Spleen
Splen (Lien)
Also known as: lien, splenic
The largest secondary lymphoid organ: a fist-sized, intraperitoneal organ in the left hypochondrium deep to ribs 9–11 that filters blood, removes old red cells, recycles iron and mounts immune responses to blood-borne antigens, especially encapsulated bacteria.
Overview
The spleen lies in the left hypochondrium, posterior to the stomach and in contact with the diaphragm, deep to ribs 9–11 with its long axis along the 10th rib; a normal spleen does not extend in front of the mid-axillary line and is not palpable. It is about the size of a clenched fist (≈ 1 × 3 × 5 inches, ≈ 150 g). Its convex diaphragmatic surface faces the diaphragm; its visceral surface bears gastric, renal, colic and pancreatic impressions and the hilum, where the splenic vessels enter and the tail of the pancreas touches it. The superior border is typically notched — a remnant of fetal lobulation that helps identify an enlarged spleen on palpation. It is almost entirely covered by peritoneum and is attached by the gastrosplenic ligament (carrying the short gastric and left gastro-omental vessels) and the splenorenal ligament (carrying the splenic vessels and the tail of the pancreas).
Internally, a capsule of dense connective tissue with some smooth muscle sends trabeculae into a soft pulp. White pulp (lymphoid tissue) surrounds the arterial tree: periarteriolar lymphoid sheaths (PALS) of T cells around central arterioles, B-cell follicles (often with germinal centres) and a marginal zone at the boundary with red pulp, where blood first percolates past macrophages and marginal-zone B cells. Red pulp (most of the volume) consists of splenic cords (of Billroth) — a reticular meshwork packed with macrophages and blood cells — and venous sinusoids lined by elongated stave cells bound by ring-like basal lamina.
Blood from central arterioles passes through penicillar arterioles and macrophage-sheathed capillaries. In the dominant open circulation, it empties into the cords and must squeeze back into the sinusoids through 1–3 µm slits between stave cells. Young, deformable red cells pass; old, rigid or abnormal red cells (and those carrying inclusions) are trapped and eaten by macrophages — culling — or have inclusions removed — pitting. A smaller closed circulation connects capillaries directly to sinusoids.
- Location
Left hypochondrium (and partly epigastric region), intraperitoneal, posterolateral to the stomach, deep to the left 9th–11th ribs, separated from them by the diaphragm and the costodiaphragmatic recess of the pleura.
Parts & subdivisions
- Capsule and trabeculaeDense irregular connective tissue with a little smooth muscle; trabeculae carry trabecular arteries and veins.
- T-cell sheath around central arterioles (thymus-dependent area).
- B-cell follicles, with germinal centres during responses, eccentrically placed on the PALS.
- Marginal zoneInterface between white and red pulp containing the marginal sinus, specialised macrophages and IgM-secreting marginal-zone B cells that respond rapidly to blood-borne polysaccharide antigens.
- Reticular meshwork with macrophages, red cells, platelets and plasma cells; site of red-cell filtering.
- Red pulp — venous sinusoidsWide channels lined by rod-shaped stave cells with slits; drain to pulp veins.
- HilumOn the visceral surface: splenic artery branches, splenic vein tributaries, lymphatics and nerves; tail of pancreas in contact.
Anatomy
- Innervation
Coeliac plexus — sympathetic (vasomotor) fibres travelling with the splenic artery.
- Arterial supply
Splenic artery — the largest branch of the coeliac trunk, running tortuously along the superior border of the pancreas within the splenorenal ligament; divides into 4–5 segmental branches at the hilum (end arteries, so infarcts are wedge-shaped and partial splenectomy is possible). It also gives the short gastric, left gastro-omental and pancreatic branches. The spleen receives ≈ 5% of the cardiac output.
- Venous drainage
Splenic vein runs behind the pancreas, usually receives the inferior mesenteric vein and unites with the superior mesenteric vein behind the neck of the pancreas to form the hepatic portal vein — so portal hypertension causes congestive splenomegaly.
- Lymphatic drainage
Hilar nodes → pancreaticosplenic nodes → coeliac nodes. (The pulp itself has no afferent lymphatics: the spleen filters blood, not lymph.)
