ANATOMY & PHYSIOLOGY II THE BODY SYSTEMS
Anatomy & Physiology II The Body Systems Michelle Cochrane 2014 All rights reserved. This material is subject to copyright and may not be reprinted or reproduced in any manner without prior consent from the author. Reproduced by Om Shanti College with permission from the author for educational purposes only. 2
LESSON 4 LYMPHATIC SYSTEM 4.1 Function 4.2 Lymphatic Circulation 4.2.1 Lymphatic Fluid 4.2.2 Lymph Capillaries 4.2.3 Lymph Vessels 4.2.4 Lymph Trunks and Ducts 4.2.5 Lymph Flow 4.3 Lymphatic Cells and Tissues 4.3.1 Cells 4.3.2 Tissues 4.4 Lymph Nodes 4.5 Lymphatic Organs 4.5.1 Thymus 4.5.2 Spleen 4.5.3 Tonsils and Adenoids 4.5.4 Appendix 3
4.1 FUNCTION The lymphatic system is part of the circulatory system, the other part being the cardiovascular system, however it is often considered separately as a system in its own right. The main role of the lymphatic system is to drain fluid from the tissues back into the bloodstream. The lymphatic system has a number of important functions that it performs, including: Maintaining fluid balance Transporting lipids around the body Transporting white blood cells around the body Removing waste products, bacteria and other toxins Filtering the blood 4.2 LYMPHATIC CIRCULATION Blood flows through the cardiovascular system delivering oxygen, nutrients and other substances to the cells and tissues of the body. When this happens, fluid moves from the blood vessels into the interstitial spaces then back into the blood vessels again. Some of the fluid remains behind in the interstitial spaces. This fluid needs to be returned to the bloodstream again in order to maintain blood pressure. This is where the lymphatic circulation comes in. Unlike the cardiovascular system, lymphatic circulation only works in one direction returning fluid back from the body to the venous system. 4.2.1 LYMPH FLUID Lymphatic fluid is similar in composition to blood plasma however it is not the same and should be considered separately. It mainly consists of water, but also contains any plasma proteins that have leaked from the blood vessels into the interstitial spaces and must be returned to the blood. 4
The lymphatic fluid also contains other substances. It can pick up bacteria and cancer cells and carry them to the lymph nodes for destruction. It also transports fat from the digestive system. We make about 3 litres of lymph fluid per day. (Pearson Education) 4.2.2 LYMPH CAPILLARIES Lymph capillaries are the smallest vessels in the lymphatic system. Unlike blood capillaries, the lymph capillaries are closed at one end. This means that fluid can flow into them but not out. The main purpose of the capillaries is to drain excess fluid from the tissues. Lymph capillaries are found in almost all of the tissues of the body except avascular tissues (those that do not contain blood vessels), bone marrow and central nervous system. 4.2.3 LYMPH VESSELS Just as blood capillaries join together to feed into the veins, so lymph capillaries join together to feed into the lymph vessels. Lymph vessels are very similar in structure to veins, except that they have thinner walls and more valves. Like veins, they are impermeable that is they do not allow anything to leak through the walls. The lymph vessels connect back to the venous system via the lymph trunks and ducts. 5
4.2.4 LYMPH TRUNKS AND DUCTS The lymph trunks and ducts provide the link between the lymphatic system and the cardiovascular system. The lymph vessels merge to form lymphatic trunks which then pass into two lymph ducts: Right lymphatic duct o Drains fluid from the upper right hand side of the body only. o Drains into the right subclavian vein (beneath the right clavicle). Thoracic duct o Drains fluid from the whole of the rest of the body. o Drains into the left subclavian vein (beneath the left clavicle). 4.2.5 LYMPH FLOW The lymph fluid flows from the lymph capillaries into the lymph vessels, then through the lymph nodes (see below) and into the lymphatic trunks and ducts. The lymphatic system does not contain an organ like the heart which pumps the fluid around, so it must rely on other ways to ensure that the fluid flows through. There are three main ways in which this is achieved: The contraction of skeletal muscles acts as a pump to push fluid through the vessels. (this is why it is important to keep moving, and why you may get swollen ankles after sitting still for a long time such as on an aeroplane) Respiration causes a difference in the pressure gradient. Since fluid will always try to move from an area of high pressure to an area of low pressure, this helps to move the fluid along. Lymph vessels have a large number of valves more than in the veins that stop the fluid from flowing back the wrong way. Otherwise gravity would cause all the fluid to pool around our ankles. 4.3 LYMPH CELLS AND TISSUES The functional parts of the lymph system the lymph nodes and organs are mainly comprised of immune cells which are supported by connective tissue. 4.3.1 CELLS The most important cells in the lymphatic system are the immune cells. There are two main types of immune cells in the lymphatic system lymphocytes and macrophages. 6
