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Spleen

Last updated: November 20, 2020

Summary

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.

Gross anatomy

Overview

Characteristics

  • 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 splenomegaly.

Location

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.

Function

Vasculature and innervation

  • Splenic hilum
    • Located on the visceral surface of the spleen
    • Entry and exit point for splenic vasculature
    • Only part of spleen that is not covered in peritoneum
Vasculature and innervation of the spleen
Structure Course Notable features
Arteries
  • Splenic artery
Veins
  • Splenic vein
Lymphatics
Innervation

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!

Ligaments

The splenic ligaments connect the spleen to neighboring structures, contain important vessels, and are part of the greater omentum.

Splenic ligaments
Gastrosplenic ligament Splenorenal ligament
Connected structure
Ligament contents
  • Short gastric vessels
  • Left gastroepiploic vessels
Notes

Microscopic anatomy

The spleen is made of red pulp and white pulp, separated by the marginal zone.

Red pulp

Red pulp is composed of splenic cords and sinuses (sinusoids).

  • 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
    • Open circulation: Blood empties from sheathed capillaries into the splenic cords and then enters the sinusoids through slits in the vessel wall.
    • Closed circulation: Blood empties from sheathed capillaries of the red pulp directly into the sinusoids.
  • Macrophages: found in the cords and around the sinusoids
  • Blood flow: splenic artery arterioles red pulp (cords → sinusoids) → venules splenic vein portal circulation

White pulp

Function

Red pulp

White pulp

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.

Splenectomy → IgM production → ↓ complement activation → ↓ C3b opsonization ↓ clearance of opsonized bacteria → ↑ susceptibility to infection by encapsulated bacteria.

Asplenic patients must be vaccinated against infection with encapsulated organisms!

Embryology

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).

Clinical significance

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).

References