Lymphatic System: Organs and Functions

• The lymphatic system has two main components:
  • A system of lymphatic capillaries and larger vessels that return interstitial fluid to the bloodstream
  • Scattered aggregations of lymphoid tissue, including lymph nodes and tonsils

Lymphatic System Overview

• The lymphatic system is part of the vascular system
• It is a key component of the immune system, providing both specific and non-specific defense mechanisms
• It consists of a network of lymphatic vessels carrying lymph directionally towards the heart, specifically the right atrium
• "Lympha" is Latin for "water"
• Circulating lymphocytes (T and B cells), lymphocytes, and plasma cells spread throughout the body, aiding in the body's protective functions

Lymphatic Organs

• Important lymphatic organs:
  • Thymus
  • Tonsils
  • Spleen
  • Lymph nodes
  • Hemal nodes
  • Diffuse lymphatic tissue
  • Lymphatic nodules

Thymus

• The thymus is a lobed organ with a temporary function
• It is located in the cervicothoracic area
• Its topography differs by species
• It extends from the dorsal sternal aspect towards the ventral cervical area

Tonsils

• Tonsils are less discrete lymphoid aggregations

Spleen

• The spleen is part of the lymphoepithelial system located in the abdominal cavity, related to the stomach
• The spleen stores, releases, and concentrates erythrocytes
• It also produces lymphocytes and monocytes

Hemal Nodes

• Hemal nodes are small lymphatic bodies that resemble lymph nodes in structure
• They have a thin capsule
• They are found along blood vessels and are most common in sheep
• Afferent and efferent lymphatic vessels are absent in hemal nodes
• Hemal nodes are found in some animals in relation to their abdominal and thoracic viscera
• They may represent an intermediate stage between a lymph node and the spleen

Diffuse Lymphatic Tissue and Lymphatic Nodules

• Diffuse lymphatic tissue is present in many mucous membranes like the larynx, intestine, prepuce, and vagina
• It lacks a capsule
• Mucosa-associated lymphatic tissue (MALT) plays a role in regulating mucosal immunity
  • MALT can be a site of non-Hodgkin lymphoma
  • MALT lymphoma has links to Helicobacter pylori in the stomach

Lymph Tissue

• Lymph tissue is of 3 Types:
  • Diffuse Lymphatic Tissue
    • No capsule present
    • Found in connective tissue of almost all organs
  • Lymphatic Nodules
    • No capsule present
    • Oval-shaped masses
    • Singly or in clusters (Peyer's patches are clusters of lymphatic nodules in the ileum's mucosa)
  • Lymphatic Organs
    • Capsule present
    • Lymph nodes, spleen and thymus gland

Lymphatic Circulatory System

• The lymphatic circulatory system consists of a network of lymph vessels and lymph nodes
• This system collects interstitial fluid, waste material, viruses, and bacteria from body tissues outside the bloodstream
• Lymph vessels carry lymph, which is a clear, watery fluid
• Lymph is the ultrafiltration of blood plasma
• Lymph is interstitial fluid after it has entered the lymphatic system
• Lymph is similar to blood plasma but contains less protein
• Lymph vessels draw up lymph fluid from around cells and send it towards the chest
• Lymph fluid collects into the chyli cistern/thoracic duct; draining into the cranial vena cava that opens in the heart
• Without drainage by the lymphatic system, lymph fluid would build up and cause swelling

Lymph Nodes

• Lymph nodes are small structures on the trajectory of the lymph vessels
• They act as filters for harmful substances
• Lymph nodes contain immune cells that fight infection by attacking and destroying germs carried in lymph fluid
• Anatomists compare lymph nodes to a pearl chain
• They are usually near arteries and veins
• Lymph nodes may swell or enlarge to filter out "bad" cells, indicating a problem like infection, injury, cancer, immune-mediated diseases, or allergies
• This swelling is called lymphadenopathy
• Ear pain, fever, and enlarged lymph nodes near the ear can be signs of an ear infection
• Cancer can spread through lymph nodes

Examination of Lymph Nodes

• Veterinarians cannot palpate all lymph nodes; only those in the loose subcutaneous tissues
• Palpable lymph nodes include Mandibular, prescapular etc
• Size varies with location, age, and species
• They are bean-shaped with a smooth contour
• Some are hard to locate
• Physical examination of palpable lymph nodes includes inspection and palpation
  • Inspection reveals changes in normal contours caused by enlargement
  • Palpation provides a more critical evaluation of any changes

Palpable Lymph Nodes

• If the mandibular lymph node (just below the jaw) is enlarged, examine the right side of the mouth carefully
  • Enlarge lymph node may indicate a tooth root abscess involving a tooth on the right side
• If all lymph nodes in the head region are enlarged, consider an upper respiratory infection or other illness

Canine Lymph Nodes

• Lymph Nodes of the Head
  • The Mandibular Lymph Center
    • Presents a single group of mandibular lymph nodes
    • Palpable at angle of jaw
    • Two to four in number caudo-ventrally to the angle of the mandible
  • The Parotidian Lymph Center
    • Represented usually by a single lymph node 1-2 centimeters long but very rare can be two also
    • Situated near the base of the ear; close to the temporo-mandibular joint
• Retropharyngeal Lymph node
  • Comprises two groups of nodes regarding their topography, medial and lateral
  • The lateral retropharyngeal lymph node appears rarely, double or triple and small
• Superficial Cervical Lymph Node
  • Placed cranial to the supraspinous muscle and to the shoulder; approachable through the regional skin
• Axillary Lymph Center
  • Comprises the proper axillary lymph node and the accessory axillary lymph node
  • The former one is placed medial to the shoulder joint, in a mass of fat tissue medially to the distal part of the teres major muscle being easily palpated
• The Ilio-femoral (deep inguinal or old term)
  • Very reduced in size and inconstant
  • When present it has two lymph nodes: the ilio-femoral lymph node placed at the termination of the external iliac artery close to the femoral artery and the femoral lymph node placed in the distal part of the femoral triangle
• The Ingino-femoral Lymph Center (superficial inguinal or old term) in dogs
  • In the scrotal dog lymph nodes - palpable on the dorsolateral surface of the penis, 1 centimeter cranial to the spermatic cord, close to the lower part of the preputial cavity, embedded in fat tissue
  • In bitches the mammary lymph - one, two or three normally palpable and placed on the dorso-lateral margin of the last mammary gland, 2-3 centimeters from the cranial margin of the pubis bone
• The Popliteal Lymph Center
  • Presents only the superficial popliteal lymph node, mostly just one
  • Sizes ranged between 1-4 centimeters long
  • Placed caudal to the stifle joint, on the caudal surface of the gastrocnemius muscle
  • Easily palpable through the skin, even they are embedded in fat tissue

Objectives

• Role of the Lymphatic System
• Two major components of the Lymphatic System
• Lymphatic organs
• 3 types of lymph tissue
• Define the Lymphatic Circulatory System (lymphatic vessels, lymph and lymph nodes)
• Why it is important to include the lymph nodes in the examinations of animals
• Describe the topography of the palpable lymph nodes in canines