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SPLEEN ANATOMY

The spleen is a secondary lymphoid organ in the upper left quadrant of the abdomen, between the 9th and 11th ribs. It is an intraperitoneal organ enveloped by the visceral peritoneum, except at the splenic hilum. The fibrous capsule that envelops it is made up of connective tissue. The capsule forms short extensions into the organ called trabeculae. Surface features of the spleen: On opposite ends of the spleen, we have the anterior and posterior extremities. On either side we see the superior and inferior borders. Here we have the lateral, or diaphragmatic surface, which is the convex surface which fits into the concavity of the left hemidiaphragm. And here we have the medial, or visceral surface. On the visceral surface, there are three impressions – the renal impression, the gastric impression, and the colic impression. The hilum allows passage of the splenic artery and splenic vein. The spleen’s primary blood supply is from the splenic artery. The splenic artery branches into smaller vessels called trabecular arteries. The gastrosplenic ligament connects the hilum with the stomach’s greater curvature and the splenorenal ligament connects the hilum with the left kidney. The splenic vessels and the tail of the pancreas are within the splenorenal ligament. Between the gastrosplenic ligament and the splenorenal ligament is the lesser sac. Note that the spleen is supported by the phrenicocolic ligament, which is a fold of the peritoneum that originates from the colon. The spleen has two tissue types: the white pulp and the red pulp. The white pulp is the main lymphoid tissue of the spleen and contains lymphoid elements, including follicles, lymphocytes, and plasma cells. Lymphocytes are produced in germinal centers, which are at the center of follicles. Macrophages reside in both the white and red pulp. The red pulp constitutes most of the spleen's volume. It consists of venous sinuses and cords of lymphatic cells, including lymphocytes and macrophages. As blood flows through the spleen, macrophages and dendritic cells within the red pulp and marginal zone capture and present antigens to T- and B-cells, initiating immune responses. T-cells recognize specific antigens presented by antigen-presenting cells. This triggers the activation and proliferation of T-cells, leading to their differentiation into effector T-cells. These effector T-cells coordinate immune responses by directing the actions of other immune cells or by attacking directly. B-cells differentiate into plasma cells, which produce and release antibodies. Antibodies attach to antigens and help remove them from your body. Individuals without a functioning spleen are prone to a higher risk of infections. The spleen is the largest lymphoid organ and the only lymphoid organ that primarily filters blood instead of lymphatic fluid. As blood passes through the spleen, the reticular fibers and macrophages within the splenic cords actively remove damaged, old, or abnormal red blood cells, cellular debris, and foreign particles from circulation. This process ensures the maintenance of a healthy RBC population and prevents circulation of abnormal or damaged cells. Additionally, the spleen’s red pulp acts as a reservoir for platelets and red blood cells, releasing them into circulation during times of increased demand, such as in cases of bleeding or injury, to aid in clot formation and hemostasis. Contraction of smooth muscle in the spleen’s vessel walls, facilitated by sympathetic nervous system signals, propels stored erythrocytes back into the bloodstream, aiding in the maintenance of erythrocyte homeostasis and ensuring adequate oxygen delivery. Splenic rupture is a medical emergency, and it can cause life-threatening internal bleeding. The red pulp’s splenic cords are important reservoirs of large quantities of macrophages. Upon severe tissue injury, the spleen releases a large quantity of monocytes, which travel to the site of injury through the bloodstream to regulate inflammation and facilitate tissue healing. The spleen also plays a role in hematopoiesis during fetal development. Although the primary site of hematopoiesis shifts from the spleen to the bone marrow during fetal development, the spleen retains its hematopoietic capacity in certain circumstances. In certain hematological disorders or situations where bone marrow function is compromised, the spleen can resume its hematopoietic role. Under these conditions, hematopoietic stem cells within the spleen differentiate into various blood cell lineages, producing erythrocytes, granulocytes, and platelets. The spleen also contributes to systemic homeostasis and metabolic regulation. It has a role in iron metabolism, storing iron derived from the degradation of hemoglobin in phagocytosed erythrocytes so it can be recycled. This stored iron can be mobilized during times of increased demand, such as erythropoiesis or iron deficiency, ensuring a steady supply for red blood cell production.