Corpus: Spleen

The spleen is the largest lymphatic organ in the human body.

2.1. Aspect

The spleen has two main surfaces:

• Diaphragmatic surface: smooth, convex outer surface facing the diaphragm.
• Visceral surface: concave inner surface facing adjacent abdominal organs.

The surrounding organs create slight indentations on the visceral surface:

• Gastric surface: near the stomach
• Renal surface: near the left kidney
• Colic surface: near the left colonic flexure

The splenic hilum, where blood vessels enter the spleen, is located at the center of the visceral surface. The superior edge of the spleen is called the superior margin, and the inferior edge is called the inferior margin. These meet at the anterior and posterior poles of the spleen.

2.2. Dimensions

A healthy spleen typically weighs 150 to 200 g, measures 11 to 14 cm in length, 7 to 8 cm in width, and 3 cm in thickness. The size of the spleen correlates with body size and weight, but gender has no significant impact.

2.3. Topography

The spleen is located in the left upper quadrant of the abdomen, in the left hypochondriac region, intraperitoneally. It is positioned between the 9th and 11th ribs, with its base lying against the left diaphragm.

2.4. Vascular supply

The spleen receives its arterial supply from the splenic artery, a branch of the celiac trunk. Inside the spleen, the splenic artery divides into trabecular arteries, which further branch into central arterioles that supply the splenic follicles. Venous blood from the spleen is drained via the splenic vein, which joins the portal vein, with both vessels converging at the splenic hilum. Lymphatic drainage from the spleen occurs through the splenic lymph nodes.

2.5. Innervation

The spleen's autonomic innervation comes from sympathetic fibers of the celiac plexus, known as the splenic plexus, which accompany the splenic artery. These adrenergic fibers control the contraction of myofibroblasts in the trabecular-capsular system. The presence of parasympathetic innervation is debated.^[1]

2.6. Ligamentous apparatus

The spleen is held in place by four main ligaments, which are folds of peritoneum:

• Splenocolic ligament
• Gastrosplenic ligament
• Phrenicocolic ligament
• Phrenicosplenic ligament

The splenorenal ligament also contributes to its stabilization.

2.7. Splenic spleen

Around 20 % of individuals have one or more accessory spleens, typically near the splenic hilum or embedded in the gastrosplenic ligament or greater omentum. These have the same histological structure as the main spleen. In cases where splenectomy is required (e.g., due to excessive destruction of red blood cells), accessory spleens must also be removed to prevent recurrence of symptoms. In contrast, accessory spleens can be beneficial when the spleen is removed due to trauma, as they may take over its functions.

Polysplenia, a condition in which multiple, usually non-functional spleens are present without a main spleen, is often associated with other congenital abnormalities.

2.8. Mnemonic

A simple rule of thumb for spleen anatomy is the saying "3 business economists with 8 suitcases drive 200 kilometers in a 911 on the 13th" - dimensions: 3 x 8 x 13 cm; weight: 200 g; location: between the 9th and 11th ribs (note: the 3rd business economist must be very small).

The spleen is enclosed by a dense connective tissue capsule, which is covered by peritoneum. Trabeculae extend from this capsule into the spleen's parenchyma, creating a meshwork of reticular fibers and myofibroblasts.

The spleen is divided into two functional regions: the white pulp and the red pulp.

4.1. White pulp

The white pulp consists of three main parts:

• The whitish, macroscopically visible splenic nodules (Malpighian corpuscles), which are lymphoid follicles containing B lymphocytes.
• The marginal zone, which is grouped together with the lymph follicles as the "B zone."
• The periarterial lymphatic sheaths (PALS) surrounding the central arteries, which are populated by T lymphocytes.

The central arteries further branch out to form brush-like vascular structures known as brush arterioles, which then continue into the sheath capillaries of the adjacent red pulp.

The white pulp is involved in immune defense and constantly monitors the blood flowing through it for antigens and toxins.

4.2. Red pulp

Located between the splenic nodules and sinuses, the red pulp contains reticular connective tissue through which blood flows. Its main function is to break down old or deformed erythrocytes. Macrophages in the red pulp phagocytose damaged cells that become trapped in the meshwork.

Unlike most other abdominal organs, the spleen arises from mesenchymal tissue in the dorsal mesogastrium. By the third month of fetal development, the spleen separates from the mesogastrium, remaining connected only at the splenic hilum. The rotation of the stomach places the spleen in the left upper abdomen.

The spleen has multiple roles in immune defense and blood cell management:

• Phagocytosis: Macrophages in the red pulp clear old or damaged blood cells, as well as microorganisms, immune complexes, and fibrin.
• Immune defense: The spleen promotes B and T cell activation and proliferation in response to antigens. It also produces opsonins, properdin, and tuftsin, which enhance immune responses.
• Hematopoiesis: In early childhood, the spleen participates in the formation of blood cells. In cases of bone marrow failure, it can resume this function.
• Blood storage: The spleen stores erythrocytes, lymphocytes, and other blood cells, releasing around 240 ml of blood during hypovolemia or hypoxia.

7.1. Diseases

Hypersplenism refers to hyperfunction of the spleen, while hyposplenism indicates reduced function. The absence of the spleen or its complete dysfunction is called asplenia. Splenomegaly describes an enlarged spleen.

Splenic rupture is a significant traumatic event. Infections can lead to splenitis or the formation of a splenic abscess. A splenic infarction occurs when the blood supply to the spleen is interrupted, causing tissue death.

7.2. Examination methods

In healthy individuals, the spleen is not palpable. However, if it is significantly enlarged, it may be felt during inspiration under the left costal arch.

The spleen is examined using imaging techniques such as:

• Sonography
• Computed tomography (CT)
• Magnetic resonance imaging (MRI).

The spleen’s vessels can be visualized using angiography, and its morphological and functional changes can be evaluated through spleen scintigraphy.

Regular apnoea diving has led to genetic adaptations in the PDE10A gene in the Bajau people, also known as "sea nomads." These adaptations cause spleen enlargement, allowing them to stay underwater for longer periods since the spleen serves as an erythrocyte reservoir.^[2]

• Sectio chirurgica: Surgery on the spleen
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• Aumüller et al. Duale Reihe Anatomie, Thieme Verlag, 5th edition, 2020