The spleen is an intraperitoneal organ located in the left upper quadrant of the abdomen that originates from the mesoderm. As it lies deep within the rib cage, the spleen is generally not palpable except in particularly slender individuals or if it is pathologically enlarged (splenomegaly). It is supplied by the splenic artery (a branch of the celiac trunk) and drained by the splenic vein, which joins the superior mesenteric vein to form the hepatic portal vein. The lymphatics drain into the celiac lymph nodes. The spleen receives its sympathetic and parasympathetic innervation via the celiac plexus and the vagus nerve respectively. The spleen is composed of the white pulp and the red pulp, which are separated by the marginal zone. The red pulp consists of blood-filled sinusoids and cords that filter abnormal red blood cells and platelets. The white pulp consists of the periarterial lymphatic sheath and lymphoid follicles, which are involved in the production of antibodies. Marginal-zone macrophages are involved in the clearance of encapsulated pathogens. In embryos, the spleen is the site of hematopoiesis. The spleen is a well-vascularized organ that is susceptible to injury and rupture from blunt abdominal injury. Asplenic patients should receive vaccinations against encapsulated organisms such as Streptococcus pneumoniae, Neisseria meningitidis, and H. influenzae.
- Coffee bean-shaped, intraperitoneal organ
- The spleen has two poles (superior and inferior) and two surfaces (diaphragmatic and visceral).
- Usually not palpable as it lies deep in the rib cage, but may be felt in individuals of slender build or those with .
- Left upper quadrant (LUQ) of the abdomen
- Protected by the left 9th to 11th ribs
- Neighboring structures of the spleen
The “odd” spleen: 1-3-5-7-9-11. The spleen is 1 inch (thick) by 3 inches (breadth) by 5 inches (length), weighs 7 ounces, and lies between the 9th and 11th ribs.
- Immune response
- Filters old or misshapen RBCs and platelets out of circulation
- In embryos: hematopoiesis
Vasculature and innervation
- Splenic hilum
|Vasculature and innervation of the spleen|
|Lymphatics|| || |
The spleen has only a thin capsule and is well-vascularized, so it is susceptible to injury and rupture, especially through trauma to the LUQ or left lower ribs. Splenic rupture should therefore always be considered in blunt abdominal injuries!
|Gastrosplenic ligament||Splenorenal ligament|
|Connected structure|| |
|Ligament contents|| |
- Splenic cords: a reticular meshwork filled with blood (open circulation system) that filters the blood from damaged erythrocytes
- Spleen sinusoids: long vessels with a fenestrated ring-like (“barrel hoop”) basement membrane that prevent old or malformed RBCs/platelets from reentering venous circulation
- Macrophages: found in the cords and around the sinusoids
- Blood flow: splenic artery → arterioles → red pulp (cords → sinusoids) → venules → splenic vein → portal circulation
- Periarteriolar lymphatic sheath (PALS)
- Marginal zone
- Filters abnormal RBCs/platelets: macrophages capture and break down defective RBCs/platelets via phagocytosis.
- Fetal erythropoiesis
- Sequestration of platelets: Up to ⅓ of the body's platelets are stored in the spleen.
The white pulp is the lymphatic tissue of the spleen, it mounts immune response to blood-borne antigens and infections. After maturation in the primary lymphatic organs, lymphocytes migrate into the spleen (as a secondary lymphoid organ), where they lie dormant until activation. After differentiation to effector cells, activated B and T lymphocytes reenter the bloodstream via the red pulp to mount an immune response in inflamed tissue.
- Macrophages and other APCs in the PALS and marginal zone trap the antigen from circulation and present it to the lymphocytes of the spleen.
- Splenic macrophages capture the opsonized encapsulated bacteria.
- B cells in follicles differentiate into plasma cells, which produce antibodies (e.g., IgM) and form memory cells
Asplenic patients must be vaccinated against infection with encapsulated organisms!
- Organ of mesodermal origin
- Arises within the dorsal mesentery, posterior to the stomach
- Has foregut blood supply (i.e., celiac trunk), although it is not a foregut derivative
- Involved in fetal erythropoiesis: produces RBCs from 10–28 weeks
Prenatally, the spleen is involved in the formation of blood. Postnatally, extramedullary hematopoiesis in the spleen would not be physiological but rather an indicator of a hematooncological disease (e.g., bone marrow involvement in leukemia or lymphoma).
A variety of disorders can cause splenomegaly, including increased immunological burden (e.g., infectious mononucleosis), increased RBC removal and destruction (e.g., hemolytic anemia), blood congestion (e.g., portal hypertension), and malignant infiltration (e.g., leukemia, lymphoma).