The lymphatic system is a network of vessels and nodes that plays a crucial role in returning excess tissue fluid to the blood, filtering lymph to remove foreign particles, transporting waste materials, and producing lymphocytes for immunity. It consists of various components, including lymphatic vessels that rely on muscle contractions for lymph flow and lymph nodes that filter lymph and produce immune cells. Disorders such as lymphangitis and lymphoma can affect the lymphatic system, with treatments ranging from antibiotics and management strategies to chemotherapy and surgery.

1. THE LYMPHATIC
SYSTEM
Veterinary Physiology

2. What is the Lymphatic System?
• The lymphatic system is a network of: capillaries, vessels, ducts, nodes and tissues
• It is part of the circulatory system
• It carries excess tissue fluid (lymph) back to the circulating blood
• It is found in all regions of the body except the central nervous system and bone marrow
• It produces lymphocytes, which in turn produce antibodies – so technically, it has a role in
immunity

3. 4 Functions of the Lymphatic System
1. Return excess lymph to the circulating blood
2. Filter lymph to remove bacteria and other foreign particles.This takes places in
the lymph nodes
3. Transport waste materials – the products of fat digestion and fat-soluble
vitamins from the lacteals of the intestinal villi to the circulation
4. To produce lymphocytes, which in turn make antibodies

4. How is Lymph formed? – Removing
ExcessTissue Fluid
The systolic force of the heart creates hydrostatic pressure and this pushes water out of the
capillaries. Certain blood proteins are unable to pass through the capillary walls and this results in
a water potential and the build up of these proteins results in osmosis in an attempt to reach
equilibrium. However, equilibrium is never reached because blood is constantly flowing.The
water that leaks out of the capillaries is known as tissue fluid.
To prevent a build up of tissue fluid surrounding the cells in the tissue, some of it enters the
lymph vessels where it becomes lymph. It eventually re-joins the blood.
It is essential for this to happen because if the build up of fluid becomes too great, this could
result in the fluid accumulating and forming an oedema, which is very painful.Therefore, the
lymphatic system prevents – it is known as lymphatic drainage.
Lymph mainly consists of water, sugar and electrolytes. It has less larger proteins than plasma as
these were unable to pass through the capillary walls.

5. LymphVessel
Lymphatic vessels have a similar
structure to veins.
Lymph flow is mainly passive and
relies on the contractions of
surrounding muscles to move the
lymph along.
The valves prevent the backflow
and pooling of lymph in the vessels.

6. Lymphatic Ducts
The lymphatic vessels enter the larger lymphatic ducts which drain the lymph back into the blood
vessels to be returned to the heart.The main ducts are…
• The right lymphatic duct – the smaller duct which empties blood from the head, neck, thorax
and right forelimb into the right side of the heart via the right subclavian vein
• The thoracic duct – the main lymphatic duct which collects blood from the rest of the body. It
empties blood into the heart via the cranial vena cava
• There is also a pair of tracheal ducts that drain the head and neck into the thoracic duct or one
of the large veins near the heart

7. Where lymph
is carried to
the node
Where
lymph is
carried
away
from the
node
Where
lymphocytes
are
produced Hilius –
where the
lymph
vessel
leave
Trabeculae –
support for the
node

8. Lymph Nodes
The lymphocytes produced by the lymph nodes are important within the immune
system as they containT and B cells.
All lymph passes through at least one node before returning to circulation.This is
important because each node acts as a mechanical filter by trapping foreign matter
(such as bacteria particles) and destroying them using phagocytic cells. These
phagocytic cells, such as macrophages, are located in the medulla of the node.This
attempts to halt the spread of disease around the body.
Lymph nodes are located around the body and some are superficial so can be
palpitated. During times of infection or disease, lymph nodes can swell – this is
known as lymphadenopathy.The swelling is a result of the active macrophage and
the lymphocytes multiplying.

9. Palatable Lymph Node’s Location

10. LymphaticTissue
Primary LymphoidTissue Secondary Lymphoid
Tissue
Bone Marrow Lymph Nodes
Thymus Gland Mucosal-associated
LymphoidTissue (MALT)
Bursa of Fabricus (Birds) Peyer’s Patch
Peyer’s Patch (cattle, sheep,
pigs, dogs, horses)
Spleen
Appendix (rabbits) Tonsil

11. Thymus Gland
The thymus gland is especially important in young animals (late
fetal and early postnatal life) who have immature immune
sytems – the thymus gland kick starts normal development. It
producesT lymphocytes which give rise to the cell mediated
immune response.
It lies caudal to the neck, in the cranial thoracic inlet.
During puberty, the thymus gland regresses and may eventually
disappear.

12. Spleen
This is the largest lymphoid organ located near the greater
curvature of the stomach, within the greater omentum. It is not
essential for life so can be surgically removed. It does however
have a number of functions which are split into red pulp and
white pulp…
• Reservoir for red blood cells and platelets
• Destroys worn out red blood cells – this is the job of the
phagocytic cells but the iron from the RBCs is preserve for re-
use in haemoglobin synthesis
• Removal of particular matter from the circulation – filtering
out foreign particles which are then destroyed by phagocytes
• Produces lymphocytes

13. Tonsils
The tonsils form a ring of lymphoid tissue around the junction of the pharynx with
the oral cavity and act as a first line of defence against microorganisms that enter
via the mouth.

14. What Can GoWrong?
Equine Lymphangitis
This occurs when the lymphatic flow is impaired so instead it accumulates and the result is an
oedema.
Causes:
• Inflammation of the lymphatic system
• Standing for too long
• Infection
• As a result of a small wound
(It is essential a fracture is ruled out as the cause!!)
Treatment:
Antibiotics, massaging, movement, cold hosing

15. What Can GoWrong?
Lymphoma  A neoplasm originating in the lymphoid tissue (outside the bone marrow)
Mean onset is 6-9 years in dogs and 8-12 years in cats and can be graded from I-V.
Owner’s will usually present their animal showing symptoms such as lethargy, anorexia and upon
examination, the vet will notice general lymphadenopathy. A fine needle aspirant can be used to aid
diagnosis.
Treatment:
• Chemotherapy – can result in total remission in 69-90% cases but a median survival time of 6-12
months. If no response to this treatment, ‘back up’ drugs can be used but approximately 40-50%
of dogs respond to this
• Surgery to prevent advancement

16. Lymphoma –WHO Grading
• I  solitary node or lymphoid tissue in a single organ
• II  multiple lymph node involvement in a single region
• III  generalised lymph node involvement
• IV  liver and/or spleen involvement
• V  Bone marrow and/or other organs involved