- Relations
Diaphragmatic surface: diaphragm (separating it from the left pleura, lung and ribs 9–11). Visceral surface: stomach (anteriorly), left kidney (posteriorly), left colic (splenic) flexure (inferiorly), tail of pancreas (at the hilum). Supported inferiorly by the phrenicocolic ligament.
- Histology
Capsule and trabeculae; white pulp with PALS around central arteries and follicles; marginal zone; red pulp of cords and sinusoids lined by stave cells with ring fibres (best seen with reticulin stains).
- Development
Develops from mesenchymal condensations in the dorsal mesogastrium from the 5th week; the fetal spleen is lobulated (hence the notched border). Rotation of the stomach brings it to the left. Accessory spleens (splenunculi) occur in ≈ 10–30% of people, usually near the hilum or in the gastrosplenic ligament — relevant when splenectomy fails to cure immune thrombocytopenia.
Function
- Filters blood: removes senescent, damaged or abnormal red cells (culling) and intracellular inclusions (pitting).
- Recycles iron and haem from destroyed red cells (iron → transferrin/ferritin; haem → bilirubin).
- Mounts immune responses to blood-borne antigens; key defence against encapsulated bacteria via opsonising antibody, IgM memory B cells and macrophages.
- Reservoir for about a third of the body's platelets and for monocytes released in inflammation.
- Haematopoiesis in the fetus (and extramedullary haematopoiesis in disease).
- Physiology
Most splenic blood flow passes through the open circulation, slowly and at high haematocrit, exposing red cells to prolonged metabolic and mechanical stress. Red cells live ≈ 120 days, and senescent cells lose deformability and are removed mainly here. The spleen also pools platelets (≈ ⅓; more in splenomegaly — hypersplenism), stores monocytes and produces IgM and tuftsin. Unlike in some animals, the human spleen stores little blood, though its capsule can contract slightly under sympathetic stimulation.
Clinical significance
Splenomegaly and hypersplenism
Causes: portal hypertension, haemolytic anaemias, myeloproliferative disorders (CML, myelofibrosis), lymphoma and leukaemia, infections (EBV, malaria, visceral leishmaniasis, infective endocarditis), autoimmune disease and storage diseases (Gaucher). A palpable spleen enlarges towards the right iliac fossa, has a notch, moves with respiration, is dull to percussion and you cannot get above it (distinguishing it from an enlarged kidney, which is ballotable and resonant anteriorly). Sequestration causes pancytopenia (hypersplenism).
Splenic rupture
The spleen is the organ most commonly injured in blunt abdominal trauma, often with left lower rib fractures. Signs: left upper quadrant pain, shock, and pain referred to the left shoulder tip (Kehr sign). Haemodynamically stable patients may be managed non-operatively or with embolisation; unstable patients need splenectomy. Pathologically enlarged spleens (infectious mononucleosis, malaria) can rupture after trivial trauma.
Asplenia and hyposplenism
After splenectomy or in functional asplenia (sickle-cell disease, coeliac disease), patients are at lifelong risk of overwhelming post-splenectomy infection — especially S. pneumoniae, H. influenzae type b, N. meningitidis — and severe malaria and babesiosis. The blood film shows Howell–Jolly bodies (nuclear remnants), target cells and Pappenheimer bodies; platelet counts rise. Prevention: vaccination, antibiotic prophylaxis, patient education and an emergency antibiotic supply.
Mnemonics
1, 3, 5, 7, 9, 11
≈ 1 inch thick, 3 inches wide, 5 inches long, weighs ≈ 7 oz, lies between ribs 9 and 11.
Test yourself
The periarteriolar lymphoid sheath (PALS) of the spleen consists mainly of…
- B lymphocytes
- T lymphocytes
- Red cells and macrophages
- Plasma cells
Answer: T lymphocytes
White pulp has a T-cell PALS around central arterioles, with B-cell follicles and a marginal zone at its edge. Red pulp (cords and sinusoids) filters red cells.
Compiled from standard anatomy and physiology references — see the Lymphatic & Immune System sources. For education only; not medical advice.