The Lymphatic System

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A scanning electron micrograph of a human lymphocyte.
A scanning electron microscope (SEM) image of a single human lymphocyte. Image:

The lymphatic system—a network of vessels, ducts, glands, and nodes—functions to carry excess fluid away from spaces within body tissues (interstitial spaces) and plays a key role in defending the body against disease-causing agents (pathogens). The organs and tissues of the lymphatic system are also considered organs of the immune system.


To understand how the lymphatic system can defend the body against pathogens, you should be able to:

  • Understand how the lymphatic pathways and components are structured and how they work with the cardiovascular system.
  • Describe the function of lymph as a disease-fighting mechanism.
  • Explain the lymphatic system’s role in the body’s defense against disease.


capillary—a small, blood vessel that connects a venule and an arteriole.

immunity—resistance to disease.

inflammation—a tissue response that includes an increase of fluid in the affected area.

interstitial fluid—fluid within cells.

lymph—fluid transported in the lymphatic system.

lymph node—a mass of lymph tissue located along lymphatic pathways in the body which function in immunity.

lymphocyte—a type of white blood cell that fights pathogens in the body.

macrophage—a large cell that engulfs and destroys disease-causing agents and cell fragments in the body.

pathogen—any disease-causing agent.

spleen—a large organ located in the upper left abdomen that functions as a blood reservoir and filter.

thymus—a two-lobed organ located behind the sternum and between the lungs which functions in providing immunity.

Lymph Formation and Function

Increasing pressure in tissue spaces forces tissue fluid into lymph capillaries. Once tissue fluid is in a lymph capillary, it is called lymph.

Lymph carries proteins, water, viruses, bacteria, toxins and cellular waste products and transports them to and from tissues. The structure of lymph vessels allows these substances to enter a lymph capillary when most would be too large to enter a blood capillary. The flow of lymph through the lymphatic system is accomplished through:

  • contraction of skeletal muscles.
  • contraction of smooth muscles in the walls of lymphatic vessels.
  • breathing muscles.

Valves in lymph vessels prevent the backflow of lymph in much the same way as backflow of blood is prevented in veins.

Did You Know?

Lymph vessels carry substances away from tissues. In this way, these vessels may transport cancer cells from one tissue to another. This is why lymph nodes in the armpit are often tested for cancer cells during breast cancer treatment.

Lymphatic System Structure and Function

Lymph is collected from different areas of the body by a network of structures that begin with lymphatic capillaries. The following description begins with the lymphatic capillaries:

Lymphatic capillaries—One-cell thick walls allow tissue fluid

Diagram of lymph capillaries in the lymphatic system

from the interstitial spaces to enter a lymphatic capillary.

Once tissue fluid is in a lymph capillary, it is called lymph.



 Afferent lymphatic vessels—These vessels are formed by lymphatic capillaries and have a structure similar to veins.

Walls have three layers (a lining, a smooth muscle layer, and an outer layer of connective
tissue) and flap-like valves.  Valves prevent backflow of lymph; lymph can only travel one
way through the pathway.

Afferent lymphatic vessels carry lymph to the lymph node.

Diagram of the structure of a lymph node.
Structure of a lymph node. Credit:


Lymph nodes are specialized organs located along the pathway of lymphatic vessels which filter lymph.



Lymph nodes vary in size but are about 2.5 cm (1in) in length and bean-shaped.

A white blood cell (large bluish structure in center) and red blood cells (doughnut-shaped structures).
A stained lymphocyte surrounded by red blood cells. Image:

They produce large numbers of lymphocytes, specialized white cells which can defend the body against bacteria and viruses. Macrophages, cells that can engulf and destroy foreign substances and damaged cells, are also present in the lymph nodes. Foreign substances and cellular debris are filtered through the node tissues.


(Remember:  Afferent vessels carry lymph TO the lymph node; efferent vessels carry lymph AWAY from the lymph node.)

Efferent lymphatic vessels—carry lymph out of the node and merge with other vessels to form:


Diagram of location of major lymph nodes in the human body.
Location of major lymph nodes. Image:

Lymphatic trunks—structures that drain lymph from large regions of the body; lymphatic trunks are named for the region they drain. Lymphatic trunks include:

  • Lumbar trunk-drains lymph from the legs, lower abdominal wall, and pelvic organs.
  • Intestinal trunk-drains lymph from the organs of the abdomen.
  • Intercostal and bronchomediastinal trunks-drain lymph from areas around the thorax.
  • Subclavian trunk-drains lymph from the arms.
  • Jugular trunk-drains lymph from sections of the neck and head.

     These trunks join one of two collecting ducts:

1) Thoracic duct—begins in the abdomen and extends upward through the diaphragm in front of the vertebral column and empties into the left subclavian vein.

This duct drains lymph from the lower body regions, left arm, and left side of the head
and neck.


2) Right lymphatic duct—begins in right thorax and empties into the right subclavian vein.

This duct drains lymph from the right side of the head and neck, right arm, and right


Lymph leaves the ducts, enters the venous system of blood (in the right subclavian vein), and becomes part of the plasma just before blood returns to the right atrium of the heart.

Besides the lymph nodes, two other organs of the lymphatic system work to defend the body against pathogens.

1) The thymus is a bi-lobed organ located

Chest x-ray showing the thymus gland.
A chest x-ray showing the thymus (circled) of a young child. Image:

in front of the aorta and behind the sternum. The thymus is much larger in children and decreases in size after puberty. It

may be mostly replaced by connective tissue in elderly people.

The thymus contains lymphocytes that developed in bone marrow.  Some of these lymphocytes further develop into T-lymphocytes which leave the thymus and provide a type of immunity.

2) The spleen is the largest lymphatic

Microscopic slide showing human spleen tissue
A photograph taken through a microscope of spleen tissue showing red pulp, white pulp, and the capsule (at the top). Image:

organ. It is located in the upper left abdominal cavity, beneath the diaphragm, and behind the stomach.  Unlike other lymphatic

organs, the spaces in the spleen are filled with blood, not lymph.

The spleen functions as a blood reservoir. Blood can be stored in the spleen, and in time of increased need, can be released into circulation.


Location of the spleen in the human body (see insert and arrow).
Location of the spleen in the body. Image:

Tissues in the spleen (white pulp) contain large numbers of lymphocytes while other tissues (red pulp) contain red blood cells, along with some lymphocytes and macrophages.

Macrophages get rid of red blood cell fragments that have ruptured during passage through the capillaries. Macrophages also engulf and destroy bacteria.




Check For Understanding

  1. How does the lymphatic system work to defend the body against disease-causing agents?




  1. Explain the disease-fighting function of the spleen.



  1. Explain the disease-fighting function of the thymus.



  1. Track the route of lymph through the body starting from the collection of lymph in the lymphatic capillaries in body tissues.