Structure and Function of the Lymphatic System in Dogs

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What Is the Lymphatic System?

The lymphatic system is a complex and vital system primarily responsible for the transportation of lymph and for participating in many immune functions of the body. The lymphatic system occurs throughout the body and is made up of small glands called lymph nodes, which are connected to each other by a series of vessels called lymphatics. Other important organs in this system include the bone marrow, spleen, thymus and gut-associated lymphoid tissue (GALT), which is the lymphatic tissue associated with the gastrointestinal (GI) tract.

Where Is the Lymphatic System?

The lymphatic system is located throughout the body and has many components:

  • Lymph nodes or glands are small round, oval or bean-shaped structures that are located at various locations throughout the body. The lymph nodes are connected to each other by a series of vessels called lymphatics, which carry lymph from place to place. Some lymph nodes lie along the surface of the body (along the neck, under the arms, in the groin, behind the knees), while others lie deep within the body (chest and abdomen).

  • The bone marrow lies within the central shaft of bones, primarily the long bones of the body.

  • The spleen is located near the stomach in the left forward part of the abdomen.

  • The thymus is located in the front part of the chest cavity, between the trachea (windpipe) and the ribs.

  • The GALT is made up of lymph tissue scattered throughout the GI tract including the tonsils and intestines.

    What Is the General Structure of the Lymphatic System?

    The lymphatic system is composed of a network of lymph vessels referred to as lymphatics, as well as certain organs and tissues, including the lymph nodes, bone marrow, spleen, thymus and GALT.

    Lymph is a milky fluid that flows throughout the system. It contains proteins, fats and a type of white blood cell called lymphocytes. Lymph is collected from the fluid of various tissues and eventually is returned to the blood circulatory system. The lymphatic system provides another route by which fluid can flow from distant tissues back into the blood stream, one that is separate from capillaries and veins. It also carries proteins and other substances away from tissues that cannot be removed or transported directly into the blood system.

    Similar to the blood circulatory system, the lymphatic system is comprised of fine channels that lie adjacent to the blood vessels. These lymphatic vessels eventually merge into a rather large vessel called the thoracic duct. As the lymph is carried from distant parts of the body, it is collected into larger and larger vessels until the vessels all converge in the chest and deposit the lymph in the large vein (cranial vena cava) leading to the right atrium of the heart.

    The lymph moves through the lymphatic vessels toward the lymph nodes. The lymph nodes lie at varying points along the course of the lymphatic chain and can form clusters in some areas of the body. Lymph nodes have a dense fibrous outer coating, called a capsule and are filled with white blood cells and spaces containing lymph fluid. Several types of white blood cells predominate in the lymph nodes, particularly lymphocytes, plasma cells, and macrophages.

    The bone marrow consists of connective tissue, the cells of which form a delicate meshwork within the marrow cavity. The marrow cavity is permeated by numerous thin-walled blood vessels. Within the spaces of this tissue, the immature and adult stages of different blood cells exist.

    The spleen is the largest body of lymphatic system. It is a dark red organ that is supplied with numerous blood vessels. A tough capsule of fibrous tissue covers the spleen. The splenic "pedicle" is located along one surface and serves as the entry and exit point for blood vessels. The internal structure of the spleen consists of:

  • Red pulp – areas for red blood cell storage and for the trapping of immune proteins called antigens

  • White pulp – areas of special immune response cells called lymphocytes and reticuloendothelial cells

  • Marginal zone – an area that separates the white and red pulp and helps to filter the blood

    The thymus is an organ that varies in size depending on the age of the individual. It is largest in young animals and shrinks to a very small size in the adult.

    GALT is present throughout the gastrointestinal tract. Peyer's Patches are aggregates of lymphoid tissue found in the small intestine and are a type of GALT.

    What Are the Functions of the Lymphatic System?

    The lymphatic system has several very important functions: absorbing excess fluid from tissues and returning it to the bloodstream, absorbing fat from the gastrointestinal tract, transporting white blood cells and certain proteins, and playing an important role in the immune system, particularly in the production of antibodies (immunoglobulins).

  • The lymphatic system filters and removes debris from the tissues of the body. Cells produce proteins and waste products. The lymph absorbs these products and carries them away from the tissues because they are often too large to be effectively absorbed and removed by the bloodstream.

  • The lymphatic system, functioning along with the circulatory system, absorbs nutrients from the small intestines. A large portion of digested fats is absorbed via the lymphatic capillaries. Fat absorbed from the small intestinal lymphatic capillaries or lacteals is termed chyle.

