Lymphatic System

• The lymphatic system consists of vessels/ducts that transport excess interstitial (tissue) fluid to blood vessels near the heart.
• Lymph nodes, lymph organs, and gut-associated lymphatic tissue (GALT) comprise the lymphatic system.
• Lymph is the fluid transported, containing few cells besides lymphocytes.
• T-lymphocytes circulate through the lymphatic system (blood > tissue > lymph > blood), while B-cells are mainly in the lymphatic organs.
• Lymph also contains nutrients, fats, and protein, with the exception of chyle in the mesenteric lymph nodes.

Lymph Formation

• Lymph starts as excess tissue fluid picked up by small lymph capillaries, which need sufficient pressure.
• More fluid exiting capillaries than being absorbed leads to excess tissue fluid.
• Arterioles deliver blood and plasma to tissues under enough pressure to force some plasma out.
• Blood pressure decreases in capillary beds, so there isn't enough pressure to force plasma out.
• Instead, plasma is drawn back into blood vessels by osmotic pressure (fluids flow from the interstitial space into the vessels).
• Edema results when osmotic pressure on the venous side isn't sufficient to compensate for fluid pushed out by the arterial side, often due to low albumin causing more fluid to leak into the interstitial space.
• Lymph capillaries collect this excess fluid and transport it through the thoracic duct back into systemic circulation, next to aorta.
• Lymph capillaries follow the route of the veins.
• Lymph vessels, like veins, rely on muscle contractions with valves to prevent backflow and propel fluid forward.
• Lymph vessels pass through at least one lymph node on their way to the thoracic duct.
• Lymph vessels merge to form the thoracic duct that empties into the vena cava.

Lymph Characteristics

• Lymph entering the thoracic duct is a translucent fluid mainly containing lymphocytes.
• Lymph contains more water, sugar, and electrolytes, possessing less protein than plasma.
• Lymph from the digestive system is called chyle.
• Chyle passes through lymph vessels in the mesentery, called lacteals, due to its milky appearance from fatty chyle.
• After meals, lymph includes microscopic fat particles called chylomicrons. These chylomicrons cause postprandial lipemia seen in serum ("after meal" fat presence in blood).

Lymphoid Compartments

• Bone marrow: Produces stem cells to create lymphocytes
• Central lymphoid organs: maturation of lymphocytes occurs within them. Examples are the thymus, bone marrow, and the gut-associated lymphoid tissue (GALT).
• Peripheral lymphoid organs: mature lymphocytes reside and respond to antigens. Examples are lymph nodes, tonsils, and bone marrow.

Functions of the Lymphatic System

• Removal of excess tissue fluid
• Inadequate lymph drainage causes fluid buildup, leading to edema.
• Waste Material Transport:
• Lymph transports waste materials from the interstitial fluid to re-enter circulation for elimination.
• Filtration of Lymph:
• Interstitial fluid carries microorganisms, cellular debris, and foreign material. The fluid must be filtered before it enters the circulation.
• Lymph nodes filter the lymph.
• Protein Transport:
• Large proteins, especially enzymes, are transported to the systemic circulation.
• These proteins are too large to enter venous circulation at the capillaries

Lymph Nodes

• Bean-shaped small structures.
• Lymph vessels drain fluid from specific body areas, ensuring lymph from a region passes through a specific node.
• Lymph nodes possess a connective tissue capsule with branches (trabeculae) extending into the node's center.
• Lymph enters the node at the hilus via afferent vessels and exits through efferent vessels.
• Nodes are divided into a cortex and medulla.
• The cortex is where lymphocytes reside, concentrated in groups called lymph nodules.
• The medulla is the node's internal structure, made of a coarse fibrous mesh with embedded macrophages.
• Lymph passes through at least one lymph node, and through its lymphatic vessels.
• Lymph picks up lymphocytes or their products and macrophages to filter foreign matter (including cancer cells).
• Lymph nodes may be removed as part of cancer treatment or for monitoring.
• Swollen lymph nodes may indicate a disease in the area they drain, acting as a sentinel.
• Macrophages become active and enlarge the lymph node when removing foreign material.
• Lymph nodes can swell from neoplastic growth when a tumor spreads to a regional lymph node.

Spleen

• Largest lymphoid organ in the body.
• Covered by a capsule of fibrous connective and smooth muscle with trabeculae branching into the parenchyma.
• In carnivores, trabeculae are muscular, contracting to squeeze blood out of the spleen back into circulation.
• Allows the spleen to act as a reservoir for blood.
• The parenchyma is divided into white pulp and red pulp.
• White pulp is localized lymphoid tissue where lymphocytes can clone themselves during an immune response.
• Red pulp contains blood vessels, tissue macrophages, and stored blood in sinuses.

Functions in The Spleen

• The spleen is for Blood storage in red pulp.
• Filtration is carried out by macrophages in the red pulp, where they destroy old cells and store iron.
• Lymphocyte cloning happens in the white pulp.
• The spleen is not required for survival and can be removed surgically due to traumatic damage or tumors.

Thymus

• It is located in the caudal neck/cranial thorax on either side of the trachea.
• It is prominent in young animals and regresses as they mature.
• Processed into T cells, thymocytes leave the thymus, distributing to the peripheral lymphoid tissue.
• Thymus helps develop the immune system in young animals.

Tonsils

• Unencapsulated nodules of lymphoid tissue.
• Found on epithelial surfaces throughout the body, especially in the pharyngeal region.
• Act as a frontline of defense against respiratory and digestive infections.
• Located at the beginning of the drainage system, "guarding" mucosal surfaces.
• Other "tonsils" are found in the larynx, prepuce, vagina, and intestine

Gut Associated Lymph Tissue (GALT)

• Unencapsulated lymphoid tissue in the lining of the intestine.
• Over 25% of the intestinal mucosa and submucosa consist of lymphoid tissue.
• It has the largest accumulation of lymphoid tissue in the body and plays a major role in the immune response.
• GALT is compared to the Bursa of Fabricus in birds, where B cells are processed before being sent to peripheral lymphoid tissue

This quiz covers key aspects of the lymphatic system. Questions focus on its functions, composition of lymph, circulation of lymphocytes, and the roles of lymphatic organs such as the spleen and thymus.