Chapter 20 lymphatic system

Lymphatic system 

• Returns leaked fluid to the blood

• It has the following parts:

  • A system of lymphatic vessels 

  • Lymph (the leaked fluid that is being returned to the blood)

  • Lymph nodes (that cleanse the lymph as it passes through them)

Lymph

• the leaked fluid that is being returned to the blood

Lymphoid organs and tissues 

• Provide the structure of bodies i’s immune system 

• Include organs like spleen, thymus, and tonsils 

• Lymph nodes are also an important part of the immune system 

Lymphatic trunks 

• The largest collecting vessels unit to for lymphatic trunks 

• These trunks drain fairly large regions of the body

Lymphatic ducts

1. Right lymphatic duct 

2. Thoracic duct 

How is lymph transported 

• The milking action of active skeletal muscles 

• Pressure changes in the thorax during breathing

• Valves to prevent backflow 

• Smooth muscle contraction in walls of lymph vessels also helps 

  • Despite all of these, the flow of lymph is very slow

  • Physical activity and movement increase lymph flow 

Lymphoid cells

• Lymphocytes 

• Macrophages 

• Dendritic cells 

Lymphocytes 

• These are the main immune system cells that help to rid the body of unwanted pathogens 

• There are 2 types

  1. B cells 

  2. T cells 

• Both protect the body against antigens 

Macrophages 

• These cells are crucial for protecting the body, as they engulf (digest) foreign and harmful substances 

Dendritic cells 

• Spiny-looking cells that capture antigens and bring them back to lymph nodes where the immune response is activated 

Lymphoid tissue 

• Is tissue that is comprised of lymphoid cells. In addition, there is also connective tissue that holds these cells together 

• It is found in all lymphoid organs except the thymus 

Lymphoid tissue It is important for 2 reasons:

1. It provides a ‘home’ for lymphocytes and provides a place where they can multiply 

2. It provides an excellent surveillance point from where lymphocytes and macrophages can ‘keep an eye on the body’

   • Because of this the lymphatic system is often called the ‘surveillance system of the body’

Lymphoid organs 

• Comprised of lymphoid tissue 

• They are classified as either:

  • Primary lymphoid organs 

  • Secondary lymphoid organs 

Primary lymphoid organs 

• Red bone marrow and the thymus 

• B cells and T cells both originate in the red bone marrow 

  • B cells mature in the red bone marrow

  • T cells mature in the thymus 

Secondary lymphoid organs 

• These are key players in the immune response because they are where mature lymphocytes are activated (encounter antigens)

• They include:

  • Lymph nodes 

  • The spleen 

  • The collections of mucosa-associated lymphoid tissues (lymphoid nodules) that form tonsils, Peyer’s patches (lymphoid nodules) in small intestine, and the appendix 

• Lymph nodes are unique in that they are the only organ that cleanses lymph 

Lymph nodes

• These are the most important secondary lymphoid organs 

• They are located throughout the body

• Larger clusters found in axillary, inguinal and cervical regions 

• 2 main functions:

• Cleanse lymph

  • They act as filters that remove microorganisms and pathogens. These are destroyed by macrophages 

• Activate the immune system 

  • They are sites where lymphocytes interact with antigens, which triggers an immune response 

• Most are bean-shaped and around 2.5cm long 

• Have a cortex and a medulla 

• Lymph nodes are filled with B and T cells 

Spleen 

• The largest lymphoid organ 

• Located just below the diaphragm 

• Cleanses the blood:

  • Removes aged and defective RBCs

  • Macrophages remove debris and foreign matter

The spleen also 

• Recycles the breakdown products from old RBCs

• Stores blood platelets and monocytes for release when needed 

• May be a site of erythrocyte production in the fetus 

Thymus

• Is the most active in the early years of life 

• It is where the precursor of T cells mature 

• These precursors go on to become mature T cells 

  • By puberty it decreases in size and is mainly fibrous by old age

MALT

• Lymphoid tissue found in many mucosal membranes 

• Include:

  • Appendix 

  • Tonsils

  • Peyers patch (small intestine)

  • Also respiratory and digestive systems 

  • Located in places where pathogens and other harmful substances normally enter the body. Help to destroy these before they can harm the body