Lymphocytes Lymphocytes are also found in the blood. In fact, they are the most common type of white blood cell. Their main purposes are the same in the lymphatic system as they are in the blood system: Destroy viruses and cancer cells Regulate the immune system Secrete antibodies The lymphocytes can be divided into: B cells found mainly in the spleen and lymph nodes. Can be activated to produce antibodies against recognised pathogens (harmful substances). T cells either activate other B cells and T cells to fight pathogens, or fight the pathogens themselves. We will look into lymphocytes in more detail when we learn about the immune system. Macrophages Macrophages are the other type of immune cell found in the lymphatic system. They destroy bacteria and other pathogens by phagocytosis. They also help to activate the T cells. Reticular cells These are the cells which generate the connective tissue which supports the immune cells. 4.3.2 TISSUES Lymph tissue is made from reticular connective tissue (see reticular cells above). It makes up all of the lymphatic organs except the thymus. Macrophages are usually found on the outside of the reticular tissue while the lymphocytes are found in the spaces in between the tissues. 4.4 LYMPH NODES Lymph nodes are small, oval or bean shaped structures made from concentrated lymphatic tissue. They are found throughout the body, usually in groups. Each node is surrounded by a layer of dense connective tissue known as a capsule. 7
The areas where the biggest concentrations of lymph nodes are found are: Throat Armpits Chest Abdomen Groin Lymph nodes are comprised of two parts an outer cortex and an inner medulla (you will also find this structure in some other organs of the body). Lymph vessels pass through the lymph nodes, where the fluid is filtered and harmful substances are removed. The lymph fluid can only move in one direction. Lymph vessels flowing into the lymph nodes are called afferent vessels and lymph vessels flowing out of the lymph nodes are called efferent vessels. The lymph nodes contain many lymphocytes both B cells and T cells. When lymphatic fluid passes through the lymph nodes, it gets filtered. Bacteria, viruses, cancer cells and other pathogens are trapped in the tissues where they are acted on by lymphocytes and macrophages. At times of infection, lymph nodes can become enlarged as they get inflamed due to the large number of pathogens. The lymph nodes have an important role to play in destroying cancer cells. Cancer cells that metastasise (travel through the body) can often travel in the lymphatic system. The lymph nodes filter out these cells to be destroyed, however some of the cells may lodge in the nodes and cause a secondary cancer. This is why breast cancer can sometimes spread to the axillary lymph nodes which are located in the armpits. 8
4.5 LYMPHATIC ORGANS The lymph nodes are just one example of a number of organs that are involved in the lymphatic system, all of which have their own role to play at different stages of the lymphatic process. 4.5.1 THYMUS The thymus is responsible for the development of our T cells. T cells are first developed in the bone marrow, then migrate to the thymus where they develop and mature. Most T cells are developed during childhood. It is most active during infancy and its function and size sharply decline after puberty. This is why, unlike most organs of the body, the thymus is larger in children than in adults. After its function is fulfilled it is no longer required and therefore atrophies (gets smaller). We will look at the development of T cells more when we learn about the immune system. The thymus is different from the other lymphatic organs in that it does not directly fight pathogens itself, but it is considered a lymphatic organ because of its role in developing T cells. 4.5.2 SPLEEN The spleen is the largest lymphatic organ in the body. It is oval in shape and about the size of a fist, situated near the stomach and liver. It is comprised of lymphatic tissue surrounded by a capsule of connective tissue. It has two main components: White pulp mostly lymphatic tissue containing B cells Red pulp contains old red blood cells and macrophages White pulp This consists of lymphatic tissue with a very high concentration of B cells and is where the immune function of the spleen takes place. 9
Red pulp This is where red blood cells are received when they are worn out (about 120 days old). It is also the site where pathogens are collected. Both the old RBCs and pathogens are then destroyed by macrophages through phagocytosis. Although the spleen is a lymphatic organ, it does not have any afferent vessels so it does not receive and filter lymph. Its main purpose is to store B cells and macrophages. Since all the functions of the spleen can also be carried out by other organs, we are able to live successfully without it. 4.5.3 TONSILS AND ADENOIDS The tonsils and adenoids are collections of lymphatic tissue that are found at the back of the mouth. They are located at the junction of the oral cavity with the oropharynx and nasopharynx. (see the section on the respiratory system above if you cannot remember where these are) Their purpose is to mount an immune response to harmful substances that are either inhaled or ingested. Tonsils are made from a type of lymphatic tissue known as mucosa associated lymphoid tissue (MALT) because of their position in the connective tissue associated with mucous membranes. Unlike lymph nodes, they are not fully encapsulated which means that pathogens landing on the mucosal tissue get absorbed into the lymphatic tissue where they can be destroyed. Remember tonsils are inside the mouth. The lumps you can feel on either side of your throat are not your tonsils, they are your cervical lymph nodes. Although they can often become inflamed when you have tonsillitis because of their close proximity. 4.5.4 APPENDIX The appendix it a small tube attached to the large intestine which contains lymphatic tissue. It helps to destroy unwanted bacteria in the large intestine. It also functions to produce memory B cells (which we will discuss later) which help to fight future infections. Like the spleen, there are other organs which perform the same functions as the appendix so we can live without it. 10