Gross Anatomy

Lymph Anatomy

On the macroscopic level, the lymph system is comprised of a network of vessels and associated tissue that is responsible for the removal of excess interstitial fluid that builds up naturally as a result of capillary leakage, to absorb and transport fatty acids and other nutrients that are too large to be effectively transported across the apical membrane of the intestinal lumen, and the production, storage and transport of immune cells; most notably lymphocytes. The structures of the lymphatic system are divided into the conducting system that transports the lymph from the interstitial space back to systemic circulation, and the lymphoid tissues that produce, transport and store immune cells, most notably, lymphocytes.

Conducting System

The conducting system of lymphatic tissue is comprised of the lymph capillaries, lacteals, lymph vessels and the left and right thoracic ducts. These structures are responsible for returning capillary filtrate from the interstitial space to the circulatory system and thus aid in maintaining constant circulating volume and thereby maintaining relatively constant blood pressure. As these structures are independent of blood flow, they lack a dedicated pumping mechanism to move fluid; rather movement of fluid through lymph vessels relies on skeletal muscle contraction. Like veins, lymph vessels have valves on the interior to ensure that the flow of fluid is unidirectional. The interstitial fluid is absorbed through specialized openings in the lymph capillary that are created by overlapping of endothelial cells. This creates a one way valve that allows for fluid flow into lymph capillaries but prevents fluid movement out. Skeletal muscle contraction conducts the lymph through increasingly larger vessels and eventually back to systemic circulation via the left and right thoracic ducts that empty into the left and right subclaivian veins. Lacteals are the lymph structures responsible for the absorption of lipids and fat soluble compounds from the intestinal lumen and transporting them back to systemic circulations. Fluid moves through lacteals by the same mechanism as the rest of the conducting system. It is important to note that any fat soluble substance absorbed in this manner bypasses the liver as the lacteals are not connected by the hepatic portal circulation and any toxins or pharmaceuticals escape possible primary degradation by hepatocytes in this manner.  

Primary Tissues

The primary lymph organs are the Thymus and the bone marrow and are sites for lymphocyte development. The thymus is a bi-lobed organ that sits in the anterior superior mediastinum, anterior to the heart and posterior to the sternum and is the principle site of T-cell production. The thymus grows from birth, reaching its greatest size during puberty, after which, the thymus begins a process of involution in response to the production of sex hormones at the onset of puberty. This involution of the organ correlates to a decrease in organ size and activity (T-cell production) and continues slowly throughout the lifetime of the individual. Each two lateral lobes of the organ are composed of multiple smaller lobules and these are further divided into follicles that are approximately 1-2 nm in diameter, histologically, these follicles are separated into cortical and medullary tissue. Cortical tissue is where primary development of T-cells occurs and then they migrate to the medulla for maturation and then transport to secondary lymph organs via systemic circulation.

Figure 6Thymus as it sits in the chest

Figure 7 Exposed bone marrow of Femur head

Bone marrow consists of flexible tissue that occurs in two types; red and yellow both located inside of bones. Yellow marrow consists mainly of fat cells located in the medullary cavity and is surrounded by red marrow. Red marrow is hematopoietic, responsible for production of platelets, red and white blood cells. All bone marrow is highly vascular to allow for quick delivery of fresh red and white cells to systemic circulation. Bone marrow is considered a key structure of the lymphatic system as it is a site for lymphocyte production.

Secondary Tissues

The secondary organs of the lymph system provide an environment for mature lymphocytes to be stored for release in a systemic immune response and activation by antigens. Theses organs also allow for lymphocytes not in systemic circulation to interact with antigens or foreign material. The main secondary organs are; lymph nodes, tonsils, spleen, adenoids, Payer’s patches and mucus associated lymphoid tissue (MALT).

Figure 8 Axillary lymph node

Lymph nodes lie along the lymph vessels and are small, bean shaped structures no greater than 2cm in size that accept fluid from multiple afferent lymph vessels and releasing fluid from a single efferent lymph vessels. The efferent vessels emerge from a small depression of the nodular surface called the hilum; it is this depression that gives the lymph nodes their bean like shape. The inside of the node consists of reticular and elastin fibers that provide not only structural support but a lattice of fibers that aids in lymphocyte motility.

Figure 9 Lymph node

Figure 10 Tonsils

The palatine tonsils are small lymph associated tissue clusters situated in the oro and nasopharynx. They aid combating any possible pathogenic material that enters the body via the nose or the mouth. Mucosa associated lymph tissue is found imbedded in the various mucus membranes throughout the body. As many mucus membranes are in direct contact with possible pathogens or in direct contact with the external environment such as the airways or the gut lumen, it is necessary for these mucus membranes to have local foci of lymph tissue for the storage of lymphocytes for quick release in the event of contact with pathogens.

Figure 11 Anatomical position of spleen and thymus

Figure 12 Excized spleen from tuberculosis patient

The spleen is the largest lymphoid organ weighing up to 200g and it lies between the 9^th and 12^th thoracic ribs in the upper left quadrant of the abdomen. The spleen is divided into two main tissue types, red and white pulp. The red pulp is the site of removal and degradation of red blood cells from circulation and retains a store of red cells so the body can respond to hemorrhagic shock by quickly replacing a portion the circulating red cells as well as removing antigens and microorganisms from the blood. The white pulp is a main storage site for monocytes and removes antibody tagged bacteria and cells from both systemic and lymphatic circulation.

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