Lymphatic Vascular System

Lymphatic Vascular System

• The endothelium in lymph capillaries and vessels is extremely thin, allowing greater permeability.
• The structure of larger lymph vessels is similar to that of veins, except that their walls are much thinner.
• Lymph vessels bring lymphocytes, fatty acids, and immunoglobulins to the systemic bloodstream.

Lymph Movement

• Contractions of surrounding skeletal muscles force lymph to move forward.
• Lymph vessels contain more valves to prevent a backflow of collected lymph.
• Lymph vessels are found in all tissues except the central nervous system, cartilage, bone, bone marrow, thymus, placenta, and teeth.

Lymphatic Capillaries

• Lymphatic capillaries are thin-walled vessels that begin as blind-ended channels adjacent to capillary beds, where they collect lymph.
• They are composed of a single layer of attenuated endothelial cells that lack fenestrae and fasciae occludentes.
• They possess a sparse basal lamina.
• Lymph enters these leaky capillaries via spaces between overlapping endothelial cells.
• Small lymphatic anchoring filaments between the surrounding connective tissue and the abluminal plasma membrane assist in maintaining luminal patency in these delicate vessels.

Lymphocytes

• Lymphocytes can be classified into three major types based on their immunologic functions: B lymphocytes (B cells), T lymphocytes (T cells), and Null cells.
• B cells and T cells are the two main cell types found in lymphoid organs.
• Lymphocytes originate in the bone marrow and develop and mature in primary lymphoid organs.

Immune Cells

• Other immune cells include macrophages, monocytes, neutrophils, eosinophils, basophils, dendritic cells, and Langerhans cells.
• B cells are both antigen-presenting and antigen-receiving cells.

Lymphatic Tissue

• Diffuse lymphatic tissue, GALT (Gut Associated Lymphatic Tissue), MALT (Mucosa Associated Lymphatic Tissue), BALT (bronchus-associated lymphatic tissue), and lymphatic nodules are types of lymphatic tissue.
• Lymphatic nodules are usually found in structures associated with the alimentary canal, such as the tonsils, ileum, and vermiform appendix.

Tonsils

• Pharyngeal tonsils, palatine tonsils, and lingual tonsils are types of tonsils.
• Tonsils are composed of lymphatic tissue.

Organs of the Immune System

• Primary lymphoid organs: thymus and bone marrow.
• Secondary lymphoid organs: lymph nodes, spleen, MALT, GALT, NALT, BALT, CALT, and urogenital tract.

Functions of the Lymphatic System

• Collects plasma continuously leaking out from blood vessels into the interstitial spaces and returns it to the blood (after filtration).
• Regulates both immune and fluid homeostasis.
• Passively collects lymph, excess tissue fluid, and proteins from the intercellular spaces of the connective tissue and returns it into the venous portion of the blood vascular system.
• Lymph is a clear fluid and an ultrafiltrate of the blood plasma.
• Numerous lymph nodes are located along the route of the lymph vessels, where they filter the lymph and engulf any foreign microorganisms and suspended matter.