Bio 202
Lymphatic System/Immunity Lab
Dr. Bruce Forciea
This lab covers the lymphatic system and immunity.
The Lymphatic System
The Big Picture
In essence the circulatory system capillaries are leaky. In fact there is a net loss of fluid of about 30 L/day from the capillaries to the interstitium. The fluid must be brought back into circulation in order to maintain fluid balance. This is one important job of the lymphatic system. That is to return the interstitial fluid back into circulation.
The lymphatic system is a vascular system that contains capillaries, vessels and lymph nodes (fig. 16.1). The lymph capillaries pick up interstitial fluid lost by the circulatory system. The fluid known as lymph moves through the system and is returned to venous circulation.
The lymphatic system also transports dietary fats from the gastrointestinal system. Small lymphatic structures called lacteals are located in the small intestine in structures called villi. Fats are broken down and packaged as structures known as chylomicrons. The fats then move through the system to the venous circulation.
A good portion of the immune system resides in the lymphatic system as well. Lymph nodes containing white blood cells work to destroy pathogens.
Lymphatic Capillaries
Lymphatic capillaries are distributed throughout the interstitium. Lymphatic capillaries are not found in the central nervous system and bone marrow. They are also not resident in tissues without blood flow such as the epidermis or cartilage. They are designed to allow one way fluid flow into the capillary (fig. 16.2).
Lymphatic capillaries consist of overlapping simple squamous epithelium. They also form one way valves. This arrangement allows for increased permeability and fluid movement toward the venous circulation.
Lymphatic Vessels
The lymphatic capillaries form larger structures called lymphatic vessels. The vessels have a similar structure to veins and contain three layers. The three layers consist of an inner endothelium, a middle smooth muscle layer and an outer layer of thin fibrous connective tissue.
Lymphatic vessels also contain valves to allow the one way flow of blood. Smooth muscle contraction moves blood from one area separated by a valve to another. Some cells in the lymph vessel walls are capable of generating action potentials that cause the smooth muscle to contract.
Skeletal muscle contraction also moves lymph fluid by means of generating pressure on the outside of the vessels causing them to constrict. The valves only allow one way flow so lymph is moved toward the venous circulation.
Lymph fluid also moves into vessels in the thoracic cavity as a result of dilation of lymph vessels resulting in decreases in thoracic cavity pressure. The thoracic cavity expands during inspiration causing a decrease in thoracic pressure. The vessels react by dilating and creating an area of lower pressure. Lymph fluid then moves toward the area of lower pressure.
Lymph Nodes
Lymph nodes are located throughout the lymphatic system. The lymphatic vessels connect with the nodes and fluid moves through them. There are numbers of nodes connected to a vessel so that lymph fluid moves from one node to another. Lymph nodes are small oval structures and are generally not felt during examinations unless enlarged or calcified (fig. 16.3).
Lymph nodes act as filters and work to remove pathogens such as bacteria and viruses. Although diffusely located throughout the body, lymph nodes tend to conglomerate in certain areas. These include the cervical, axillary, inguinal, popliteal and mammary glands (fig. 16.4).
Lymph nodes consist of a dense connective tissue covering and a trabeculated internal structure. The nodes contain reticular connective tissue that forms an interconnected web like structure. Vessels entering the nodes are known as afferent vessels. Likewise vessels exiting the nodes are known as efferent vessels.
Lymph nodes consist of an outer cortex and an inner medulla. The cortex contains open areas called sinuses. The medulla contains medullary cords which are branching structures of lymphatic tissue. Open areas called medullary sinuses are also present.
Lymph nodes contain white blood cells called macrophages and lymphocytes. Macrophages are located in the sinuses and phagocytize bacteria and debris. Lymphocytes are located in germinal centers and when activated can move into the bloodstream.
The lymphatic vessels eventually form larger structures known as lymphatic trunks. The lymphatic trunks drain specific portions of the body. The subclavian trunks drain the upper extremities. The jugular trunks drain the head and neck. The bronchomediastinal trunks drain the thoracic area. The intestinal trunks drain the abdomen. The lumbar trunks drain the lower extremities and pelvic area.
The lymphatic trunks connect with larger structures called lymphatic ducts which connect with the venous system at the subclavian veins. There are two ducts including the thoracic duct and right lymphatic duct. The jugular, subclavian and bronchomediastinal trunks connect to either the right internal jugular, right subclavian, or right brachiocephalic trunk. In some people the three trunks merge to form the right lymphatic duct.
The remaining trunks connect with the thoracic duct. The drainage of lymph fluid is therefore asymmetrical with respect to the arrangement of the right lymphatic and thoracic duct. In other words the right lymphatic duct drains the right side of the head, neck and trunk while the thoracic duct drains the left side of the head, neck and trunk as well as both lower extremities.
In some cases the intestinal and lumbar trunks merge to form a sac like structure called the cisterna chili.
Lymphatic Organs
Two organs are associated with the lymphatic system. These are the spleen and the thymus. The organs contain lymphatic tissue consisting primarily of white blood cells known as macrophages and lymphocytes as well as some other types of cells. There are two general types of lymphocytes. These are the T and B lymphocytes. Both are produced in the bone marrow and carried to the lymphatic system. Activation of the immune system causes these cells to divide and attach pathogens.
Lymphatic tissue also contains reticular cells that produce reticular fibers. White blood cells connect with these fibers so that fluid moving through the tissue is exposed to the cells. The white blood cells can then destroy bacteria and debris.
Lymphatic tissue resides throughout the lymphatic system. When it is not located in a lymph node or organ such as in the mucous membranes of the digestive, urinary, respiratory and reproductive systems it is known as Mucosa associated lymphoid tissue (MALT). The tonsils are another example of MALT.
The spleen is located in the left upper quadrant of the abdominal area generally close to the diaphragm and is about as large as an adult fist (fig. 16.5). It consists of an outer connective tissue capsule. The inner portion has a trabeculated structure containing areas of red and white pulp. The spleen also contains venous sinuses.
White pulp consists of lymphatic tissue associated with arteries within lymphatic organs. Red pulp contains both white and red blood cells and is associated with veins.
The splenic artery and vein enter and exit the spleen at the hilum. Blood flows into the spleen and through the trabeculated network. The cells in the spleen work to destroy pathogens. Lymphocytes in the spleen can react to pathogens and trigger the immune system. The spleen also acts as a blood reservoir.
The thymus is a gland located just deep to the sternum in the superior portion of the mediastinum (fig. 16.6). Early in life the thymus is larger and decreases in size with age although it continues to produce white blood cells.
The thymus has two lobes each surrounded by a connective tissue capsule. It contains an outer cortex and inner medulla. The internal region of the thymus is trabeculated and filled with lymphocytes. The thymus produces large numbers of T-lymphocytes that can travel to the blood.
Practice naming lymphatic structures by working through the following learning object:
http://www.wisc-online.com/Objects/ViewObject.aspx?ID=MEA104
View the following immune system learning objects:
http://www.wisc-online.com/objects/index_tj.asp?objID=AP2604
http://www.wisc-online.com/objects/index_tj.asp?objID=MBY601
http://www.wisc-online.com/objects/index_tj.asp?objID=MBY501
Complete and submit the lab assessment.