Spleen

Abstract

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

  • Left upper quadrant (LUQ) of the abdomen
  • Protected by the left 9th to 11th ribs
  • Neighboring structures of the spleen
    • Diaphragm (posterior and superior)
    • Stomach (anterior and medial)
    • Left kidney (inferior and medial)
    • Left colic flexure (inferior)
    • Tail of the pancreas (at the hilum of the spleen)

The “odd” spleen: 1-3-5-7-9-11 --> The spleen is 1 inch (thickness) 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
Vessel Course Notable features
Arteries
  • Celiac trunk → superior border of pancreashilum of spleen → branches into central and penicillar arterioles
Veins
  • Veins that empty into the splenic vein
    • Short gastric vein
    • Left gastroepiploic vein
    • Pancreatic veins from body and tail of pancreas
    • Inferior mesenteric vein
Lymphatics
  • Celiac nodes
  • Efferent lymphatics only
Innervation

The spleen has only a thin capsule and is a well-vascularized organ, making it 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.

Gastrosplenic ligament Splenorenal ligament
Connected structure
Ligament contents
  • Short gastric vessels, gastroepiploic vessels
Notes

Microscopic anatomy

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

Red pulp (75%)

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

  • Splenic cords: reticular meshwork filled with blood (open circulation system); filters blood from damaged erythrocytes
  • Spleen sinusoids: fenestrated vessels that prevent old or malformed RBCs/platelets from re-entering 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 (25%)

Function

Red pulp

White pulp

The white pulp is the lymphatic tissue of the spleen. 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.

The spleen is the major site of IgM production in the body. It also contains a large store of macrophages capable of capturing opsonized bacteria. Opsonization is the body's primary defense against encapsulated bacteria. Therefore, splenectomy → IgM production (→ ↓ complement activation → opsonization) → ↓ bacterial 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 hemato-oncological disease (e.g., bone marrow involvement in leukemia or lymphoma)!

Clinical significance

The spleen can become enlarged (splenomegaly) due to various disorders: 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).

last updated 10/23/2018
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