The Anatomy of the Lymphatic Vessels

Lymphatic vessels (also known as lymph vessels or lymphatics) are part of the body’s lymphatic system. The lymphatic system also includes lymph (the fluid found within the vessels) and lymphatic organs, such as lymph nodes.

Together with the rest of the system, lymphatic vessels help maintain the body’s fluid balance, absorb fats from the digestive tract, and provide immune defense against microorganisms and disease. Impairment in these functions can cause issues like lymphedema and the spread of cancer cells. 

Anatomy

Here’s a breakdown of how the lymphatic vessels are set up as part of the total lymphatic system.

Structure 

Lymphatic vessels are tube-like structures that carry fluid (called lymph) away from the tissues to deliver it back into the blood’s circulation. Unlike blood vessels that circulate blood in a continuous, closed-loop system, lymphatics carry fluid in one direction. 

The lymphatic pathway begins with lymph capillaries, the smallest type of vessel. Lymph capillaries are made up of a single layer of overlapping cells. This arrangement allows fluid to flow into the vessel but not out.

Lymph capillaries merge to form progressively larger lymphatic vessels. The largest of these are lymphatic trunks and ducts. Ultimately, the lymphatic vessels feed into the subclavian vein, returning the fluid to the blood. Along the way, the lymphatic vessels pass lymph through lymph nodes for filtration. 

A layer of smooth muscle surrounds lymphatic vessels. As this muscle contracts, it propels the lymph forward. Lymphatic vessels also contain valves that prevent backflow. 

Location

Lymphatics span throughout most of the body, except for the bone marrow, brain, and spinal cord. Lymph nodes are distributed along the lymphatic pathway, connected by vessels. Clusters of lymph nodes are found in the armpit, groin, and neck.

Anatomical Variations 

Certain congenital disorders can affect the normal development of lymphatic vessels.

Milroy disease, caused by a genetic mutation, results in small or absent lymphatic vessels. This interferes with the transportation of fluid away from the tissues. As such, lymphedema (swelling), particularly in the legs and feet, can develop. Individuals affected with this disorder are also at increased risk of cellulitis, a type of skin infection. 

Lymphangiomatosis describes the abnormal overgrowth of lymphatic vessels. This condition can affect one or multiple organs and usually impacts the lungs and bones. Individuals with lung issues may cough and have difficulty breathing.

When lymphangiomatosis affects the bone, fractures and pain can occur. The severity of the disease can range from asymptomatic or mild symptoms to severe impairment and even death. 

Function

The primary functions of the lymphatic vessels and system include:

• Aiding absorption of fats and fat-soluble vitamins from the digestive tract
• Providing defense against invading microorganisms and disease
• Returning excess tissue fluid to the blood circulation

Fluid Balance

As blood circulates, fluid leaks out of blood capillaries and surrounds tissue cells. The fluid located around the cells is called interstitial fluid. This fluid delivers oxygen and nutrients to the cells.

Most of the fluid flows back into the bloodstream, but some is left behind. The remaining fluid is taken up by lymphatic capillaries and is now referred to as lymph.

The lymphatic system transports the lymph and delivers it back into the blood circulation at the subclavian vein. This balance of fluid is vital. Removing interstitial fluid at the same rate it is produced helps maintain blood pressure and prevent edema (swelling in the tissues).

Fat Absorption

The absorption of nutrients from the digestive tract happens in the small intestine. The lining of the small intestine is covered with hair-like projections called villi. In the center of each villus, there are blood and lymphatic capillaries.

Blood capillaries are responsible for most nutrient absorption. However, specialized lymphatic capillaries, called lacteals, absorb fats and fat-soluble vitamins (vitamins A, D, E, and K). Once absorbed, the fats and vitamins make their way through the lymphatic system and are delivered to the bloodstream. 

Immune Functions

As lymph travels along the lymphatic pathway, it passes through bean-shaped structures called lymph nodes. Lymph nodes are responsible for filtering the lymph and killing any microorganisms or damaged (potentially cancerous) cells.

Lymph nodes contain immune cells such as lymphocytes. When a microorganism is detected in the lymph, lymphocytes multiply and travel to the infection site to help destroy the invading agent. This activation of lymphocytes is responsible for the swollen lymph nodes you may experience during an illness. 

Associated Conditions

Here are some issues that are directly related to problems with the lymphatic system.

Lymphedema 

Lymphedema describes the buildup of fluid within tissues due to insufficient lymph transport. Swelling usually happens in the arms or legs but can also occur in other body parts. 

Lymph nodes may be removed or damaged by surgery, radiation, or trauma, disrupting the flow of lymph through the vessels. The resulting lymphedema can be mild or severe. Affected areas are also more prone to infections and sores.

Lymphedema is a common complication after breast cancer treatment. Treatment often involves the surgical removal of lymph nodes under the arm and radiation. Arm swelling develops where the lymph nodes were removed.

Early diagnosis is crucial to prevent lymphedema from getting worse. Compression garments, such as compression stockings or a compression sleeve, can help move the fluid and keep it from pooling. Exercise also helps with circulation. Manual lymphatic drainage, a specialized massage performed by a trained therapist, may also be recommended.

Spread of Malignant Tumors

Cancer spreads when cells from a primary tumor separate and travel to other parts of the body. These cancerous cells are taken up and transported by the lymphatic system.

The immune system can get rid of mutated cells occasionally, but it can get overwhelmed by a large number of cancer cells. When this happens, cancer can grow in another part of the body.

Tests

Imaging tests used to visualize the lymphatic system can help:  

• Detect cancer spread 
• Diagnose and monitor lymphedema 
• Identify the location of lymphatic structures to minimize damage in surgery or to find lymph nodes for removal

Testing involves injecting a dye or radioactive tracer into the skin or muscle. The dye or tracer is taken up by the lymphatic vessels, allowing the structures to be seen when scanned in a specialized machine.

Some imaging tests include:

• Lymphoscintigraphy 
• Magnetic resonance (MR) lymphography 
• Near-infrared (NIR) lymphography 
• X-ray lymphography

Diagnosing lymphedema may also involve a computed tomography (CT) scan or doppler ultrasound. These tests can identify obstructions and rule out other causes of swelling, such as a blood clot.

Based on your risk factors for lymphedema, your healthcare provider may be able to make a diagnosis with a physical exam alone. Once diagnosed, you can move forward with treatment to improve your quality of life.