Contents
Learning Objectives
By the end of this section, you should be able to:
- List the functions of the lymphatic system.
- Explain how lymph forms and returns to the bloodstream.
- Name the major cells of the lymphatic system and state their functions.
- Name and describe the types of lymphatic tissue or organs.
- Describe the structure and function of the red bone marrow, thymus, lymph nodes, tonsils, and spleen.
Introduction
In our previous lecture, we discussed the circulatory system and how it functions. We discussed that Blood move from the arteries to the capillaries where it interact with tissues, and then it is collected in veins through which it is transferred back to the heart.
One thing is that this process is not always 100% efficient. When circulating blood reaches the capillaries, part of its fluid passes into surrounding tissues as Tissue fluid. Most of the fluid re-enters the capillaries at their venous ends. However, some of it return to the circulation through a separate system called LYMPHATIC SYSTEM.
What is Lymphatic System
Lymphatic comes from the Latin word lymphaticus, meaning "connected to water," as lymph is clear.
The Lymphatic system, Is a subsystem of the circulatory system that consists of a complex network of vessels, tissues, and organs. This network of lymphatic vessels carry a clear fluid called lymph in one direction towards the heart.
The circulatory system processes an average of 20 litres of blood per day through capillary filtration which removes plasma while leaving the blood cells. Roughly 17 litres of the filtered plasma actually get reabsorbed directly into the blood vessels, while the remaining 3 litres are left behind in the interstitial fluid. The primary function of the lymph system is to provide an assistant route for these excess 3 litres per day to get returned to the blood.
I won't be spending so much time in explaining this topic because the lymphatic system is not meant to be fully discussed in ANP 102, you will be doing it in later courses, however it is good if you should know its basics and the reason for it.
Functions of the Lymphatic System
- The lymphatic system removes interstitial fluid from tissues.
- The lymphatic system absorbs and transports fatty acids and fats from the digestive system
- The lymphatic system transports white blood cells in the body
- The lymphatic system inspects body fluids for disease agents
Components of the Lymphatic System
- Lymph
- Lymphatic Vessels
- Lymphatic Cells
- Lymphatic organs
Lymph
Lymph is a clear fluid that flows around the body in the lymphatic system. It is formed from plasma. Plasma is carried around the body in the blood vessels. It leaks out of the blood vessels and bathes the tissues and supplies the cells of the body with nutrients. Most of this plasma then drains back into the blood vessels.
A small amount is left behind, together with:
- waste products from the cells
- fat that is broken down in the bowel and needs to be carried to larger blood vessels
- things that have got into the body and might be harmful, such as germs and toxins
- damaged or abnormal cells, including cancer cells.
This all drains into tiny lymph vessels. Lymph vessels in the small intestine also absorb fats and fat-soluble vitamins. When it is in lymph vessels, the fluid is known as ‘lymph’.
Lymphatic Vessels
Lymphatic Capillaries
Lymph capillaries surround and collect Lymph from the various tissues in the body.
Lymph capillaries are absent in central nervous system, cartilage, cornea, bone, and bone marrow.
Lymph collected by capillaries are collected into two ducts
- Thoracic duct - collects from left side head, neck, and chest, left upper extremity, and entire body below ribs.
- Right Lymphatic duct – collects from upper right side of body
Lymphatic Trunk
Lymphatic trunks are formed from the convergence of lymphatic vessels or capillaries.
There are Five major lymph trunks in the human bodies:
- Lumbar trunks: receives lymph from lower limbs
- Intestinal trunk: receives chyle (lymph that comes from digestive organs e.g small intestine) from digestive organs
- Bronchomediastinal trunks: collects lymph from chest region.
- Subclavian trunks: receive lymph from upper limbs and thoracic wall.
- Jugular trunks: drain lymph from the head and neck
Lymphatic Cells
Lymphatic cells comprise of Lymphocytes, macrophages, dendritic cells and reticular cells.
They play a major role in the immune function of the Lymphatic System. They provide defense against specific pathogens or toxins. They can live for months, even decades. They are produced in bone marrow and lymphatic tissues.
Important Lymphocytes in the Lymphatic System include:
- T-Cells: Directly attack foreign cells or body cells infected by viruses. Majority of circulating lymphocytes are T-Cells.
- B-Cells – Produce antibodies which react with antigens (foreign substances e.g bacteria). Antibodies join with antigens, begin destruction of target.
- NK cells – Natural killer cells. Attack foreign cells, normal cells infected with viruses, cancer cells.
Dendritic cells
These cells alert immune system to pathogens that have breached the body surface.
Reticular cells
Branched stationary cells that contribute to the stroma of a lymphatic organ.
Lymphatic organs
Lymphatic organs are split into two types:
- Primary organs: These are sites of development and maturation of Lymphatic cells (bone marrow & thymus).
- Secondary organs: These are sites of mass activated lymphocyte populations (lymph nodes, lymphatic nodules, spleen, tonsils).
Red Bone marrow
Red bone marrow is involved in haematopoiesis (blood formation) and immunity
Lymph Nodes
Lymph nodes are small, round organs, range from small to big.
Lymph nodes filter lymph before it is returned to blood stream. 99% of pathogens (bacteria, toxins etc.) are removed
Lymph nodes are located in ideal spots to protect vital organs of body
Thymus
Thymus lies behind the sternum in the chest region
T-cells divide and mature there.
Thymus | photo credit: Wikimedia commons
Your Thymus is most probably shrinked by now. It is only very active during young age.
Thymus is also the site of thymosins production. Thymosins are hormones that regulate T-cell development.
Spleen
Spleen is the largest lymphatic organ.
Spleen | Photo credit: OpenStaxCollege
It has two different pulps know as White pulp (thick sleeves of lymphatic tissue) and Red pulp (surrounds white pulp).
White pulp
- Resembles lymphoid nodules
- Removes antigens
- Initiates immune response
Red pulp
- Contains red blood cells
- Recycles damaged or outdated Red Blood Cells (RBCs)
- Stores iron from recycled RBCs
Lymphatic nodules (follicles)
Lymphatic nodules are dense masses of lymphocytes and macrophages that congregate in response to pathogens.
Peyer patches: dense clusters of lymphocytes and macrophages in the ileum, a part of the small intestine.
Tonsils
Simplest lymphatic organs majorly found in the throat area. They contain thousands of white blood cells that fight bacteria that enter the body through the nose and mouth.
There are Four groups of tonsils:
- Palatine tonsils
- Lingual tonsils
- Pharyngeal tonsils
- Tubal tonsils
Tonsils are arranged in a ring to gather and remove pathogens.
Appendix
Appendix has an immune function, especially early in life.
It assists with the maturation of B- cells and in the production of antibodies known as immunoglobulin A.
Researchers have also shown that appendix is involved in the production of molecules that help direct the movement of lymphocytes to various other locations in the body.
Disorders of the Lymphatic and Immune Systems
Chylothorax: leakage of fatty lymph into the thorax.
Lymphangitis: inflammation of a lymph vessel
Mononucleosis: viral disease caused by Epstein-Barr virus, Attacks B lymphocytes.
Elephantiasis: blockage of lymphatic vessels by filaria worms.
Lymphedema: occurs when the lymph vessels are not able to adequately drain lymph fluid, usually from an arm or leg.
You don't understand Lymphatic System after reading this?
Watch this video below and then read the article once again.
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