The Lymphatic System


Diagram of the lymphatic system. Lymph notes are swellings on tubes (called lymph vessels) that travel throughout the body. The right lymphatic duct and entering vein are in the neck. A tonsil is a swelling on the lymph vessel in the mouth. The thymus is a lumpy structure on the heart. A close-up of a lymph node shows a roundish structure with many tubes attached to it. The central area has a box labeled “masses of lymphocytes and macrophages”. A close-up of this area shows tissue cells in the background with a blood capillary network. Lymph vessels run between the  blood capillary network. Lymphatic capillaries are the ends of the lymph vessels. Fluid from around the cells (called interstitial fluid) enters the lymphatic capillaries and travels through the lymphatic vessels.
The essential components of the human lymphatic system drain fluid away from tissues.

Source: OpenStax Microbiology

OpenStax Microbiology

The lymphatic system is also a network of vessels that run throughout the bod. However, these vessels do not form a full circulating system and are not pressurized by the heart. Rather, the lymphatic system is an open system with the fluid moving in one direction from the extremities toward two drainage points into veins just above the heart. Lymphatic fluids move more slowly than blood because they are not pressurized. Small lymph capillaries interact with blood capillaries in the interstitial spaces in tissues. Fluids from the tissues enter the lymph capillaries and are drained away. These fluids, termed lymph, also contain large numbers of white blood cells.

Blood enters the capillaries from an arteriole (red) and leaves through venules (blue). Interstitial fluids may drain into the lymph capillaries (green) and proceed to lymph nodes. A close-up of tissue cells in interstitial fluid. An arteriole and a venule are connected by a network of capillaries. Lymphatic vessels are also a network  in this region and end in lymph capillarie.
Blood enters the capillaries from an arteriole (red) and leaves through venules (blue). Interstitial fluids may drain into the lymph capillaries (green) and proceed to lymph nodes. (credit: modification of work by National Cancer Institute, National Institutes of Health)

The lymphatic system contains two types of lymphoid tissues. The primary lymphoid tissue includes bone marrow and the thymus. Bone marrow contains the hematopoietic stem cells (HSC) that differentiate and mature into the various types of blood cells and lymphocytes. The secondary lymphoid tissues include the spleen, lymph nodes, and several areas of diffuse lymphoid tissues underlying epithelial membranes. The spleen, an encapsulated structure, filters blood and captures pathogens and antigens that pass into it. The spleen contains specialized macrophages and dendritic cells that are crucial for antigen presentation, a mechanism critical for activation of T lymphocytes and B lymphocytes. Lymph nodes are bean-shaped organs situated throughout the body. These structures contain areas called germinal centers that are rich in B and T lymphocytes. The lymph nodes also contain macrophages and dendritic cells for antigen presentation. Lymph from nearby tissues enters the lymph node through afferent lymphatic vessels and encounters these lymphocytes as it passes through; the lymph exits the lymph node through the efferent lymphatic vessels.

a) A diagram showing that the spleen is found at the end of the pancreas just under the diaphragm. The part closest to the pancreas is the hilium and contains the splenic artery and vein. B) a close-up of the spleen. Afferent lymphatic vessels are tubes that bring material in. Efferent lymphatic vessels are tubes that take material out of the spleen. Connective tissue forms a capsule around the outside; and a space under the capsule is labeled subcapsular sinus. The cortex of the spleen is the outer layer. And trabeculae are regions of cortex that fold in towards the center. Germinal centers are red structures in the cortex.
(a) The spleen is a lymphatic organ located in the upper left quadrant of the abdomen near the stomach and left kidney. It contains numerous phagocytes and lymphocytes that combat and prevent circulatory infections by killing and removing pathogens from the blood. (b) Lymph nodes are masses of lymphatic tissue located along the larger lymph vessels. They contain numerous lymphocytes that kill and remove pathogens from lymphatic fluid that drains from surrounding tissues.

Source: OpenStax Microbiology


Parker, N., Schneegurt, M., Thi Tu, A.-H., Forster, B. M., & Lister, P. (n.d.). Microbiology. Houston, Texas: OpenStax. Access for free at:


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