Lymphatic System Overview - Anatomy & Physiology

Deep Lymphatic drainage - Copyright B.Catchpole RVC

Introduction

The lymphatic system can be divided into two anatomical and functional subsets: lymphatic vessels that carry lymph around the body, and the lymphoreticular system which describes the lymphoid tissues. The lymphatic system has three functions - immune defence, removal of interstitial fluid from tissues and the transport of fats.

Lymphatic Vessels and Lymph

Due to their structure lymphatic capillaries are more permeable than vascular capillaries which means that not only can they remove fluid more effectively from tissues but they can also take up large molecules as well as chylomicrons. Chylomicrons transport fats and enter the lymph to eventually join the circulatory system via the thoracic duct; this enables the lipid soluble triacylglycerols (TAGs) to be transported into the bloodstream. The interstitial fluid or lymph within the lymphatic vessels passes through lymph nodes where it is surveyed by immune cells before returning to the circulation, ensuring that tissue pathogens are removed.

Lymphoreticular system

The lymphoreticular system produces immune cells and removes senescent cells. Primary (or central) lymphoid tissues can also be referred to as primary lymphoid organs. Maturation of lymphocytes and lymphopoiesis occurs in the primary lymphoid tissues, with different tissues responsible for maturing different types of lymphocyte. The primary lymphoid tissues are:

• Bone marrow
• The Bursa of Fabricius
• The Thymus
• The Foetal Liver

Secondary (or peripheral) lymphoid tissues or secondary lymphoid organs provide a site for immune responses to occur and are populated by relatively mature T cells and B cells, macrophages and dendritic cells; each tissue seems to be preferentially populated by lymphocyte types that specialise in the antigens that are most likely to be presented at that site. The secondary lymphoid tissues are:

• The Lymph nodes
• The Spleen
• Mucosal Associated Lymphoid Tissue or MALT
• The Tonsils
• The Appendix/caecal pouch
• The Ileal Peyer's Patch
• Regional lymphoid tissue

Functions

The lymphatic system has three functions:

1. Immune defence: This is a broad topic area.

   Immune functions are covered broadly under the primary and secondary lymphoid tissues of the lymphoreticular system with further information under the immunology section.

2. Removal of interstitial fluid from tissue: Details can be found at lymphatic vessel function.
3. Transport of fat: Chylomicrons transport digested lipids, however these are too large to enter the blood stream directly and thus enter the lymphatic system before being released into the blood stream. Further details can be found at Triacylglycerol Digestion and Absorption.

Lymphatic System Overview - Anatomy & Physiology Learning Resources
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Flashcards Test your knowledge using flashcard type questions	Lymphoreticular Flashcards

References

Texts:

• Dyce, K.M., Sack, W.O. and Wensing, C.J.G. (2002) Textbook of Veterinary Anatomy. 3rd ed. Philadelphia: Saunders.
• Janeway, C.A., Travers, P., Walport, M. and Shlomchik, M.J. (2005) Immunobiology: The immune system in health and disease. 6th ed. New York: Garland Science Publishing.
• McGeady, T.A., Quinn, P.J., FitzPatrick, E.S. and Ryan, M.T. (2006) Veterinary Embryology. Oxford: Blackwell Publishing.
• Ross, M.H. and Pawlina, W. (2006) Histology: A text and atlas. 5th ed. Baltimore: Lippincott Williams & Wilkins.
• Sjaastad, O.V., Hove, K. and Sand, O. (2004) Physiology of Domestic Animals. Oslo: Scandinavian Veterinary Press.
• Tizard, I.R. (2004) Veterinary Immunology: An Introduction. 7th ed. Philadelphia: Saunders.

Literature:

• Dasso, J.F., Obiakor, H., Bach, H., Anderson, and Mage, R.G. (2000) A morphological and immunohistological study of the human and rabbit appendix for comparison with the avian bursa. Developmental & Comparative Immunology 24(8): pp.797-814.
• Metcalfe, D.D., Baram, D. and Mekori, Y. (1997) Mast Cells. Physiological Reviews 77(4): pp.1033-1064.
• Nance, D.M. and Sanders, V.M. (2007) Autonomic innervation and regulation of the immune system (1987-2007). Brain, Behavior, and Immunity 21(6): pp.736-745.

Originally funded by the RVC Jim Bee Award 2007

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