  • The lymph nodes filter out cellular waste products and foreign material in the lymph fluid, including potentially dangerous infectious particles like bacteria and viruses. They trap material received from the lymphatic vessels and provide a site for white blood cells to mount an immune response. They act as a barrier against the entrance of these foreign substances into the bloodstream.

  • The chief function of the bone marrow is the production of various red and white blood cells.

  • The spleen is an integral part of the immune system and it filters abnormal cells from the blood. It also helps make and store blood cells.

  • The thymus is a very important part of the immune system in the newborn. It is the site where the earliest immune cells are made and where immune functions take place in the young animal.

  • GALT's main function is to provide immunologic defenses at the surface of certain areas of the body, such as the tonsil and the lining of the intestinal tract. These are areas where the body is often exposed to foreign materials and infectious agents.


  • What Are Common Diseases of the Lymphatic System?

    Due to the distribution and complexity of the lymphatic system, many disorders may affect all or some part of it. The most common disorders seen in dogs include the following:

  • Lymphoma or lymphosarcoma is a tumor of white blood cells. It is a malignant cancer, and it may affect one or more parts of the lymphatic system. Lymphoma may occur as a solid tumor associated with the lymph nodes, the intestines, kidneys, liver, spleen, thymus or other parts of the body. It may also develop as a circulating form that is confined largely to the bone marrow and blood stream. Lymphoma is one of the most common cancers seen in dogs, and has been treated with chemotherapy protocols for a number of years.

  • Lymphadenopathy is enlargement of the lymph nodes. It may represent lymphosarcoma, but may also develop for other reasons. Lymph nodes may enlarge when they are reacting to foreign substances or infection. They become larger as white blood cells proliferate within the nodes. Such reactions may also occur following vaccination or with any chronic inflammation within the body.

  • Lymphadenitis is inflammation of the lymph nodes. It may involve one or several lymph nodes, depending upon the cause. Common causes include wounds, skin infections, infections within the soft tissues of the body, nonlymphatic tumors, and areas of active healing.

  • Chylothorax is the accumulation of chyle in the chest cavity from rupture, obstruction, or abnormal development of the thoracic duct. It may develop secondary to heart disease, tumors of the thorax, diaphragmatic hernias, trauma, fungal infections, heartworm disease, and for unknown reasons. It is more common in Afghan hounds and Shiba Inu dogs than in other breeds.

  • Lymphangitis is an inflammation of the lymph vessel. It often arises from trauma, foreign bodies, and infections. It may occur at the same time as lymphadenitis.

  • Lymphangiectasia is an obstructive (blockage) disorder that causes dilation of the lymph vessels, particularly in the intestinal tract.

  • Lymphedema is an accumulation of lymph in the soft tissues of one or more of the limbs. Congenital forms occur in some dogs (e.g. poodles, Labrador retriever, Great Dane) due to deformities in either the lymphatic channels or the lymph nodes themselves. Acquired forms may occur with blockage or destruction of lymph vessels from trauma, surgery, inflammation, infection, tumors, or radiation therapy. In some cases the soft tissue swells so much that the limb may be painful or dysfunctional.

    What Types of Diagnostic Tests Are Used to Evaluate the Lymphatic System?

    Several tests are particularly helpful in evaluating the lymphatic system. Depending on the part or parts of the lymphatic system involved, a combination of tests may be recommended by your veterinarian.

  • A complete blood count (CBC), biochemical profile, and urinalysis are recommended to help evaluate organ functions, to detect evidence of infection or inflammation throughout the body, and to assess the types of white blood cells present in the circulation.

  • Blood tests that screen for infectious diseases such as the systemic fungal infections, the tick borne diseases and certain bacterial infections (e.g. brucellosis) may be of benefit in cases with lymphadenopathy or abnormal circulating numbers of white blood cells.

  • Chest and abdominal X-rays and ultrasonography are useful to evaluate abdominal organs including the spleen, intestines, liver, kidney and abdominal lymph nodes. They also detect the presence of fluid in the chest, such as in the case of chylothorax.

  • A bone marrow aspirate or biopsy is performed for diseases involving the bone marrow.

  • Fine-needle aspiration of enlarged lymph nodes or other abnormal tissues, followed by cytology (microscopic analysis of the cells) may be diagnostic for lymphoma, reactive lymphadenopathy or lymphadenitis. In some cases an actual biopsy (piece of tissue) is needed for diagnosis.

  • Analysis of fluid retrieved from the chest can confirm the presence of chylothorax.

  • Bacterial cultures may be submitted if bacterial infections are suspected.

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