Marieb Human Anatomy & Physiology Chapter 20 - The Lymphatic System

📚 Chapter 20 Notes

The Lymphatic System and Lymphoid Organs and Tissues

The lymphatic system is a drainage and defense system for the body. Its main job is to return extra fluid that leaks out of blood vessels back into the bloodstream. Every day, about 3 liters of fluid leak out of capillaries and are not reabsorbed. If this fluid stayed in tissues, we would swell severely. The lymphatic system collects this excess fluid, now called lymph, and returns it to the blood. It also plays a major role in immunity by filtering pathogens and housing immune cells.

The lymphatic system consists of lymphatic vessels, lymph, lymph nodes, and lymphoid organs such as the spleen, thymus, tonsils, Peyer’s patches, and appendix. Together, these structures protect the body from infection and disease.

Lymphatic Vessels and Lymph

Lymphatic capillaries are tiny, blind-ended vessels that weave between tissue cells and blood capillaries. They are more permeable than blood capillaries because their endothelial cells overlap loosely to form one-way mini valves. When interstitial fluid pressure increases, these mini valves open and allow fluid to enter. When pressure inside rises, they close to prevent backflow. This design ensures fluid moves in only one direction.

Unlike blood capillaries, lymphatic capillaries can absorb large particles such as proteins, cell debris, pathogens, and even cancer cells. In the small intestine, specialized lymphatic capillaries called lacteals absorb digested fats. The fatty lymph produced here is called chyle.

Lymphatic capillaries merge into larger lymphatic vessels that resemble veins. These vessels have thinner walls and more valves than veins. Lymph is transported through nine major lymphatic trunks, which then drain into one of two ducts. The right lymphatic duct drains the right upper portion of the body. The thoracic duct drains the rest of the body and is larger. Both ducts empty into the venous circulation near the junction of the internal jugular and subclavian veins.

Lymph flow is slow because the system operates under low pressure. It depends on skeletal muscle contractions, breathing movements, nearby arterial pulses, and smooth muscle contractions in the vessel walls. Physical activity increases lymph flow, while immobilization slows it.

Lymphedema occurs when lymph cannot properly return to the blood, causing swelling. This may result from tumors blocking vessels or removal of lymph nodes during cancer treatment.

Lymphoid Cells and Tissue

Lymphoid tissues contain immune cells and supporting cells. The main immune cells are lymphocytes, which include T cells and B cells. T cells directly attack infected or abnormal cells and manage immune responses. B cells become plasma cells that produce antibodies to mark pathogens for destruction.

Macrophages engulf pathogens and help activate T cells. Dendritic cells capture antigens and present them to T cells to trigger an immune response. Supporting cells called reticular cells form a network of fibers that provide structure to lymphoid tissue.

Lymphoid tissue is made primarily of reticular connective tissue and serves as a surveillance site for immune cells. It can appear as diffuse tissue or organized into lymphoid follicles. Many follicles contain germinal centers, where B cells rapidly divide.

Primary lymphoid organs are where lymphocytes mature. B cells mature in red bone marrow. T cells mature in the thymus. Secondary lymphoid organs are where mature lymphocytes first encounter antigens and become activated. These include lymph nodes, spleen, and mucosa-associated lymphoid tissue (MALT).

Lymph Nodes

Lymph nodes are small, bean-shaped structures located along lymphatic vessels. They filter lymph and activate immune responses. Hundreds are located throughout the body, especially in the cervical, axillary, and inguinal regions.

Lymph nodes have an outer capsule and internal trabeculae dividing them into compartments. The cortex contains lymphoid follicles filled with B cells. The deeper cortex contains T cells. The medulla contains plasma cells, B cells, and T cells arranged in medullary cords.

Lymph enters through afferent lymphatic vessels, flows through sinuses where macrophages filter it, and exits through fewer efferent vessels at the hilum. Because fewer vessels exit than enter, lymph flow slows, allowing immune cells time to act.

Swollen lymph nodes, called lymphadenopathy, usually indicate infection. Infected nodes may become tender and inflamed. Cancer cells can also spread to lymph nodes; these nodes are typically swollen but not painful.

Spleen

The spleen is the largest lymphoid organ and is located in the left upper abdomen under the diaphragm. It filters blood instead of lymph. It removes aged red blood cells, pathogens, and debris.

The spleen contains white pulp and red pulp. White pulp is involved in immune responses and contains lymphocytes surrounding central arteries. Red pulp contains macrophages that destroy old red blood cells and recycle iron.

The spleen also stores platelets and monocytes and can produce red blood cells during fetal development. If removed, the liver and bone marrow take over many of its functions.

MALT (Mucosa-Associated Lymphoid Tissue)

MALT protects body entryways such as the respiratory, digestive, and genitourinary tracts. Major structures include tonsils, Peyer’s patches, and the appendix.

Tonsils form a ring around the pharynx and trap pathogens from food and air. The palatine tonsils are the largest and most often infected. Tonsils have crypts that trap debris and bacteria, helping build immune memory.

Peyer’s patches are large clusters of lymphoid follicles in the small intestine. They prevent bacteria from penetrating the intestinal wall and generate memory lymphocytes.

The appendix contains many lymphoid follicles and also helps prevent bacterial invasion.

Thymus

The thymus is a primary lymphoid organ located in the lower neck and upper chest. It is most active during childhood and shrinks after puberty. It is responsible for T cell maturation.

The thymus is divided into lobules with an outer cortex and inner medulla. The cortex contains rapidly dividing T cells. The medulla contains thymic corpuscles and helps develop regulatory T cells to prevent autoimmune disease.

The thymus does not directly fight antigens. Instead, it prepares T cells to recognize self versus non-self. It has a blood-thymus barrier that protects immature T cells from premature exposure to antigens.