Understanding the lymphatic system

Acknowledgements to F.V. Collins

The blood stream is an irrigation system conveying food and oxygen to the body. This irrigation system requires a drainage system. The body is provided with such a system, the lymphatic system, it keeps body fluid levels in balance and removes waste products, bacteria and harmful substances from the tissues and cell activity it’s composed of (a) a fluid called lymph (b) lymph vessels or ducts and (c) lymph nodes or glands.

Blood does not normally come in contact with the tissues; to convey nutrients and oxygen to the body, a fluid called lymph is formed which seeps from the capillary walls into the surrounding body tissue. Lymph is collected by tiny lymph vessels or ducts like blood capillaries that run alongside blood capillaries, they unite to form small lymphatic vessels which in term unite to form larger vessels.

The lymphatic system ends with the discharge of lymph into the veins through the thoracic or cervical duct just prior to it entering the heart. The lumbar lymphatic trunk collects efferent vessels from the iliac and deep inguinal lymph nodes. The cisterna chyli collects lymph from the renal, hepatic, intestinal and lumbar lymph nodes. 

 
Lymph System
 

Lymph
Lymph is derived from blood plasma. As blood passes through the capillaries some of the plasma oozes through the endothelial wall, the resultant fluid now called lymph is thinner and waterier than the original plasma.

Cells take in nutrients from lymph, and as they carry out metabolism, their waste is released back into the lymph and discharged out of the body.

Injuries such as burns, poisons and bacterial invasion of the broken skin stimulate the production of lymph. Blisters and oedematous areas are an example.

The simple squeezing of a pimple will reveal a clear fluid, this is lymph.

The composition of lymph varies, and white blood corpuscles are always present when an infection has occurred in the body, it’s part of the body’s defense system.

The flow of lymph in the body is sluggish and is governed by three factors.
 
a)    Pressure from the capillaries and suction from the vena cava
b)    Muscular movement squeezing the vessels forcing the contents onward
c)    Presence of valves (non-return valves) preventing back flow.

Lymph nodes

Throughout the body at definite points are numerous round fleshy bodies known as lymph nodes.
There are two types of lymph nodes, regional and terminal lymph nodes.
Regional lymph nodes drain a region of the body. Terminal lymph nodes are at the end of the lymph flow and discharge lymph into one of the lymphatic ducts and ultimately the venous system. It’s important to know the difference as an infected regional lymph node indicates the infection is contained within the drainage region of the node whereas if the infection has spread to the terminal lymph node, then the infection is considered systemic.

Nodes are composed of masses of cells, mostly lymphocytes (white blood corpuscles).
Lymph vessels that enter the node from their drainage area or from another lymph node are called afferent vessels. Vessels that leave the lymph node to either enter another node or discharge into the venous system are termed efferent vessels.

When fighting infection lymph nodes attempt to close the efferent vessels to limit the spread of the infection and contain it within the drainage area or lymph node.

Because of the tendency of organisms to become trapped in lymph nodes, the nodes are valuable indicators of infections within the node’s drainage area.

The result of the struggle between the white corpuscles and the invading organisms is the formation of pus which is dead white cells, tissue and invading organisms.
Some organisms can escape from the first node and be trapped by the next node in the flow system or even reach the terminal node. The infection is then termed systemic.

Each node that an inspector is required to inspect is the largest one of a group of nodes in that location. If the larger node is removed or destroyed by disease, smaller nodes in the vicinity will enlarge and take over the function of the destroyed lymph node.
Often observed in the mesenteric lymph nodes is a milky fluid called “chyli” consisting of lymph and emulsified fats which are absorbed from the small intestine during digestion. The cisterna chyli, a dilated lymphatic sac, between the first and second lumbar vertebrae, is a key structure in this process, serving as a reservoir for chyle before it enters the thoracic duct.

Drainage areas of the chief lymphatic nodes 

Popliteal lymph nodes
Regional lymph node
Afferent vessels drain the adjacent muscles and the lower leg and foot 
Efferent vessels go to the deep inguinal, ischiatic and internal iliac lymph nodes.

Ischiatic lymph nodes 
Regional lymph node
Afferent vessels drain the adjacent sexual organs, adjacent muscles and the skin of the pelvis (pin bone area) and tail.
Efferent vessels go to the internal iliac lymph node

Superficial inguinal or supra mammary in females
Regional lymph node
Afferent vessels drain the external genital organs (except the testicles which drain into the lumber lymph nodes), the median posterior surfaces of the thigh and median surface of the leg and in females the mammary glands.
The efferent vessels go to the deep inguinal lymph nodes.

Deep inguinal lymph node 
Terminal lymph node
Its afferent vessels drain most of the muscles and bones of the upper thigh, lower leg and abdominal muscles, the sexual organs, the kidneys and the posterior portion of the peritoneum, also receives lymph from the popliteal, superficial inguinal, precrural and external iliac lymph nodes. 
Efferent vessels leads to the lumbar trunk and internal iliac lymph nodes.

