Spleen

Location

To best depict the location of the spleen, we’ll describe its relations. The spleen is found in the left hypochondriac region of the abdomen (left upper quadrant). More precisely, the spleen is located posterior to the stomach and anterior to the left hemidiaphragm at the level of ribs 9-10. Medial to the spleen is the left kidney; superior is the diaphragm, while inferiorly it rests directly on the left colic flexure (splenic flexure).

Although the spleen can descend as far as the pubic symphysis, as seen in mantle cell lymphoma, it normally does not move beyond the left rib arch and so is unavailable for palpation in healthy individuals.

Anatomy

The spleen is a purple, fist-sized organ. It is wrapped by a fibroelastic capsule which allows the spleen to significantly increase its size when necessary. The spleen is an intraperitoneal organ, so all of its surfaces are covered with visceral peritoneum. Only the hilum of the spleen, the site through which the splenic artery and vein pass, is peritoneum-free. 

Organs near to the spleen leave their impressions on its surfaces which, together with spleen borders, can easily be observed and described. 

• Diaphragmatic (lateral) surface leans onto the adjacent part of the diaphragm, thus it is slightly convexed to perfectly fit into the concavity of the left hemidiaphragm. This surface also shows impressions from ribs 9-11. 
• Medial surface of the spleen shows three areas of impression. The colic area is the impression of the left colic flexure, the gastric area is the impression of the stomach, and the renal area is the impression of the left kidney. The splenic hilum is found in the central part of this surface.

The spleen has three borders (superior, inferior, and anterior) as well as two extremities (anterior and posterior). The superior border bounds the gastric area, the inferior border bounds the renal area and the anterior border bounds the colic area.

Microscopic anatomy

Understanding the microscopic anatomy of the spleen is important for understanding its function. Numerous septa called trabeculae extend from the dense irregular fibroelastic connective tissue of the capsule into the parenchyma of the spleen. Both the capsule and trabeculae contain myoepithelial cells which have the ability to contract. As the spleen stores a significant amount of blood, the contraction of myoepithelial cells pumps stored blood into the circulatory system when the body is in need; for example during intense physical activity or massive hemorrhage.

The parenchyma of the spleen is called pulp. Based on the color of the pulp on fresh sections, white and red pulp can be distinguished. White pulp is the main lymphoid tissue of the spleen. It is the accumulation of lymphocytes around an arterial vessel. This aggregation of lymphocytes constitutes the lymphoid tissue known as periarterial lymphoidsheath (PALS) and it is the first to react if microbes reach the spleen through the bloodstream. The central arterial vessels in PALS nodules are branches of the splenic artery.Red pulp consists of splenic venous sinuses and cords (of Billroth), linings of splenic macrophages around the sinuses. The central artery of PALS continues from the white pulp and enters the red pulp as a capillary. These capillaries empty into the splenic cords, where macrophages phagocyte old and damaged erythrocytes. From there, blood diffuses into the splenic sinuses, thus returning to the venous circulation.

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Blood vessels

The arterial supply of the spleen comes from the tortuous splenic artery, which reaches the spleen as it travels through the splenorenal ligament. This artery emerges from the celiac trunk, which is a branch of the abdominal aorta. 

The venous drainage of the spleen occurs via the splenic vein, which also receives blood from the inferior mesenteric vein. Posterior to the neck of the pancreas, the splenic vein unites with the superior mesenteric vein to form the hepatic portal vein. 

Innervation

The spleen is innervated by autonomic nerves from the celiac plexus, which supply the spleen with both sympathetic and parasympathetic nerves. These nerves form the splenic plexus which reaches the splenic hilum traveling along the splenic artery and its branches.

Function

The spleen is a secondary lymphoid organ. This means that the spleen filters blood and presents foreign particles (antigens) to the lymphocytes it houses. In this way, the spleen stimulates the maturation and activation of lymphocytes.

By filtering blood, the spleen also recycles senescent and damaged erythrocytes. In healthy individuals, approximately one-third of total platelets (thrombocytes) are stored in the spleen. In health disturbances followed by spleen enlargement (splenomegaly), the amount of platelets sequestrated in the spleen increases up to 90%, resulting in thrombocytopenia (a low number of platelets in circulating blood). When thrombocytopenia is severe, it can result in spontaneous bleeding which can be very dangerous, especially if it occurs within the central nervous system. Note that in fetuses the spleen is the site where hematopoiesis occurs, meaning that the spleen is the source of blood cell formation until the bone marrow becomes competent to take over that process.

Related articles

Articles within this topic:

• Histology of the spleen
• Splenic artery
• Splenic vein