The lymphatic system is a frequently overlooked body system, despite being responsible for a ton of important roles within the body! It is closely associated with the circulatory system and our white blood cells, so we couldn’t finish up haematology without talking lymphatics too. Here are my top 10 things the veterinary nurse needs to know about the lymphatic system!

1. Function

The lymphatic system has four key functions: to remove excessive tissue fluid, to transport waste materials to the bloodstream, to filter lymphatic fluid, and to transport proteins and enzymes to the bloodstream.

 2. Structure

The lymphatic system includes several different structures, which are divided into primary lymphoid tissue and secondary lymphoid tissue. Primary lymphoid tissue includes the bone marrow and the thymus, and secondary lymphoid tissue includes the lymph nodes, spleen, and gastrointestinal-associated lymphatic tissue (GALT). These structures are connected via a network of vessels that carry lymph (lymphatic fluid) back to the bloodstream.

3. Transport

Lymph vessels are thin-walled channels with multiple one-way valves, to maintain a normal flow of lymphatic fluid around the body. They are surrounded by smooth muscle tissue; the contraction of this muscle aids the movement of lymphatic fluid through these vessels.

Lymph vessels marge into two large vessels called lymphatic ducts. At the level of the thoracic duct, the lymphatic fluid enters the bloodstream via the vena cava.

4. Lymph formation

Lymphatic fluid is formed from the interstitial fluid which has accumulated within tissues. The lymphatic fluid which drains from the gastrointestinal tract is called chyle and contains multiple small fat particles known as chylomicrons. 

5. The bone marrow

The bone marrow is made up of red and yellow marrow. Red marrow is where the majority of blood cells are made, from haematopoietic stem cells. Yellow marrow contains increased amounts of fat tissue, hence the more yellow appearance. As animals age, less red marrow is present. 

6. The thymus

The thymus is an organ which plays an important role in the immune function of younger animals. It is located within the cranial thorax and is gradually replaced by fat tissue as the animal ages.

6. Lymphocytes

Lymphocytes are a type of white blood cell closely associated with the lymphatic system. These cells play an important role in the immune system within the body and develop from haematopoietic stem cells within the bone marrow into either a B- or T-lymphocyte. B-lymphocytes mature in the bone marrow and T-lymphocytes mature in the thymus.

8. The lymph nodes

Lymph nodes are small, kidney-bean shaped structures which are located along lymph vessels (the channels through which lymph fluid flows). Lymph nodes are responsible for the filtering of lymphatic fluid, and stimulation of lymphocytes to create antibodies and destroy cells. They are made up of a cortex and medulla, and contain B- and T-lymphocytes, as well as macrophages. Several lymph nodes can be palpated in a healthy patient, including the submandibular, prescapular and popliteal nodes.

9. The spleen

The spleen is another organ with important lymphatic functions; it is the largest lymphoid organ in the body! It is made up of both red and white pulp – the red pulp deals with red blood cells and the white pulp contains lymphoid tissue.

The red pulp is responsible for storing blood, and removing dead, dying or abnormal red blood cells, and is made up of blood vessels, blood storage sinuses and macrophages. The white pulp is involved in the cloning of lymphocytes during an immune response.

The spleen can actually store up to 20% of the body’s blood volume, which it can then release via splenic contraction when red blood cells are required.

10. The gut-associated lymphoid tissue

Lymphatics are also present within the mucosa and submucosa of the gastrointestinal tract; this is known as GALT (gut-associated lymphoid tissue). The lacteals are the lymphatic vessels which run through the GI tract and are present through the centre of each villus within the intestine. These vessels absorb dietary fats as well as lymphatic fluid.

So that’s your 10 need-to-know points about the lymphatic system, a body system we don’t often consider, but a vitally important one!

References

1. Merrill, L. 2012. Small Animal Internal Medicine for Veterinary Technicians and Nurses. Iowa: Wiley-Blackwell.