Internal iliac lymph nodes 
Terminal lymph node
Afferent vessels come from the sub lumbar muscles, pelvis, tail and thigh, the femur and hip joint, from the urinary and genital organs and also from the deep inguinal, precrural, popliteal, external iliac and ischiatic lymph nodes.
Efferent vessels go to the lumbar lymphatic trunk which discharges into the venous system.

External Iliac lymph nodes
Terminal lymph node
Afferent vessels drain the pelvis, the adjacent abdominal and flank muscles together with the posterior part of the peritoneum also receives vessels from the precrural lymph nodes.
Efferent vessels go to the deep inguinal, internal iliac nodes and the lumbar trunk.

Precrural lymph nodes 
Regional lymph node
Afferent vessels drain the adjacent muscles and skin.
Efferent vessels go to the deep inguinal and internal iliac lymph nodes.

Lumbar lymph nodes 
Terminal lymph node
Afferent vessels come from adjacent sub lumbar and abdominal muscles, the lumbar vertebrae, kidneys, adrenal glands and peritoneum. 
The efferent vessels go to the lumbar trunk which discharges into the venous system.

Renal lymph nodes 
Terminal lymph node
 Afferent vessels drain the kidneys and adrenal glands also some vessels come from the peritoneum and certain visceral organs.
Efferent vessels go to the cisterna chyli which drains into the thoracic duct and then into venous system.

Sternal lymph nodes
Regional lymph node
Afferent vessels come from surrounding structures together with the costal (relating to the ribs) and mediastinal pleura, peritoneum, pericardium, liver diaphragm and adjacent muscles.
Efferent vessels go to the anterior sternal lymph node.

Prepectoral (Ventral superficial cervical)
Terminal lymph node
Afferent vessels come from the pharynx, larynx, tongue and surrounding tissue of the mouth and the submaxillary salivary gland. 
Efferent vessels go to the tracheal lymph duct.

Prescapular (Dorsal  superficial cervical)
Terminal lymph node
Afferent vessels which vary from 150 to 200 come from the adjacent skin which includes the skin from the poll (top of head) to a vertical line passing through the back of the elbow and the muscles of the neck, shoulder, chest, fore limb and foot.
Efferent vessels go to the thoracic duct

Mesenteric lymph nodes
Terminal lymph nodes
Afferent vessels come from the duodenum the jejunum and ileum, the three sections that make up the small intestine.
The efferent vessels of the duodenal nodes go to the hepatic lymph node, those of the ileo-jejunal nodes unite to form the intestinal lymph trunk so in this section they are terminal nodes.

Pancreatic and gastric lymph nodes (EU inspection requirements).
Terminal lymph node
Afferent vessels come from the pancreas, duodenum, liver, omentum and stomachs.
The efferent vessels go into the intestinal trunk, which flows into the cisterna chyli, and ultimately into the thoracic duct. 

Hepatic or portal lymph node
Regional lymph node
Afferent vessels come from the liver and pancreas. The efferent vessels of the duodenal nodes go to the hepatic lymph node,
The efferent vessels unite to form a large collecting duct which joins the common efferent vessels of the intestinal lymph nodes.

    
Lymph node location of porcine

Cervical lymph nodes
Regional lymph node
Afferent vessels come from the head.
The efferent vessels go to the prepectoral (ventral superficial cervical) lymph node

Prepectoral lymph node
Terminal lymph node
Afferent vessels come from the cervical lymph node
The efferent vessels go to the tracheal duct.

Lymph node location of macropods
 

Axillary lymph node
Terminal lymph node
Afferent vessels come from the foreleg, superficial tissues of the thorax and abdomen.
The efferent vessels go to the thoracic duct.

Inguinofemoral lymph node (Inguinal node in domestic animals)
Terminal lymph node
Afferent vessels come from the hind leg, pouch scrotum, cloacal region.
The efferent vessels go to the inguino-axillary lymphatic trunk.

Iliosacral lymph node (Iliac node in domestic animals)
Terminal lymph node
Afferent vessels come from the pelvis and tail
The efferent vessels go to the inguino-axillary lymphatic trunk.

In a paper published by P.R.Hopwood titled - An investigation of the topography of the lymphatic system of the grey kangaroo it states “From the viewpoint of the meat inspection of the carcass, the presence of inguinoaxillary lymphatic trunks in the kangaroo is the most important anatomical variation in the drainage pattern of the superficial lymphatic vessels found between
kangaroos and the domestic species. Lymph that in the domestic species would have passed from the inguinal lymphocentre to the lumbar trunks instead passed to the axillary lymphocentre in the kangaroo. Thus, at meat inspection of a carcass, pathology of the axillary lymphocentre in the kangaroo can reflect disease in a much wider range of body regions than it would in a domestic animal.”