Structure and Functions of Lymphatic Tissue (incl. lymph)
1.0 Introduction to Lymphatic Tissues:
What is the Lymphatic System ?
The Lymphatic System is one of the two systems of circulation of fluid around the body (the other being blood circulation).
The lymphatic system consists of lymphatic fluid (called lymph) flowing through a system of lymphatic vessels, including lymphatic capillaries, other lymphatic vessels of various sizes (which can be compared with blood vessels), and lymph nodes (which are encapsulated masses of B Cells and T Cells).
The Blood System and the Lymphatic System are interconnected. The clear fluid that is known as lymph when flowing through the lympatic vessels initially passes into those vessels as interstitial fluid contained in spaces between tissue cells, spaces into which it had been filtered from blood. Ultimately, after travelling through lymphatic vessels, the same lymph passes back into the blood system at the junctions of the jugular and subclavian veins (on both the right- and left- sides of the body).
2.0 The tissues that form the Lymphatic System
Several different types of tissues form the structures of the lymphatic system. They are listed according to the parts of of the lymphatic system that they form:
(a) Primary Lymphatic Organs
The locations at which stem cells divide and mature into B Cells and T Cells:
- Red Bone Marrow
- The Thymus - This is a two-lobed organ located in the chest. The tissue of the thymus itself consists of T cells, macrophages and epithelial cells. Each lobe is encapsulated by a layer of connective tissue.
(b) Secondary Lymphatic Organs and Tissues
The locations at which most immune responses occur:
- Lymph Nodes - These are small bean-shaped organs located throughout the body and consisting of B cells that develop into plasma cells - which secrete antibodies, T cells, and macrophages. Each node is covered by a capsule of dense connective tissue.
- The Spleen - This is the largest single mass of lymphatic tissue in the human body.
The outer covering of the spleen is formed by dense connective tissue.
The spleen itself consists of two types of tissue, called white pulp and red pulp.
- White Pulp: lymphatic tissue that consists mostly of lymphoctes and macrophages.
- Red Pulp: Blood-filled sinuses and cords of splenic tissues that consists of lymphocytes, macrophages, erythrocytes, granulocytes, and plasma cells.
- Lymphatic Nodules - Lymphatic Nodules differ from Lymph Nodes in that lymphatic nodules are not surrounded by capsules (of dense connective tissue). They are also known as mucosa-associated lymphatic tissue (MALT).
(c) Lymph (Fluid)
The fluid lymph can be described as a tissue in its own right - in the same way as the fluid blood can be described as blood tissue. Lymph is a clear fluid that is similar to plasma but contains less protein. It flows through lymphatic vessels throughout the body and includes chemicals and cells whose composition varies according to location within the body.
Despite being a fluid, lymph is classified as a connective tissue.
3.0 Where in the body are the lymphatic tissues ?
As explained above, there are lymphatic tissues throughout the body.
However, the structure of lymphatic tissues vary according to the particular type of lymphatic tissue or organ they form a part of - (a), (b) or (c) above, and possibly also its location in the body - especially in the case of (c) Lymph (Fluid).
4.0 The Structure (Physical Description) of the lymphatic tissues
Lymph is a clear fluid that is similar to (blood) plasma, but contains less protein.
5.0 The Functions of lymphatic tissues
The lymphatic tissues throughout the body operate in conjunction with each other to perform the functions of the lymphatic system. The lymphatic system is part of the body's immune system. The functions of the lymphatic system include:
- Draining interstitial fluid.
- Transporting dietry lipids.
- Protecting the body (organism) against invasion / infection
See also related pages about: The structure and functions of blood, the components of blood, the structure and functions of blood vessels, blood pressure and the Glossary pages about the components of blood.