draining the node than afferent vessels feeding it, so the flow of lymph through the node stagnates somewhat, allowing time for the lymphocytes and macrophages to carry out their protective functions. Lymph passes through several nodes before it is completely cleansed.

 

Sometimes lymph nodes are overwhelmed by the agents they are trying to destroy. For example, when large numbers of bacteria are trapped in the nodes, the nodes become inflamed, swollen, and tender to the touch, a condition often referred to (erroneously) as swollen “glands”. 

Lymph nodes are just one example of the many types of lymphoid organs or aggregates of lymphatic tissue in the body. Others are the spleen, thymus, tonsils, and Peyer’s patches of the intestine, as well as bits of lymphatic tissue scattered in the connective tissues. Except for the thymus, the common feature of all these organs is their tissue makeup: all are composed of reticular connective tissue. Although all lymphoid organs help protect the body, only the lymph nodes filter lymph. The other lymphoid organs and tissues typically have efferent lymphatics draining them but lack afferent lymphatics.

Other Lymphoid Organs

Spleen

The spleen is the largest organ in the lymphatic system. The spleen is located under the ribcage and above the stomach in the left upper quadrant of the abdomen. A spleen is soft and generally looks purple. It is made up of two different types of tissue. The red pulp tissue filters the blood and gets rid of old or damaged red blood cells. The white pulp tissue consists of immune cells (T cells and B cells) and helps the immune system fight infection.

 

A helpful tip to remember the size of the spleen is the 1x3x5x7x9x11 rule:

  • An adult spleen measures around 1 inch by 3 inches by 5 inches.

  • It weighs around 7 oz.

  • It is located between the 9th and 11th ribs.

 

The spleen performs several physiological functions namely, phagocytosis of aging erythrocytes and platelets, recycling iron, inducing immune response against blood antigens, and defending against invading bacteria, fungi, viruses, prions and other infective agents. Because the spleen has only the efferent lymphatic vessels, it is, therefore, involved in the filtration of blood, but not lymph. 

The spleen has a very complex lymphatic and vascular organization. It is an organ with multiple functions. Although its primary functions are to filter blood and protect the body from invading pathogens and antigens, it also plays a significant role in maintaining the immunologic homeostasis and tissue regeneration processes. When spleen is surgically removed due to trauma or a pathological condition, Kupffer cells from the liver may act as phagocytic cells to remove aging RBCs from the circulation with limited capacity.

The bilobed thymus has important functions primarily during the early years of life. It is found in the inferior neck and extends into the superior thorax, where it partially overlies the heart deep to the sternum. The thymus is the site where 
T-lymphocyte precursors mature to become immunocomptetent lymphocytes. In other words, the thymus is where T -ymphocytes become able to defend us against specific pathogens in the immune response. 

 

Prominent in newborns, the thymus continues to increase in size during the first year, when it is highly active. After puberty, it starts to atrophy gradually and by old age it has been replaced almost entirely by fibrous and fatty tissue and is difficult to distinguish from surrounding connective tissue. 

 

To understand thymic histology, it helps to compare the thymus to a cauliflower head - the flowerets represent thymic lobules, each containing an outer cortex and an inner medulla. Most thymic cells are lymphocytes. In the cortical regions, the rapidly dividing lymphocytes are densely packed, but a few macrophages are scattered among them. The thymus has no follicles, because it lacks B cells. The lighter-staining medullary areas contain fewer lymphocytes plus some bizarre structures called thymic (Hassall’s) corpuscles. Consisting of concentric whorls of keratinized epithelial cells, they were thought to be sites of T cell destruction.

 

Recent evidence suggests that Hassall’s corpuscles are involved in the development of a class of T-lymphocytes called regulatory T cells, which are important for preventing autoimmune responses. 

In addition to its lack of follicles, the thymus differs from other lymphoid organs in two other important ways.

 

First, it functions strictly in maturation of T-lymphocyte precursors and thus is the only lymphoid organ that does not directly fight antigens. In fact, the so-called blood-thymus barrier keeps bloodborne antigens from leaking into the cortical regions to prevent premature activation of the immature lymphocytes.

 

Second, the stroma of the thymus consists of epithelial cells rather than reticular fibers. These epithelial cells provide the physical and chemical environment in which T lymphocytes can become immunocompetent.

Thymus

To understand thymic histology, it helps to compare the thymus to a cauliflower head.

Tonsils

6-A-Lymph_edited.jpg

The Lymphatic System - watercolor by Sky-David

The tonsils are the simplest lymphoid organs. They form a ring of lymphatic tissue around the entrance to the pharynx (throat), where they appear as swellings of the mucosa.

 

The tonsils are named according to location. The paired palatine tonsils are located on either side at the posterior end of the oral cavity. These are the largest of the tonsils and the ones most often infected. A lumpy collection of lymphoid follicles at the base of the tongue is referred to collectively as the lingual tonsil. The pharyngeal tonsil (referred to as the adenoids if enlarged) is in the posterior wall of the nasopharynx. The tiny tubal tonsils surround the openings of the auditory tubes into the pharynx.

 

The tonsils gather and remove many of the pathogens entering the pharynx in food or in inhaled air.

 

The lymphoid tissue of the tonsils contains follicles with obvious germinal centers surrounded by diffusely scattered lymphocytes. The tonsils are not fully encapsulated, and the epithelium overlying them invaginates deep into their interior, forming blind-ended tonsillar crypts. The crypts trap bacteria and particulate matter, and the bacteria work their way through the mucosal epithelium into the lymphoid tissue, where most are destroyed. It seems a bit dangerous to “invite” infection this way, but this strategy produces a wide variety of immune cells that have a “memory” for the trapped pathogens. In other words, the body takes a calculated risk early on (during childhood) for the benefits of heightened immunity and better health later.

Peyer’s patches, or aggregated lymphoid nodules, are large clusters of lymphoid follicles, structurally like the tonsils. They are in the wall of the distal portion of the small intestine. Lymphoid follicles are also heavily concentrated in the wall of the appendix, a tubular offshoot of the first part of the large intestine. Peyer’s patches and the appendix are in an ideal position 

(1) to destroy bacteria (which are present in large numbers in the intestine) before these pathogens can breach the intestinal wall, and 

(2) to generate many “memory” lymphocytes for long-term immunity.

Peyer’s patches, the appendix, and the tonsils - all located in the digestive tract - and lymphoid follicles in the walls of the bronchi (organs of the respiratory tract) and in the mucosa of genitourinary organs are part of the collection of small lymphoid tissues referred to as mucosa-associated lymphatic tissue (MALT). MALT protects passages that are open to the exterior from the never-ending onslaughts of foreign matter entering them.

In short, the study of lymphatic drainage of various organs is important in the diagnosis, prognosis, and treatment of all illnesses.

Silent Waves: Theory and Practice of Lymph Drainage Therapy, an Osteopathic Lymphatic Technique by Bruno Chikly

Recommended Reading

Aggregates of Lymphoid Follicles

Sources

•  Atlas of Anatomy (Thieme Anatomy) 
2nd edition, Kindle edition by Anne Gilroy, Brian MacPherson

 

•  Atlas of Human Anatomy -
7th edition by Frank H. Netter, MD

•  Gray's Anatomy for Students -
With Access - 3rd edition

•  Pearson Prentice Hall
Educational Materials

•  Anatomy of the Human Body -
Book VIII - The Lymphatic System 
by Henry Gray

Other References

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