Lymphatic System

What is the Definition of the Lymphatic system in Biology?

Let's start by looking at the definition of the lymphatic system. The lymphatic system is a network of vessels, nodes, and organs that transport lymph from tissues to the bloodstream.

Lymphatic System Diagram

The main components of the lymphatic system are presented in the diagram below (Fig. 1). Take some time to review this before we begin discussing the form and function of each component.

What is the Structure of the Lymphatic System?

Much like arteries, veins, and capillaries that transport blood throughout the body, lymph vessels are tiny tubes that weave into almost every tissue of the body, transporting lymph.

Muscle contractions transport the lymph within the vessels throughout the body, while valves prevent the lymph from going backward.

The lymph vessels are punctuated with tiny, bean-shaped structures called lymph nodes. The lymph nodes contain a network of tissues, and between the gaps in the tissue are lymphocytes that fight infections and destroy foreign substances. The lymph nodes remove foreign materials from the lymph. In the human body, these can be found in the armpits, groin, neck, abdomen, pelvis, and chest.

Lymphocytes are white blood cells that help the body fight against infections. There are two types of lymphocytes:

• B-lymphocytes (or B-cells), which produce Antibodies.

• T-lymphocytes (or T-cells), which fight infections.

Organs of the Lymphatic System

The organs of the lymphatic system can be divided into primary and secondary lymphoid organs. Let’s go through each one.

Primary Lymphoid Organs

The primary lymphoid organs are the bone marrow and the thymus. It is in these organs that lymphocytes are produced and matured.

The Bone Marrow

The bone marrow is a spongy tissue containing Blood vessels found at the center of most bones. There are two types of bone marrow:

• Red bone marrow contains blood Stem Cells that mature into red or white blood cells (including lymphocytes) or platelets.

• Yellow bone marrow contains fat and Stem Cells that mature into cartilage, fat, or bone cells.

Some lymphocytes stay in the bone marrow and mature into B-cells. Others move to the thymus to develop into T-cells.

The Thymus

The thymus is a small two-lobed organ found behind the breastbone in the upper chest. T-cells develop and mature in the thymus.

Before leaving the thymus, developing T-cells undergo positive and negative selection to test if they are capable of distinguishing between the body’s own components (called “self-molecules”) and foreign substances (called the “non-self”).

• During positive selection, the developing T-cells are exposed to self-molecules. If they recognize and bind to the self-molecules, they pass the test and move into the inner region of the thymus.

• The developing T-cells then undergo negative selection. Developing T-cells that attack the self molecules fail the test and are subsequently destroyed.

Only 5 to 10 percent pass this test and are able to exit the thymus. Those that leave the thymus move into the bloodstream and the secondary lymphoid organs, where they help fight against infections and disease.

Secondary Lymphoid Organs

The secondary lymphoid organs include the lymph nodes, spleen, and several other lymph tissues, like the tonsils. In the secondary lymphoid organs, the lymphocytes mature and differentiate further.

Lymph Nodes

As mentioned earlier, lymphocytes move into secondary lymphoid organs like the lymph nodes. T cells gather in the inner region, while B cells gather in the outer region of the lymph nodes.

When lymph enters the node, it interacts with and activates the lymphocytes. Activated lymphocytes are then transported by the lymph, which exits the node through outgoing lymphatic vessels and enters the bloodstream, distributing them throughout the body.

The Spleen

The spleen is an organ that filters blood. Two types of cells make up the functional tissues of the spleen:

• The red pulp contains macrophages that remove Bacteria, damaged or defective blood cells, and debris from the bloodstream.

• The white pulp contains numerous B-cells and T-cells that fight infections.

Tonsils

The tonsils are aggregated lymphatic tissues that can be found at the entryway of the digestive and respiratory tracts. It is mostly made up of B-cell lymphoid tissues.

Lymphatic System Function

The main functions of the lymphatic system are:

1. To recover leaked interstitial fluid into the bloodstream, which maintains fluid balance in the body;

2. To absorb fats and fat-soluble vitamins from the digestive system and bring them into the bloodstream; and

3. To filter out Microorganisms and foreign particles in the lymph.

Let's go through each one in the following section.

Recovery of Leaked Interstitial Fluid into the Bloodstream

As blood circulates in an organism’s body, blood plasma seeps into tissues through the capillaries. Part of this blood plasma is the interstitial or extracellular fluid, which contains oxygen, amino acids, energy-rich glucose, as well as other nutrients that tissue cells need.

While most of the interstitial fluid usually returns to the bloodstream, a portion of it remains in the tissues. The lymphatic system picks up this fluid and these components from tissues and returns them to the bloodstream through lymphatic vessels.

The interstitial fluid within the tissues passes into lymphatic capillaries; once inside the lymphatic system, the fluid is now referred to as lymph. The lymph then flows into larger lymphatic vessels, which eventually join together to form a pair of lymphatic ducts found at the base of the neck (Fig. 2):

• The right lymphatic duct moves the lymph from the upper right side of the body to the bloodstream through the right subclavian vein.

• The thoracic duct moves the lymph from the rest of the body to the bloodstream through the left subclavian vein.

Absorption of Fats and Fat-soluble Vitamins from the Digestive System and bringing them into the Bloodstream

The small intestine is lined with mucosa, which is covered with villi (singular: villus). Each villus contains blood capillaries and lymph capillaries called lacteals. Whereas the blood capillaries absorb the majority of nutrients from food, the lacteals absorb the fats and fat-soluble vitamins.

Because of its high fat content, the lymph in the lacteals appears milky. The combination of lymphatic fluid and fat molecules is called chyle.

Filtering out Microorganisms and Foreign Substances in the Lymph

As we have discussed earlier, when lymphatic fluids pass through the lymph nodes, the lymphocytes in the lymph nodes destroy invading Microorganisms–such as Bacteria–and foreign substances. This is why the lymphatic system plays an important role in protecting the body from substances and Microorganisms that can potentially cause diseases and infections.

What are some Disorders of the Lymphatic System?

Some ailments that affect the lymphatic system include:

• Tonsillitis: an inflammation, or infection of the tonsils.

• Lymphangitis: an infection of lymphatic vessels, usually caused by bacterial infection.

• Lymphocytosis: an abnormal increase in the number of lymphocytes (>4000 lymphocytes/microliter), caused by viral, bacterial, or parasitic infections.

• Chronic lymphocytic leukemia: a type of Cancer where the bone marrow produces abnormal lymphocytes that can outnumber healthy cells, leading to infection, anemia, and bleeding disorders.

• Non-Hodgkin lymphoma: a type of Cancer affecting lymph tissues–typically starting with B cells–with no known causes.